Chapter 2.2. Electrons

The three unit-cell vectors , and define the parallelepiped of the unit cell. We define

From the seven possible crystal systems one arrives at the 14 possible space lattices, based on both primitive and non-primitive (body-centred, face-centred and base-centred) cells, called the Bravais lattices [see Chapter 9.1 of International Tables for Crystallography, Volume A (2005)]. Instead of describing these cells as parallelepipeds, we can find several types of polyhedra with which we can fill space by translation. A very important type of space filling is obtained by the Dirichlet construction. Each lattice point is connected to its nearest neighbours and the corresponding bisecting (perpendicular) planes will delimit a region of space which is called the Dirichlet region, the Wigner–Seitz cell or the Voronoi cell. This cell is uniquely defined and has additional symmetry properties.

When we add a basis to the lattice (i.e. the atomic positions in the unit cell) we arrive at the well known 230 space groups [see Part 3 of International Tables for Crystallography, Volume A (2005)].

Owing to the translational symmetry of a crystal, it is convenient to define a reciprocal lattice, which plays a dominating role in describing electrons in a solid. The three unit vectors of the reciprocal lattice are given according to the standard definition by where the factor is commonly used in solid-state physics in order to simplify many expressions. Strictly speaking (in terms of mathematics) this factor should not be included [see Section 1.1.2.4 of the present volume and Chapter 1.1 of International Tables for Crystallography, Volume B (2001)], since the (complete) reciprocity is lost, i.e. the reciprocal lattice of the reciprocal lattice is no longer the direct lattice.

In analogy to the direct lattice we define

A construction identical to the Wigner–Seitz cell delimits in reciprocal space a cell conventionally known as the first Brillouin zone (BZ), which is very important in the band theory of solids. There are 14 first Brillouin zones according to the 14 Bravais lattices.

Often we are interested in a function (e.g. a wavefunction) and wish to know how it transforms under the configuration operator g which acts on . For this purpose it is useful to introduce a function-space operator which defines how to modify the function in the transformed configuration space so that it agrees with the original function at the original coordinate :This must be valid for all points and thus also for , leading to the alternative formulation The symmetry operations form a group G of configuration-space operations with the related group of the function-shape operators . Since the multiplication rules are preserved, these two groups are isomorphic.

In a quantum-mechanical treatment of the electronic states in a solid we have the following different entities: points in configuration space, functions defined at these points and (quantum-mechanical) operators acting on these functions. A symmetry operation transforms the points, the functions and the operators in a clearly defined way.

Consider an eigenvalue equation of operator (e.g. the Hamiltonian):where is a function of . When g acts on , the function-space operator acts [according to (2.2.3.4)] on yielding : By putting from (2.2.3.7) into (2.2.3.6), we obtain Multiplication from the left by yields This defines the transformed operator which acts on the transformed function that is given by the original function but at position .

The most general space-group operation is of the form with the point-group operation p (a rotation, reflection or inversion) followed by a translation w: With the definition it is easy to prove the multiplication rule and define the inverse of a Seitz operator as which satisfies where does not change anything and thus is the identity of the space group G.

Using the Seitz operators, we can classify the most important groups as we need them at the beginning of this chapter:

In order to derive the Bloch theorem, we can simplify the problem by considering a one-dimensional case with a lattice constant a. [The generalization to the three-dimensional case can be done easily according to (2.2.3.15).] The one-dimensional Schrödinger equation is where is invariant under translations, i.e. . We define a translation operator t according to (2.2.3.1) for the translation by one lattice constant as and apply its functional counterpart to the potential, which gives [according to (2.2.3.4)]The first part in corresponds to the kinetic energy operator, which is also invariant under translations. Therefore, since (the lattice translation subgroup) and (the Schrödinger group), commutes with , i.e. the commutator vanishes, or . This situation was described above [see (2.2.3.16)–(2.2.3.18)] and leads to the fundamental theorem of quantum mechanics which states that when two operators commute the eigenvectors of the first must also be eigenvectors of the second. Consequently we havewhere is the eigenvalue corresponding to the translation by the lattice constant a. The second equation can be written explicitly as and tells us how the wavefunction changes from one unit cell to the neighbouring unit cell. Notice that the electron density must be translationally invariant and thus it follows which is a necessary (but not sufficient) condition for defining .

We can expect the bulk properties of a crystal to be insensitive to the surface and also to the boundary conditions imposed, which we therefore may choose to be of the most convenient form. Symmetry operations are covering transformations and thus we have an infinite number of translations in T, which is most inconvenient. A way of avoiding this is provided by periodic boundary conditions (Born–von Karman). In the present one-dimensional case this means that the wavefunction becomes periodic in a domain (with integer N number of lattice constants a), i.e. According to our operator notation (2.2.4.6), we have the following situation when the translation t is applied n times: It follows immediately from the periodic boundary condition (2.2.4.9) that with the obvious solution Here it is convenient to introduce a notation so that we can write . Note that k is quantized due to the periodic boundary conditions according to (2.2.4.13). Summarizing, we have the Bloch condition (for the one-dimensional case): i.e. when we change x by one lattice constant a the wavefunction at x is multiplied by a phase factor . At the moment (2.2.4.13) suggests the use of k as label for the wavefunction .

Generalization to three dimensions leads to the exponential with and thus to the Bloch condition or written in terms of the translational operator [see (2.2.3.15)] The eigenfunctions that satisfy (2.2.4.17) are called Bloch functions and have the form where is a periodic function in the lattice, and is a vector in the reciprocal lattice [see (2.2.2.6)] that plays the role of the quantum number in solids. The vector can be chosen in the first BZ, because any that differs from by just a lattice vector of the reciprocal lattice has the same Bloch factor and the corresponding wavefunction satisfies the Bloch condition again, since where the factor is unity according to (2.2.2.7). Since these two functions, and , belong to the same Bloch factor they are equivalent. A physical interpretation of the Bloch states will be given in Section 2.2.8.

Let us repeat a few fundamental definitions of group theory: For any symmetry operation , the product can always be formed for any and defines the conjugate element of by g. Given any operation , its class is defined as the set of all its conjugates under all operations . What we need here is an important property of classes, namely that no two classes have any element in common so that any group can be considered as a sum of classes.

Assuming periodic boundary conditions with number of primitive cells along the axes , respectively, a lump of crystal with unit cells is studied. The translation subgroup T contains the general translation operators , which [using (2.2.3.15)] can be written as where each factor belongs to one of the three axes. Since T is commutative (Abelian), each operation of T is its own class and thus the number of classes equals its order, namely N. From the general theorem that the squares of the dimensions of all irreducible representations of a group must equal the order of the group, it follows immediately that all N irreducible representations of T must be one-dimensional (see also Section 1.2.3.2 of the present volume). Taking the subgroup along the axis, we must have different irreducible representations, which we label (for later convenience) by and denote as These representations are one-dimensional matrices, i.e. numbers, and must be exponentials, often chosen of the form . The constant must be related to the corresponding label of the irreducible representation. In the three-dimensional case, we have the corresponding representation where we have used the definitions (2.2.2.6) and (2.2.2.1). Within the present derivation, the vector corresponds to the label of the irreducible representation of the lattice translation subgroup.

The effect of a space-group operation on a Bloch function, labelled by , is to transform it into a Bloch function that corresponds to a vector ,which can be proven by using the multiplication rule of Seitz operators (2.2.3.12) and the definition of a Bloch state (2.2.4.17).

A special case is the inversion operator, which leads to The Bloch functions and , where p is any operation of the point group P, belong to the same basis for a representation of the space group G. The same cannot appear in two different bases, thus the two bases and are either identical or have no in common.

Irreducible representations of T are labelled by the N distinct vectors in the BZ, which separate in disjoint bases of G (with no vector in common). If a vector falls on the BZ edge, application of the point-group operation p can lead to an equivalent vector that differs from the original by (a vector of the reciprocal lattice). The set of all mutually inequivalent vectors of () define the star of the k vector () (see also Section 1.2.3.3 of the present volume).

The set of all operations that leave a vector invariant (or transform it into an equivalent ) forms the group of the vector. Application of q, an element of , to a Bloch function (Section 2.2.8) gives where the band index j (described below) may change to . The Bloch factor stays constant under the operation of q and thus the periodic cell function must show this symmetry, namely For example, a -like orbital may be transformed into a -like orbital if the two are degenerate, as in a tetragonal lattice.

A star of determines an irreducible basis, provided that the functions of the star are symmetrized with respect to the irreducible representation of the group of vectors, which are called small representations. The basis functions for the irreducible representations are given according to Seitz (1937) by written as a row vector with , where n is the dimension of the irreducible representation of with the order . Such a basis consists of functions and forms an -dimensional irreducible representation of the space group. The degeneracies of these representations come from the star of (not crucial for band calculations except for determining the weight of the vector) and the degeneracy from . The latter is essential for characterizing the energy bands and using the compatibility relations (Bouckaert et al., 1930; Bradley & Cracknell, 1972).

Each irreducible representation of the space group, labelled by , denotes an energy , where varies quasi-continuously over the BZ and the superscript j numbers the band states. The quantization of according to (2.2.4.13) and (2.2.4.15) can be done in arbitrary fine steps by choosing corresponding periodic boundary conditions (see Section 2.2.4.2). Since and belong to the same Bloch state, the energy is periodic in reciprocal space: Therefore it is sufficient to consider vectors within the first BZ. For a given , two bands will not have the same energy unless there is a multidimensional small representation in the group of or the bands belong to different irreducible representations and thus can have an accidental degeneracy. Consequently, this can not occur for a general vector (without symmetry).

The vector plays a fundamental role in the electronic structure of a solid. In the above, several interpretations have been given for the vector that

Starting with one of the 14 Bravais lattices, one can define the reciprocal lattice [according to (2.2.2.4)] by the Wigner–Seitz construction as discussed in Section 2.2.2.2. The advantage of using the BZ instead of the parallelepiped spanned by the three unit vectors is its symmetry. Let us take a simple example first, namely an element (say copper) that crystallizes in the face-centred-cubic (f.c.c.) structure. With (2.2.2.4) we easily find that the reciprocal lattice is body-centred-cubic (bcc) and the corresponding BZ is shown in Fig. 2.2.7.1. In this case, f.c.c. Cu has symmetry with 48 symmetry operations (point group). The energy eigenvalues within a star of (i.e. ) are the same, and therefore it is sufficient to calculate one member in the star. Consequently, it is enough to consider the irreducible wedge of the BZ (called the IBZ). In the present example, this corresponds to 1/48th of the BZ shown in Fig. 2.2.7.1. To count the number of states in the BZ, one counts each point in the IBZ with a proper weight to represent the star of this vector.

Figure 2.2.7.1 | top | pdf | The Brillouin zone (BZ) and the irreducible wedge of the BZ for the f.c.c. direct lattice. After the corresponding figure from the Bilbao Crystallographic Server (http://www.cryst.ehu.es/ ). The IBZ for any space group can be obtained by using the option KVEC and specifying the space group (in this case No. 225).

The BZ is purely constructed from the reciprocal lattice and thus only follows from the translational symmetry (of the 14 Bravais lattices). However, the energy bands , with lying within the first BZ, possess a symmetry associated with one of the 230 space groups. Therefore one can not simply use the geometrical symmetry of the BZ to find its irreducible wedge, although this is tempting. Since the effort of computing energy eigenvalues increases with the number of points, one wishes to restrict such calculations to the basic domain, but the latter can only be found by considering the space group of the corresponding crystal (including the basis with all atomic positions).

One possible procedure for finding the IBZ is the following. First a uniform grid in reciprocal space is generated by dividing the three unit-cell vectors by an integer number of times. This is easy to do in the parallelepiped, spanned by the three unit-cell vectors, and yields a (more-or-less) uniform grid of points. Now one must go through the complete grid of points and extract a list of non-equivalent points by applying to each point in the grid the point-group operations. If a point is found that is already in the list, its weight is increased by 1, otherwise it is added to the list. This procedure can easily be programmed and is often used when integrations are needed. The disadvantage of this scheme is that the generated points in the IBZ are not necessarily in a connected region of the BZ, since one member of the star of is chosen arbitrarily, namely the first that is found by going through the complete list.

Hartree–Fock-based (HF-based) methods (for a general description see, for example, Pisani, 1996) are based on a wavefunction description (with one Slater determinant in the HF method). The single-particle HF equations (written for an atom in Rydberg atomic units) can be written in the following form, which is convenient for further discussions:with terms for the kinetic energy, the nuclear electronic potential, the classical electrostatic Coulomb potential and the exchange, a function potential which involves the permutation operator , which interchanges the arguments of the subsequent product of two functions. This exchange term can not be rewritten as a potential times the function but is truly non-local (i.e. depends on and ). The interaction of orbital j with itself (contained in the third term) is unphysical, but this self-interaction is exactly cancelled in the fourth term. This is no longer true in the approximate DFT method discussed below. The HF method treats exchange exactly but contains – by definition – no correlation effects. The latter can be added in an approximate form in post-HF procedures such as that proposed by Colle & Salvetti (1990).

Density functional theory (DFT) is an alternative approach in which both effects, exchange and correlation, are treated in a combined scheme but both approximately. Several forms of DFT functionals are available now that have reached high accuracy, so many structural problems can be solved adequately. Further details will be given in Section 2.2.10.

Most calculations of the electronic structure in solids (Pisani, 1996; Singh, 1994; Altmann, 1994) use a linear combination of basis functions in one form or another but differ in the basis sets. Some use a linear combination of atomic orbitals (LCAO) where the AOs are given as Gaussian- or Slater-type orbitals (GTOs or STOs); others use plane-wave (PW) basis sets with or without augmentations; and still others make use of muffin-tin orbitals (MTOs) as in LMTO (linear combination of MTOs; Skriver, 1984) or ASW (augmented spherical wave; Williams et al., 1979). In the former cases, the basis functions are given in analytic form, but in the latter the radial wavefunctions are obtained numerically by integrating the radial Schrödinger equation (Singh, 1994) (see Section 2.2.11).

Closely related to the choice of basis sets is the explicit form of the wavefunctions, which can be well represented by them, whether they are nodeless pseudo-wavefunctions or all-electron wavefunctions including the complete radial nodal structure and a proper description close to the nucleus.

In the muffin-tin or the atomic sphere approximation (MTA or ASA), each atom in the crystal is surrounded by an atomic sphere in which the potential is assumed to be spherically symmetric [see (2.2.12.5) and the discussion thereof]. While these schemes work reasonably well in highly coordinated, closely packed systems (such as face-centred-cubic metals), they become very approximate in all non-isotropic cases (e.g. layered compounds, semiconductors, open structures or molecular crystals). Schemes that make no shape approximation in the form of the potential are termed full-potential schemes (Singh, 1994; Blaha et al., 1990; Schwarz & Blaha, 1996).

With a proper choice of pseudo-potential one can focus on the valence electrons, which are relevant for chemical bonding, and replace the inner part of their wavefunctions by a nodeless pseudo-function that can be expanded in PWs with good convergence.

If a solid contains only light elements, non-relativistic calculations are well justified, but as soon as heavier elements are present in the system of interest relativistic effects can no longer be neglected. In the medium range of atomic numbers (up to about 54), so-called scalar relativistic schemes are often used (Koelling & Harmon, 1977), which describe the main contraction or expansion of various orbitals (due to the Darwin s-shift or the mass–velocity term) but omit spin–orbit splitting. Unfortunately, the spin–orbit term couples spin-up and spin-down wavefunctions. If one has n basis functions without spin–orbit coupling, then including spin–orbit coupling in the Hamiltonian would lead to a matrix equation, which requires about eight times as much computer time to solve it (due to the scaling). Since the spin–orbit effect is generally small (at least for the valence states), one can simplify the procedure by diagonalizing the Hamiltonian including spin–orbit coupling in the space of the low-lying bands as obtained in a scalar relativistic step. This version is called second variational method (see e.g. Singh, 1994). For very heavy elements it may be necessary to solve Dirac's equation, which has all these terms (Darwin s-shift, mass–velocity and spin–orbit) included. Additional aspects are illustrated in Section 2.2.14 in connection with the uranium atom.

For the description of crystalline wavefunctions (Bloch functions), one often starts with a simple concept of placing atomic orbitals (AOs) at each site in a crystal denoted by , from which one forms Bloch sums in order to have proper translational symmetry: Then Bloch functions can be constructed by taking a linear combination of such Bloch sums, where the linear-combination coefficients are determined by the variational principle in which a secular equation must be solved. The LCAO can be used in combination with both the Hartree–Fock method and DFT.

A simple version of the LCAO is found by parameterizing the matrix elements and in a way similar to the Hückel molecular orbital (HMO) method, where the only non-vanishing matrix elements are the on-site integrals and the nearest-neighbour interactions (hopping integrals). For a particular class of solids the parameters can be adjusted to fit experimental values. With these parameters, the electronic structures of rather complicated solids can be described and yield quite satisfactory results, but only for the class of materials for which such a parametrization is available. Chemical bonding and symmetry aspects can be well described with such schemes, as Hoffmann has illustrated in many applications (Hoffmann, 1988). In more complicated situations, however, such a simple scheme fails.

In many respects, core electrons are unimportant for determining the stability, structure and low-energy response properties of crystals. It is a well established practice to modify the one-electron part of the Hamiltonian by replacing the bare nuclear attraction with a pseudo-potential (PP) operator, which allows us to restrict our calculation to the valence electrons. The PP operator must reproduce screened nuclear attractions, but must also account for the Pauli exclusion principle, which requires that valence orbitals are orthogonal to core ones. The PPs are not uniquely defined and thus one seeks to satisfy the following characteristics as well as possible:

There are many versions of the PP method (norm-conserving, ultrasoft etc.) and the actual accuracy of a calculation is governed by which is used. For standard applications, PP techniques can be quite successful in solid-state calculations. However, there are cases that require higher accuracy, e.g. when core electrons are involved, as in high-pressure studies or electric field gradient calculations (see Section 2.2.15), where the polarization of the charge density close to the nucleus is crucial for describing the physical effects properly.

The partition of space (i.e. the unit cell) between (non-overlapping) atomic spheres and an interstitial region (see Fig. 2.2.12.1) is used in several schemes, one of which is the augmented plane wave (APW) method, originally proposed by Slater (Slater, 1937) and described by Loucks (1967), and its linearized version (the LAPW method), which is chosen as the one representative method that is described in detail in Section 2.2.12.

The basis set is constructed using the muffin-tin approximation (MTA) for the potential [see the discussion below in connection with (2.2.12.5)]. In the interstitial region the wavefunction is well described by plane waves, but inside the spheres atomic-like functions are used which are matched continuously (at the sphere boundary) to each plane wave.

In the KKR scheme (Korringa, 1947; Kohn & Rostocker, 1954), the solution of the KS equations (2.2.10.3) uses a Green-function technique and solves a Lippman–Schwinger integral equation. The basic concepts come from a multiple scattering approach which is conceptually different but mathematically equivalent to the APW method. The building blocks are spherical waves which are products of spherical harmonics and spherical Hankel, Bessel and Neumann functions. Like plane waves, they solve the KS equations for a constant potential. Augmenting the spherical waves with numerical solutions inside the atomic spheres as in the APW method yields the KKR basis set. Compared with methods based on plane waves, spherical waves require fewer basis functions and thus smaller secular equations.

The radial functions in the APW and KKR methods are energy-dependent and so are the corresponding basis functions. This leads to a nonlinear eigenvalue problem that is computationally demanding. Andersen (1975) modelled the weak energy dependence by a Taylor expansion where only the first term is kept and thereby arrived at the so-called linear methods LMTO and LAPW.

The LMTO method (Andersen, 1975; Skriver, 1984) is the linearized counterpart to the KKR method, in the same way as the LAPW method is the linearized counterpart to the APW method. This widely used method originally adopted the atomic sphere approximation (ASA) with overlapping atomic spheres in which the potential was assumed to be spherically symmetric. Although the ASA simplified the computation so that systems with many atoms could be studied, the accuracy was not high enough for application to certain questions in solid-state physics.

Following the ideas of Andersen, the augmented spherical wave (ASW) method was developed by Williams et al. (1979). The ASW method is quite similar to the LMTO scheme.

It should be noted that the MTA and the ASA are not really a restriction on the method. In particular, when employing the MTA only for the construction of the basis functions but including a generally shaped potential in the construction of the matrix elements, one arrives at a scheme of very high accuracy which allows, for instance, the evaluation of elastic properties. Methods using the unrestricted potential together with basis functions developed from the muffin-tin potential are called full-potential methods. Now for almost every method based on the MTA (or ASA) there exists a counterpart employing the full potential.

Conventional quantum-mechanical calculations are done using the Born–Oppenheimer approximation, in which one assumes (in most cases to a very good approximation) that the electrons are decoupled from the nuclear motion. Therefore the electronic structure is calculated for fixed atomic (nuclear) positions. Car & Parrinello (1985) suggested a new method in which they combined the motion of the nuclei (at finite temperature) with the electronic degrees of freedom. They started with a fictitious Lagrangian in which the wavefunctions follow a dynamics equation of motion. Therefore, the CP method combines the motion of the nuclei (following Newton's equation) with the electrons (described within DFT) into one formalism by solving equations of motion for both subsystems. This simplifies the computational effort and allows ab initio molecular dynamics calculations to be performed in which the forces acting on the atoms are calculated from the wavefunctions within DFT. The CP method has attracted much interest and is widely used, with a plane-wave basis, extended with pseudo-potentials and recently enhanced into an all-electron method using the projector augmented wave (PAW) method (Blöchl, 1994). Such CP schemes can also be used to find equilibrium structures and to explore the electronic structure.

The various techniques outlined so far have one thing in common, namely the scaling. In a system containing N atoms the computational effort scales as , since one must determine a number of orbitals that is proportional to N which requires diagonalization of matrices, where the prefactor k depends on the basis set and the method used. In recent years much work has been done to devise algorithms that vary linearly with N, at least for very large N (Ordejon et al., 1995). First results are already available and look promising. When such schemes become generally available, it will be possible to study very large systems with relatively little computational effort. This interesting development could drastically change the accessibility of electronic structure results for large systems.

All spatial observables of the bound atom (e.g. the potential or the charge density) must have the crystal symmetry, i.e. the point-group symmetry around an atom. Therefore they must be representable as an expansion in terms of site-symmetrized spherical harmonics. Any point-symmetry operation transforms a spherical harmonic into another of the same . We start with the usual complex spherical harmonics, which satisfy Laplacian's differential equation. The are the associated Legendre polynomials and the normalization is according to the convention of Condon & Shortley (1953). For the -dependent part one can use the real and imaginary part and thus use and instead of the functions, but we must introduce a parity p to distinguish the functions with the same . For convenience we take real spherical harmonics, since physical observables are real. The even and odd polynomials are given by the combination of the complex spherical harmonics with the parity p either or − by

The expansion of – for example – the charge density around an atomic site can be written using the LAPW method [see the analogous equation (2.2.12.5) for the potential] in the form where we use capital letters for the indices (i) to distinguish this expansion from that of the wavefunctions in which complex spherical harmonics are used [see (2.2.12.1)] and (ii) to include the parity p in the index M (which represents the combined index ). With these conventions, can be written as a linear combination of real spherical harmonics which are symmetry-adapted to the site symmetry,i.e. they are either [(2.2.13.2)] in the non-cubic cases (Table 2.2.13.1) or are well defined combinations of 's in the five cubic cases (Table 2.2.13.2), where the coefficients depend on the normalization of the spherical harmonics and can be found in Kurki-Suonio (1977).

Table 2.2.13.1 | top | pdf | Picking rules for the local coordinate axes and the corresponding combinations () of non-cubic groups taken from Kurki-Suonio (1977) Symmetry Coordinate axes of Crystal system 1 Any All Triclinic Any 2 Monoclinic 222 Orthorhombic 4 Tetragonal 422 3 Rhombohedral 32 6 Hexagonal 622

Table 2.2.13.2 | top | pdf | LM combinations of cubic groups as linear cominations of 's (given in parentheses) The linear-combination coefficients can be found in Kurki-Suonio (1977). Symmetry LM combinations 23 (0 0), (3 2−), (4 0, 4 4), (6 0, 6 4), (6 2, 6 6) (0 0), (4 0, 4 4), (6 0, 6 4) (6 2, 6 6) 432 (0 0), (4 0, 4 4), (6 0, 6 4) (0 0), (3 2−), (4 0, 4 4), (6 0, 6 4), (0 0), (4 0, 4 4), (6 0, 6 4)

According to Kurki-Suonio, the number of (non-vanishing) terms [e.g. in (2.2.13.3)] is minimized by choosing for each atom a local Cartesian coordinate system adapted to its site symmetry. In this case, other terms would vanish, so using only these terms corresponds to the application of a projection operator, i.e. equivalent to averaging the quantity of interest [e.g. ] over the star of . Note that in another coordinate system (for the L values listed) additional M terms could appear. The group-theoretical derivation led to rules as to how the local coordinate system must be chosen. For example, the z axis is taken along the highest symmetry axis, or the x and y axes are chosen in or perpendicular to mirror planes. Since these coordinate systems are specific for each atom and may differ from the (global) crystal axes, we call them `local' coordinate systems, which can be related by a transformation matrix to the global coordinate system of the crystal.

The symmetry constraints according to (2.2.13.4) are summarized by Kurki-Suonio, who has defined picking rules to choose the local coordinate system for any of the 27 non-cubic site symmetries (Table 2.2.13.1) and has listed the combinations, which are defined by (a linear combination of) functions [see (2.2.13.2)]. If the parity appears, both the and the − combination must be taken. An application of a local coordinate system to rutile TiO₂ is described in Section 2.2.16.2.

In the case of the five cubic site symmetries, which all have a threefold axis in (111), a well defined linear combination of functions (given in Table 2.2.13.2) leads to the cubic harmonics.

Chemical bonding is often described by considering orbitals (e.g. a or a atomic orbital) which are defined in polar coordinates, where the z axis is special, in contrast to Cartesian coordinates, where x, y and z are equivalent. Consider for example an atom coordinated by ligands (e.g. forming an octahedron). Then the application of group theory, ligand-field theory etc. requires a certain coordinate system provided one wishes to keep the standard notation of the corresponding spherical harmonics. If this octahedron is rotated or tilted with respect to the global (unit-cell) coordinate system, a local coordinate system is needed to allow an easy orbital interpretation of the inter­actions between the central atom and its ligands. This applies also to spectroscopy or electric field gradients.

The two types of reasons mentioned above may or may not lead to the same choice of a local coordinate system, as is illustrated for the example of rutile in Section 2.2.16.2.

The energy bands are primarily characterized by the wavevector in the first BZ that is associated with the translational symmetry according to (2.2.4.23). The star of determines an irreducible basis provided that the functions of the star are symmetrized with respect to the small representations, as discussed in Section 2.2.6. Along symmetry lines in the BZ (e.g. from along towards X in the BZ shown in Fig. 2.2.7.1), the corresponding group of the vector may show a group–subgroup relation, as for example for and . The corresponding irreducible representations can then be found by deduction (or by induction in the case of a group–supergroup relation). These concepts define the compatibility relations (Bouckaert et al., 1930; Bradley & Cracknell, 1972), which tell us how to connect energy bands. For example, the twofold degenerate representation (the symmetry in a cubic system) splits into the and manifold in the direction, both of which are one-dimensional. The compatibility relations tell us how to connect bands. In addition, one can also find an orbital representation and thus knows from the group-theoretical analysis which orbitals belong to a certain energy band. This is very useful for interpretations.

In chemistry and physics it is quite common to separate the electronic states of an atom into those from core and valence electrons, but sometimes this distinction is not well defined, as will be discussed in connection with the so-called semi-core states. For the sake of argument, let us discuss the situation in a solid using the concepts of the LAPW method, keeping in mind that very similar considerations hold for all other band-structure schemes.

A core state is characterized by a low-lying energy (i.e. with a large negative energy value with respect to the Fermi energy) and a corresponding wavefunction that is completely confined inside the sphere of the respective atom. Therefore there is effectively no overlap with the wavefunctions from neighbouring atoms and, consequently, the associated band width is practically zero.

The valence electrons occupy the highest states and have wavefunctions that strongly overlap with their counterparts at adjacent sites, leading to chemical bonding, large dispersion (i.e. a strong variation of the band energy with ) and a significant band width.

The semi-core states are in between these two categories. For example, the 3s and 3p states of the 3d transition metals belong here. They are about 2–6 Ry (1 Ry = 13.6 eV) below the valence bands and have most of the wavefunctions inside their atomic spheres, but a small fraction (a few per cent) of the corresponding charge lies outside this sphere. This causes weak interactions with neighbouring atoms and a finite width of the corresponding energy bands.

Above the valence states are the unoccupied states, which often (e.g. in DFT or the HF method) require special attention.

For interpreting chemical bonding or the physical origin of a given Bloch state at , a decomposition according to its wavefunction is extremely useful but always model-dependent. The charge density corresponding to the Bloch state at can be normalized to one per unit cell and is (in principle) an observable, while its decomposition depends on the model used. The following considerations are useful in this context:

Simple metals with valence electrons originating from s- and p-type orbitals form wide bands which are approximately free-electron like (with a large band width W). Such a case corresponds to itinerant electrons that are delocalized and thus cause metallic conductivity.

The other extreme case is a system with 4f (and some 5f) electrons, such as the lanthanides. Although the orbital energies of these electrons are in the energy range of the valence electrons, they act more like core electrons and thus are tightly bound to the corresponding atomic site. Such electrons are termed localized, since they do not hop to neighbouring sites (controlled by a hopping parameter t) and thus do not contribute to metallic conductivity. Adding another of these electrons to a given site would increase the Coulomb repulsion U. A large U (i.e. t) prevents them from hopping.

There are – as usual – borderline cases (e.g. the late 3d transition-metal oxides) in which a delicate balance between t and U, the energy gain by delocalizing electrons and the Coulomb repulsion, determines whether a system is metallic or insulating. This problem of metal/insulator transitions is an active field of research of solid-state physics which shall not be discussed here.

In one example, however, the dual role of f electrons is illustrated for the uranium atom using relativistic wavefunctions (with a large and a small component) characterized by the quantum numbers n, and j. Fig. 2.2.14.1 shows the outermost lobe (the large component) of the electrons beyond the [Xe] core without the 4f and 5d core-like states. One can see the , and (semi-core) electrons, and the and (valence) electrons.

Figure 2.2.14.1 | top | pdf | Relativistic radial wavefunctions (large component) of the uranium atom. Shown are the outer lobes of valence and semi-core states excluding the [Xe] core, and the 4f and 5d core states.

On the one hand, the radial wavefunction of the orbital has its peak closer to the nucleus than the main lobes of the semi-core states , and , and thus demonstrates the core nature of these 5f electrons. On the other hand, the orbital decays (with distance) much less than the semi-core states and electrons in this orbital can thus also play the role of valence electrons, like electrons in the and orbitals. This dual role of the f electrons has been discussed, for example, by Schwarz & Herzig (1979).

In a non-fully-relativistic treatment, spin remains a good quantum number. Associated with the spin is a spin magnetic moment. If atoms have net magnetic moments they can couple in various orders in a solid. The simplest cases are the collinear spin alignments as found in ferromagnetic (FM) or antiferromagnetic (AF) systems with parallel (FM) and antiparallel (AF) moments on neighbouring sites. Ferrimagnets have opposite spin alignments but differ in the magnitude of their moments on neighbouring sites, leading to a finite net magnetization. These cases are characterized by the electronic structure of spin-up and spin-down electrons. More complicated spin structures (e.g. canted spins, spin spirals, spin glasses) often require a special treatment beyond simple spin-polarized calculations. In favourable cases, however, as in spin spirals, it is possible to formulate a generalized Bloch theorem and treat such systems by band theory (Sandratskii, 1990).

In a fully relativistic formalism, an additional orbital moment may occur. Note that the orientation of the total magnetic moment (spin and orbital moment) with respect to the crystal axis is only defined in a relativistic treatment including spin–orbit interactions. In a spin-polarized calculation without spin–orbit coupling this is not the case and only the relative orientation (majority-spin and minority-spin) is known. The magnetic structures may lead to a lowering of symmetry, a topic beyond this book.

The density of states (DOS) is the number of one-electron states (in the HF method or DFT) per unit energy interval and per unit cell volume. It is better to start with the integral quantity , the number of states below a certain energy ,where is the volume of the BZ, the factor 2 accounts for the occupation with spin-up and spin-down electrons (in a non-spin-polarized case), and is the step function, the value of which is 1 if is less than and 0 otherwise. The sum over points has been replaced by an integral over the BZ, since the points are uniformly distributed. Both expressions, sum and integral, are used in different derivations or applications. The Fermi energy is defined by imposing that , the number of (valence) electrons per unit cell.

The total DOS is defined as the energy derivative of as with the normalization where the integral is taken from if all core states are included or from the bottom of the valence bands, often taken to be at zero. This defines the Fermi energy (note that the energy range must be consistent with N). In a bulk material, the origin of the energy scale is arbitrary and thus only relative energies are important. In a realistic case with a surface (i.e. a vacuum) one can take the potential at infinity as the energy zero, but this situation is not discussed here.

The total DOS can be decomposed into a partial (or projected) DOS by using information from the wavefunctions as described above in Section 2.2.14.3. If the charge corresponding to the wavefunction of an energy state is partitioned into contributions from the atoms, a site-projected DOS can be defined as , where the superscript t labels the atom t. These quantities can be further decomposed into -like contributions within each atom to give . As discussed above for the partial charges, a further partitioning of the -like terms according to the site symmetry (point group) can be done (in certain cases) by taking the proper m combinations, e.g. the and manifold of the fivefold degenerate d orbitals in a octahedral ligand field. The latter scheme requires a local coordinate system in which the spherical harmonics are defined (see Section 2.2.13). In this context all considerations as discussed above for the partial charges apply again. Note in particular the difference between site-centred and spatially decomposed wavefunctions, which affects the partition of the DOS into its wavefunction-dependent contributions. For example, in atomic sphere representations as in LAPW we have the decomposition In the case of spin-polarized calculations, one can also define a spin-projected DOS for spin-up and spin-down electrons.

The study of hyperfine interactions is a powerful way to characterize different atomic sites in a given sample. There are many experimental techniques, such as Mössbauer spectroscopy, nuclear magnetic and nuclear quadrupole resonance (NMR and NQR), perturbed angular correlations (PAC) measurements etc., which access hyperfine parameters in fundamentally different ways. Hyperfine parameters describe the interaction of a nucleus with the electric and magnetic fields created by the chemical environment of the corresponding atom. Hence the resulting level splitting of the nucleus is determined by the product of a nuclear and an extra-nuclear quantity. In the case of quadrupole interactions, the nuclear quantity is the nuclear quadrupole moment (Q) that interacts with the electric field gradient (EFG) produced by the charges outside the nucleus. For a review see, for example, Kaufmann & Vianden (1979).

The EFG tensor is defined by the second derivative of the electrostatic potential V with respect to the Cartesian coordinates , i = 1, 2, 3, taken at the nuclear site n, where the second term is included to make it a traceless tensor. This is more appropriate, since there is no interaction of a nuclear quadrupole and a potential caused by s electrons. From a theoretical point of view it is more convenient to use the spherical tensor notation because electrostatic potentials (the negative of the potential energy of the electron) and the charge densities are usually given as expansions in terms of spherical harmonics. In this way one automatically deals with traceless tensors (for further details see Herzig, 1985).

The analysis of experimental results faces two obstacles: (i) The nuclear quadrupole moments (Pyykkö, 1992) are often known only with a large uncertainty, as this is still an active research field of nuclear physics. (ii) EFGs depend very sensitively on the anisotropy of the charge density close to the nucleus, and thus pose a severe challenge to electronic structure methods, since an accuracy of the density in the per cent range is required.

In the absence of a better tool, a simple point-charge model was used in combination with so-called Sternheimer (anti-) shielding factors in order to interpret the experimental results. However, these early model calculations depended on empirical parameters, were not very reliable and often showed large deviations from experimental values.

In their pioneering work, Blaha et al. (1985) showed that the LAPW method was able to calculate EFGs in solids accurately without empirical parameters. Since then, this method has been applied to a large variety of systems (Schwarz & Blaha, 1992) from insulators (Blaha et al., 1985), metals (Blaha et al., 1988) and superconductors (Schwarz et al., 1990) to minerals (Winkler et al., 1996).

Several other electronic structure methods have been applied to the calculation of EFGs in solids, for example the LMTO method for periodic (Methfessl & Frota-Pessoa, 1990) or non-periodic (Petrilli & Frota-Pessoa, 1990) systems, the KKR method (Akai et al., 1990), the DVM (discrete variational method; Ellis et al., 1983), the PAW method (Petrilli et al., 1998) and others (Meyer et al., 1995). These methods achieve different degrees of accuracy and are more or less suitable for different classes of systems.

As pointed out above, measured EFGs have an intrinsic uncertainty related to the accuracy with which the nuclear quadrupole moment is known. On the other hand, the quadrupole moment can be obtained by comparing experimental hyperfine splittings with very accurate electronic structure calculations. This has recently been done by Dufek et al. (1995a) to determine the quadrupole moment of ⁵⁷Fe. Hence the calculation of accurate EFGs is to date an active and challenging research field.

The nuclear quadrupole interaction (NQI) represents the interaction of Q (the nuclear quadrupole moment) with the electric field gradient (EFG) created by the charges surrounding the nucleus, as described above. Here we briefly summarize the main ideas (following Petrilli et al., 1998) and provide conversions between experimental NQI splittings and electric field gradients.

Let us consider a nucleus in a state with nuclear spin quantum number with the corresponding nuclear quadrupole moment , where is the nuclear charge density around point and e is the proton's charge. The interaction of this Q with an electric field gradient tensor , splits the energy levels for different magnetic spin quantum numbers of the nucleus according to in first order of , where Q represents the largest component of the nuclear quadrupole moment tensor in the state characterized by . (Note that the quantum-mechanical expectation value of the charge distribution in an angular momentum eigenstate is cylindrical, which renders the expectation value of the remaining two components with half the value and opposite sign.) The conventional choice is . Hence, is the principal component (largest eigenvalue) of the electric field gradient tensor and the asymmetry parameter is defined by the remaining two eigenvalues through (2.2.15.3) shows that the electric quadrupole interaction splits the ()-fold degenerate energy levels of a nuclear state with spin quantum number I () into I doubly degenerate substates (and one singly degenerate state for integer I). Experiments determine the energy difference between the levels, which is called the quadrupole splitting. The remaining degeneracy can be lifted further using magnetic fields.

Next we illustrate these definitions for ⁵⁷Fe, which is the most common probe nucleus in Mössbauer spectroscopy measurements and thus deserves special attention. For this probe, the nuclear transition occurs between the excited state and ground state, with a 14.4 KeV radiation emission. The quadrupole splitting between the and the state can be obtained by exploiting the Doppler shift of the radiation of the vibrating sample. For systems in which the ⁵⁷Fe nucleus has a crystalline environment with axial symmetry (a threefold or fourfold rotation axis), the asymmetry parameter is zero and is given directly by As can never be greater than unity, the difference between the values of given by equation (2.2.15.5) and equation (2.2.15.6) cannot be more than about 15%. In the remainder of this section we simplify the expressions, as is often done, by assuming that . As Mössbauer experiments exploit the Doppler shift of the radiation, the splitting is expressed in terms of the velocity between sample and detector. The quadrupole splitting can be obtained from the velocity, which we denote here by , by where c = 2.9979245580 × 10⁸ m s⁻¹ is the speed of light and E_γ = 14.41 × 10³ eV is the energy of the emitted radiation of the ⁵⁷Fe nucleus.

Finally, we still need to know the nuclear quadrupole moment Q of the Fe nucleus itself. Despite its utmost importance, its value has been heavily debated. Recently, however, Dufek et al. (1995b) have determined the value Q = 0.16 b for ⁵⁷Fe (1 b = 10⁻²⁸ m²) by comparing for fifteen different compounds theoretical values, which were obtained using the linearized augmented plane wave (LAPW) method, with the measured quadrupole splitting at the Fe site.

Now we relate the electric field gradient to the Doppler velocity via In the special case of the ⁵⁷Fe nucleus, we obtain EFGs can also be obtained by techniques like NMR or NQR, where a convenient measure of the strength of the quadrupole interaction is expressed as a frequency , related to by The value can then be calculated from the frequency in MHz by where (h/e) = 4.1356692 × 10⁻¹⁵ V Hz⁻¹. The principal component is also often denoted as .

In the literature, two conflicting definitions of are in use. One is given by (2.2.15.10), and the other, defined as differs from the first by a factor of 2 and assumes the value . Finally, the definition of has been introduced here. In order to avoid confusion, we will refer here only to the definition given by (2.2.15.10). Furthermore, we also adopt the same sign convention for as Schwarz et al. (1990) because it has been found to be consistent with the majority of experimental results.

Since the EFG is a ground-state property that is uniquely determined by the charge density distribution (of electrons and nuclei), it can be calculated within DFT without further approximations. Here we describe the basic formalism to calculate EFGs with the LAPW method (see Section 2.2.12). In the LAPW method, the unit cell is divided into non-overlapping atomic spheres and an interstitial region. Inside each sphere the charge density (and analogously the potential) is written as radial functions times crystal harmonics (2.2.13.4) and in the interstitial region as Fourier series: The charge density coefficients can be obtained from the wavefunctions (KS orbitals) by (in shorthand notation) where are Gaunt numbers (integrals over three spherical harmonics) and denote the LAPW radial functions [see (2.2.12.1)] of the occupied states below the Fermi energy . The dependence on the energy bands in has been omitted in order to simplify the notation.

For a given charge density, the Coulomb potential is obtained numerically by solving Poisson's equation in form of a boundary-value problem using a method proposed by Weinert (1981). This yields the Coulomb potential coefficients in analogy to (2.2.15.13) [see also (2.2.12.5)]. The most important contribution to the EFG comes from a region close to the nucleus of interest, where only the terms are needed (Herzig, 1985). In the limit (the position of the nucleus), the asymptotic form of the potential can be used and this procedure yields (Schwarz et al., 1990) for : with , and the spherical Bessel function . The first term in (2.2.15.15) (called the valence EFG) corresponds to the integral over the respective atomic sphere (with radius R). The second and third terms in (2.2.15.15) (called the lattice EFG) arise from the boundary-value problem and from the charge distribution outside the sphere considered. Note that our definition of the lattice EFG differs from that based on the point-charge model (Kaufmann & Vianden, 1979). With these definitions the tensor components are given as where and the index M combines m and the partity p (e.g. ). Note that the prefactors depend on the normalization used for the spherical harmonics.

The non-spherical components of the potential come from the non-spherical charge density . For the EFG only the terms (in the potential) are needed. If the site symmetry does not contain such a non-vanishing term (as for example in a cubic system with in the lowest combination), the corresponding EFG vanishes by definition. According to the Gaunt numbers in (2.2.15.14) only a few non-vanishing terms remain (ignoring f orbitals), such as the p–p, d–d or s–d combinations (for f orbitals, p–f and f–f would appear), where this shorthand notation denotes the products of the two radial functions . The s–d term is often small and thus is not relevant to the interpretation. This decomposition of the density can be used to partition the EFG (illustrated for the component), where the superscripts p and d are a shorthand notation for the product of two p- or d-like functions.

From our experience we find that the first term in (2.2.15.15) is usually by far the most important and often a radial range up to the first node in the corresponding radial function is all that contributes. In this case the contribution from the other two terms is rather small (a few per cent). For first-row elements, however, which have no node in their 2p functions, this is no longer true and thus the first term amounts only to about 50–70%.

In some cases interpretation is simplified by defining a so-called asymmetry count, illustrated below for the oxygen sites in YBa₂Cu₃O₇ (Schwarz et al., 1990), the unit cell of which is shown in Fig. 2.2.15.1.

Figure 2.2.15.1 | top | pdf | Unit cell of the high-temperature superconductor YBa2Cu3O7 with four non-equivalent oxygen sites.

In this case essentially only the O 2p orbitals contribute to the O EFG. Inside the oxygen spheres (all taken with a radius of 0.82 Å) we can determine the partial charges corresponding to the , and orbitals, denoted in short as , and charges.

With these definitions we can define the p-like asymmetry count as and obtain the proportionality where is the expectation value taken with the p orbitals. A similar equation can be defined for the d orbitals. The factor enhances the EFG contribution from the density anisotropies close to the nucleus. Since the radial wavefunctions have an asymptotic behaviour near the origin as , the p orbitals are more sensitive than the d orbitals. Therefore even a very small p anisotropy can cause an EFG contribution, provided that the asymmetry count is enhanced by a large expectation value.

Often the anisotropy in the , and occupation numbers can be traced back to the electronic structure. Such a physical interpretation is illustrated below for the four non-equivalent oxygen sites in YBa₂Cu₃O₇ (Table 2.2.15.1). Let us focus first on O1, the oxygen atom that forms the linear chain with the Cu1 atoms along the b axis. In this case, the orbital of O1 points towards Cu1 and forms a covalent bond, leading to bonding and antibonding states, whereas the other two p orbitals have no bonding partner and thus are essentially nonbonding. Part of the corresponding antibonding states lies above the Fermi energy and thus is not occupied, leading to a smaller charge of 0.91 e, in contrast to the fully occupied nonbonding states with occupation numbers around 1.2 e. (Note that only a fraction of the charge stemming from the oxygen 2p orbitals is found inside the relatively small oxygen sphere.) This anisotropy causes a finite asymmetry count [(2.2.15.18)] that leads – according to (2.2.15.19) – to a corresponding EFG.

Table 2.2.15.1 | top | pdf | Partial O 2p charges (in electrons) and electric field gradient tensor O EFG (in 1021 V m−2) for YBa2Cu3O7 Numbers in bold represent the main deviation from spherical symmetry in the charges and the related principal component of the EFG tensor. Atom O1 1.18 0.91 1.25 −6.1 18.3 −12.2 O2 1.01 1.21 1.18 11.8 −7.0 −4.8 O3 1.21 1.00 1.18 −7.0 11.9 −4.9 O4 1.18 1.19 0.99 −4.7 −7.0 11.7

In this simple case, the anisotropy in the charge distribution, given here by the different p occupation numbers, is directly proportional to the EFG, which is given with respect to the crystal axes and is thus labelled , and (Table 2.2.15.1). The principal component of the EFG is in the direction where the p occupation number is smallest, i.e. where the density has its highest anisotropy. The other oxygen atoms behave very similarly: O2, O3 and O4 have a near neighbour in the a, b and c direction, respectively, but not in the other two directions. Consequently, the occupation number is lower in the direction in which the bond is formed, whereas it is normal (around 1.2 e) in the other two directions. The principal axis falls in the direction of the low occupation. The higher the anisotropy, the larger the EFG (compare O1 with the other three oxygen sites). Excellent agreement with experiment is found (Schwarz et al., 1990). In a more complicated situation, where p and d contributions to the EFG occur [see (2.2.15.17)], which often have opposite sign, the interpretation can be more difficult [see e.g. the copper sites in YBa₂Cu₃O₇; Schwarz et al. (1990)].

The importance of semi-core states has been illustrated for rutile, where the proper treatment of 3p and 4p states is essential to finding good agreement with experiment (Blaha et al., 1992). The orthogonality between -like bands belonging to different principal quantum numbers (3p and 4p) is important and can be treated, for example, by means of local orbitals [see (2.2.12.4)].

In many simple cases, the off-diagonal elements of the EFG tensor vanish due to symmetry, but if they don't, diagonalization of the EFG tensor is required, which defines the orientation of the principal axis of the tensor. Note that in this case the orientation is given with respect to the local coordinate axes (see Section 2.2.13) in which the components are defined.

For the simple case of an element, namely copper in the f.c.c. structure, the band structure is shown in Fig. 2.2.16.1 along the symmetry direction from to X. The character of the bands can be illustrated by showing for each band state the crucial information that is contained in the wavefunctions. In the LAPW method (Section 2.2.12), the wavefunction is expanded in atomic like functions inside the atomic spheres (partial waves), and thus a spatial decomposition of the associated charge and its portion of -like charge (s-, p-, d-like) inside the Cu sphere, , provides such a quantity. Fig. 2.2.16.1 shows for each state a circle the radius of which is proportional to the -like charge of that state. The band originating from the Cu 4s and 4p orbitals shows an approximately free-electron behaviour and thus a energy dependence, but it hybridizes with one of the d bands in the middle of the direction and thus the -like character changes along the direction.

Figure 2.2.16.1 | top | pdf | Character of energy bands of f.c.c. copper in the direction. The radius of each circle is proportional to the respective partial charge of the given state.

This can easily be understood from a group-theoretical point of view. Since the d states in an octahedral environment split into the and manifold, the d bands can be further partitioned into the two subsets as illustrated in Fig. 2.2.16.2. The s band ranges from about −9.5 eV below to about 2 eV above. In the direction, the s band has symmetry, the same as one of the d bands from the manifold, which consists of and . As a consequence of the `non-crossing rule', the two states, both with symmetry, must split due to the quantum-mechanical interaction between states with the same symmetry. This leads to the avoided crossing seen in the middle of the direction (Fig. 2.2.16.1). Therefore the lowest band starts out as an `s band' but ends near X as a `d band'. This also shows that bands belonging to different irreducible representations (small representations) may cross. The fact that splits into the subgroups and is an example of the compatibility relations. In addition, group-theoretical arguments can be used (Altmann, 1994) to show that in certain symmetry directions the bands must enter the face of the BZ with zero slope.

Figure 2.2.16.2 | top | pdf | Decomposition of the Cu d bands into the and manifold. The radius of each circle is proportional to the corresponding partial charge.

Note that in a site-centred description of the wavefunctions a similar -like decomposition of the charge can be defined as (without the term), but here the partial charges have a different meaning than in the spatial decomposition. In one case (e.g. LAPW), refers to the partial charge of -like character inside sphere t, while in the other case (LCAO), it means -like charge coming from orbitals centred at site t. For the main components (for example Cu d) these two procedures will give roughly similar results, but the small components have quite a different interpretation. For this purpose consider an orbital that is centred on the neighbouring site j, but whose tail enters the atomic sphere i. In the spatial representation this tail coming from the j site must be represented by the (s, p, d etc.) partial waves inside sphere i and consequently will be associated with site i, leading to a small partial charge component. This situation is sometimes called the off-site component, in contrast to the on-site component, which will appear at its own site or in its own sphere, depending on the representation, site-centred or spatially confined.

The well known rutile structure (e.g. TiO₂) is tetragonal (see Fig. 2.2.16.3) with the basis consisting of the metal atoms at the Wyckoff positions, () and (), and anions at the position, located at () and () with a typical value of about 0.3 for the internal coordinate u. Rutile belongs to the non-symmorphic space group () in which the metal positions are transformed into each other by a rotation by 90° around the crystal c axis followed by a non-primitive translation of (). The two metal positions at the centre and at the corner of the unit cell are equivalent when the surrounding octahedra are properly rotated. The metal atoms are octahedrally coordinated by anions which, however, do not form an ideal octahedron. The distortion depends on the structure parameters a, and u, and results in two different metal–anion distances, namely the apical distance and the equatorial distance , the height (z axis) and the basal spacing of the octahedron. For a certain value the two distances and become equal:For this special value and an ideal ratio, the basal plane of the octahedron is quadratic and the two distances are equal. An ideal octahedral coordination is thus obtained with Although the actual coordination of the metal atoms deviates from the ideal octahedron (as in all other systems that crystallize in the rutile structure), we still use this concept for symmetry arguments and call it octahedral coordination.

Figure 2.2.16.3 | top | pdf | The local coordinate system in rutile for titanium (small spheres) and oxygen (large spheres).

The concept of a local coordinate system is illustrated for rutile (TiO₂) from two different aspects, namely the crystal harmonics expansion (see Section 2.2.13) and the interpretation of chemical bonding (for further details see Sorantin & Schwarz, 1992).

In excitations involving core electrons, simplifications are possible that allow an easier interpretation. As one example, (soft) X-ray emission (XES) or absorption (XAS) spectra are briefly discussed. In the one-electron picture, the XES process can be described as sketched in Fig. 2.2.16.4. First a core electron of atom A in state is knocked out (by electrons or photons), and then a transition occurs between the occupied valence states at energy and the core hole (the transitions between inner core levels are ignored).

Figure 2.2.16.4 | top | pdf | Schematic transitions in X-ray emission and absorption spectra.

According to Fermi's golden rule, the intensity of such a transition can be described bywhere comes from the integral over the angular components (Table 2.2.16.1) and contains the selection rule, is the local (within atomic sphere A) partial (-like) DOS, is the radial transition probability [see (2.2.16.6) below], and the last term takes the energy conservation into account.

Table 2.2.16.1 | top | pdf | factors for X-ray emission spectra showing the selection rule 0 1 2 3 4 0   1/3       1 1   2/5     2   2/3   3/7   3     3/5   4/11

The are defined as the dipole transition (with the dipole operator r) probability between the valence state at and the core state characterized by quantum numbers ,In this derivation one makes use of the fact that core states are completely confined inside the atomic sphere. Therefore the integral, which should be taken over the entire space, can be restricted to one atomic sphere (namely A), since the core wavefunction and thus the integrand vanishes outside this sphere. This is also the reason why XES (or XAS) are related to , the local DOS weighted with the -like charge within the atomic sphere A.

The interpretation of XES intensities is as follows. Besides the factor from Fermi's golden rule, the intensity is governed by the selection rule and the energy conservation. In addition, it depends on the number of available states at which reside inside sphere A and have an -like contribution, times the probability for the transition to take place from the valence and to the core hole under energy conservation. For an application, see for example the comparison between theory and experiment for the compounds NbC and NbN (Schwarz, 1977).

Note again that the present description is based on an atomic sphere representation with partial waves inside the spheres, in contrast to an LCAO-like treatment with site-centred basis functions. In the latter, an equivalent formalism can be defined which differs in details, especially for the small components (off-site contributions). If the tails of an orbital enter a neighbouring sphere and are crucial for the interpretation of XES, there is a semantic difference between the two schemes as discussed above in connection with f.c.c. Cu in Section 2.2.16.1. In the present framework, all contributions come exclusively from the sphere where the core hole resides, whereas in an LCAO representation `cross transitions' from the valence states on one atom to the core hole of a neighbouring atom may be important. The latter contributions must be (and are) included in the partial waves within the sphere in schemes such as LAPW. There is no physical difference between the two descriptions.

In XES, spectra are interpreted on the basis of results from ground-state calculations, although there could be relaxations due to the presence of a core hole. As early as 1979, von Barth and Grossmann formulated a `final state rule' for XES in metallic systems (von Barth & Grossmann, 1979). In this case, the initial state is one with a missing core electron (core hole), whereas the final state is close to the ground state, since the hole in the valence bands (after a valence electron has filled the core hole) has a very short lifetime and is very quickly filled by other valence electrons. They applied time-dependent perturbation theory and could show by model calculations that the main XES spectrum can be explained by ground-state results, whereas the satellite spectrum (starting with two core holes and ending with one) requires a treatment of the core-hole relaxation. This example illustrates the importance of the relevant physical process in experiments related to the energy-band structure: it may not always be the just the ground states that are involved and sometimes excited states must be considered.