Chapter 3.4. Domain structures

It was demonstrated in Section 3.1.2 that a characteristic feature of structural phase transitions connected with a lowering of crystal symmetry is an anomalous behaviour near the transition, namely unusually large values of certain physical properties that vary strongly with temperature. In this chapter, we shall deal with another fundamental feature of structural phase transitions: the formation of a non-homogeneous, textured low-symmetry phase called a domain structure.

When a crystal homogeneous in the parent (prototypic) phase undergoes a phase transition into a ferroic phase with lower point-group symmetry, then this ferroic phase is almost always formed as a non-homogeneous structure consisting of homogeneous regions called domains and contact regions between domains called domain walls. All domains have the same or the enantiomorphous crystal structure of the ferroic phase, but this structure has in different domains a different orientation, and sometimes also a different position in space. When a domain structure is observed by a measuring instrument, different domains can exhibit different tensor properties, different diffraction patterns and can differ in other physical properties. The domain structure can be visualized optically (see Fig. 3.4.1.1) or by other experimental techniques. Powerful high-resolution electron microscopy (HREM) techniques have made it possible to visualize atomic arrangements in domain structures (see Fig. 3.4.1.2). The appearance of a domain structure, detected by any reliable technique, provides the simplest unambiguous experimental proof of a structural phase transition.

Figure 3.4.1.1 | top | pdf | Domain structure of tetragonal barium titanate (BaTiO3). A thin section of barium titanate ceramic observed at room temperature in a polarized-light microscope (transmitted light, crossed polarizers). Courtesy of U. Täffner, Max-Planck-Institut für Metallforschung, Stuttgart. Different colours correspond to different ferroelastic domain states, connected areas of the same colour are ferroelastic domains and sharp boundaries between these areas are domain walls. Areas of continuously changing colour correspond to gradually changing thickness of wedge-shaped domains. An average distance between parallel ferroelastic domain walls is of the order of 1–10 µm.

Figure 3.4.1.2 | top | pdf | Domain structure of a BaGa2O4 crystal seen by high-resolution transmission electron microscopy. Parallel rows are atomic layers. Different directions correspond to different ferroelastic domain states of domains, connected areas with parallel layers are different ferroelastic domains and boundaries between these areas are ferroelastic domain walls. Courtesy of H. Lemmens, EMAT, University of Antwerp.

Under the influence of external fields (mechanical stress, electric or magnetic fields, or combinations thereof), the domain structure can change; usually some domains grow while others decrease in size or eventually vanish. This process is called domain switching. After removing or decreasing the field a domain structure might not change considerably, i.e. the form of a domain pattern depends upon the field history: the domain structure exhibits hysteresis (see Fig. 3.4.1.3). In large enough fields, switching results in a reduction of the number of domains. Such a procedure is called detwinning. In rare cases, the crystal may consist of one domain only. Then we speak of a single-domain crystal.

Figure 3.4.1.3 | top | pdf | Elastic hysteresis of ferroelastic lead phosphate Pb3(PO4)2 (Salje, 1990). Courtesy of E. K. H. Salje, University of Cambridge. The dependence of strain on applied stress has the form of a loop. The states at the extreme left and right correspond to two ferroelastic domain states, steep parts of the loop represent switching of one state into the other by applied stress. The strain at zero stress corresponds to the last single-domain state formed in a field larger than the coercive stress defined by the stress at zero strain (the intersection of the loop with the axis of the applied stress). Similar dielectric hysteresis loops of polarization versus applied electric field are observed in ferroelectric phases (see e.g. Jona & Shirane, 1962).

There are two basic types of domain structures:

Another important kind of domain structure is a ferroelectric domain structure, in which domains differ in the direction of the spontaneous polarization. Such a domain structure is formed at ferroelectric phase transitions that are characterized by the appearance of a new polar direction in the ferroic phase. Ferroelectric domains can usually be switched by external electric fields. Two ferroelectric domains with different directions of spontaneous polarization can have different spontaneous strain [e.g. in dihydrogen phosphate (KDP) crystals, two ferroelectric domains with opposite directions of the spontaneous polarization have different spontaneous shear strain], or two ferroelectric domains with antiparallel spontaneous polarization can possess the same strain [e.g. in triglycine sulfate (TGS) crystals].

The physical properties of polydomain crystals are significantly influenced by their domain structure. The values of important material property tensor components, e.g. permittivity, piezoelectric and elastic constants, may be enhanced or diminished by the presence of a domain structure. Owing to switching and detwinning phenomena, polydomain materials exhibit hysteresis of material properties. These features have important practical implications, e.g. the production of anisotropic ceramic materials or ferroelectric memories.

The domain structure resulting from a structural phase transition belongs to a special type of twinning referred to as transformation twinning (see Section 3.3.7.2 ). Despite this, the current terminology used in domain-structure studies is different. The main terms were coined during the first investigations of ferroelectric materials, where striking similarities with the behaviour of ferromagnetic materials led researchers to introduce terms analogous to those used in studies of ferromagnetic domain structures that had been examined well at that time.

Bicrystallography (see Section 3.2.2 ) provides another possible frame for discussing domain structures. Bicrystallography and domain structure analysis have developed independently and almost simultaneously but different language has again precluded deeper confrontation. Nevertheless, there are common features in the methodology of both approaches, in particular, the principle of symmetry compensation (see Section 3.2.2 ), which plays a fundamental role in both theories.

In Chapter 3.1 , it is shown that the anomalous behaviour near phase transitions can be explained in the framework of the Landau theory. In this theory, the formation of the domain structures follows from the existence of several equivalent solutions for the order parameter. This result is a direct consequence of a symmetry reduction at a ferroic phase transition. It is this dissymmetrization which is the genuine origin of the domain structure formation and which determines the basic static features of all domain structures.

This chapter is devoted to the crystallographic aspects of static domain structures, especially to the symmetry analysis of these structures. The main aim is to explain basic concepts, derive relations that govern the formation of domain structures and provide tables with useful ready-to-use data on domain structures of ferroic phases. The exposition uses algebraic tools that are explained in Section 3.2.3 , but the important points are illustrated with simple examples comprehensible even without mathematical details. The synoptic tables in Sections 3.4.2 and 3.4.3 present the main results of the analysis for all possible ferroic domain structures. More detailed information on certain points can be found in the software GIKoBo-1.

All these results are definite – their validity does not depend on any particular model or approximation – and form thus a firm basis for further more detailed quantitative treatments. `For the most part, the only exact statements which can be made about a solid state system are those which arise as a direct consequence of symmetry alone.' (Knox & Gold, 1967.)

The exposition starts with domain states, continues with pairs of domain states and domain distinction, and terminates with domain twins and walls. This is also the sequence of steps in domain-structure analysis, which proceeds from the simplest to more complicated objects.

In Section 3.4.2, we explain the concept of domain states (also called variants or orientational states), define different types of domain states (principal, ferroelastic, ferroelectric, basic), find simple formulae for their number, and disclose their hierarchy and relation with symmetry lowering and with order parameters of the transition. Particular results for all possible ferroic phase transitions can be found in synoptic Table 3.4.2.7, which lists all possible crystallographically non-equivalent point-group symmetry descents that may appear at a ferroic phase transition. For each descent, all independent twinning groups (characterizing the relation between two domain states) are given together with the number of principal, ferroelastic and ferroelectric domain states and other data needed in further analysis.

Section 3.4.3 deals with pairs of domain states and with the relationship between two domain states in a pair. This relationship, in mineralogy called a `twin law', determines the distinction between domain states, specifies switching processes between two domain states and forms a starting point for discussing domain walls and twins. We show different ways of expressing the relation between two domain states of a domain pair, derive a classification of domain pairs, find non-equivalent domain pairs and determine which tensor properties are different and which are the same in two domain states of a domain pair.

The presentation of non-equivalent domain pairs is divided into two parts. Synoptic Table 3.4.3.4 lists all representative non-equivalent non-ferroelastic domain pairs, and for each pair gives the twinning groups, and the number of tensor components that are different and that are the same in two domain states. These numbers are given for all important property tensors up to rank four. We also show how these data can be used to determine switching forces between two non-ferroelastic domain states.

Then we explain specific features of ferroelastic domain pairs: compatible (permissible) domain walls and disorientation of domain states in ferroelastic domain twins. A list of all non-equivalent ferroelastic domain pairs is presented in two tables. Synoptic Table 3.4.3.6 contains all non-equivalent ferroelastic domain pairs with compatible (coherent) domain walls. This table gives the orientation of compatible walls and their symmetry properties. Table 3.4.3.7 lists all non-equivalent ferroelastic domain pairs with no compatible ferroelastic domain walls.

Column K_1j in Table 3.4.2.7 specifies all representative non-equivalent domain pairs that can appear in each particular phase transition; in combination with Tables 3.4.3.4 and 3.4.3.6, it allows one to determine the main features of any ferroic domain structure.

Section 3.4.4 is devoted to domain twins and domain walls. We demonstrate that the symmetry of domain twins and domain walls is described by layer groups, give a classification of domain twins and walls based on their symmetry, and present possible layer groups of non-ferroelastic and ferroelastic domain twins and walls. Then we discuss the properties of finite-thickness domain walls. In an example, we illustrate the symmetry analysis of microscopic domain walls and present conclusions that can be drawn from this analysis about the microscopic structure of domain walls.

The exposition is given in the continuum description with crystallographic point groups and property tensors. In this approach, all possible cases are often treatable and where possible are covered in synoptic tables or – in a more detailed form – in the software GIKoBo-1. Although the group-theoretical tools are almost readily transferable to the microscopic description (using the space groups and atomic positions), the treatment of an inexhaustible variety of microscopic situations can only be illustrated by particular examples.

Our attempt to work with well defined notions calls for introducing several new, and generalizing some accepted, concepts. Also an extended notation for the symmetry operations and groups has turned out to be indispensable. Since there is no generally accepted terminology on domain structures yet, we often have to choose a term from several existing more-or-less equivalent variants.

The specialized scope of this chapter does not cover several important aspects of domain structures. More information can be found in the following references. There are only two monographs on domain structures (both in Russian): Fesenko et al. (1990) and Sidorkin (2002). The main concepts of domain structures of ferroic materials are explained in the book by Wadhawan (2000) and in a review by Schranz (1995). Ferroelastic domain structures are reviewed in Boulesteix (1984) and Wadhawan (1991), and are treated in detail by Salje (1990, 1991, 2000a,b). Different aspects of ferroelectric domain structures are covered in books or reviews on ferroelectric crystals: Känzig (1957), Jona & Shirane (1962), Fatuzzo & Merz (1967), Mitsui et al. (1976), Lines & Glass (1977), Smolenskii et al. (1984), Zheludev (1988) and Strukov & Levanyuk (1998). Applications of ferroelectrics are described in the books by Xu (1991) and Uchino (2000). Principles and technical aspects of ferroelectric memories are reviewed by Scott (1998, 2000).

As for all crystalline materials, domain structures can be approached in two ways: In the microscopic description, a crystal is treated as a regular arrangement of atoms. Domains differ in tiny differences of atomic positions which can be determined only indirectly, e.g. by diffraction techniques. In what follows, we shall pay main attention to the continuum description, in which a crystal is treated as an anisotropic continuum. Then the crystal properties are described by property tensors (see Section 1.1.1 ) and the crystal symmetry is expressed by crystallographic point groups. In this approach, domains exhibit different tensor properties that enable one to visualize domains by optical or other methods.

The domain structure observed in a microscope appears to be a patchwork of homogeneous regions – domains – that have various colours and shapes (see Fig. 3.4.1.1). Indeed, the usual description considers a domain structure as a collection of domains and contact regions of domains called domain walls. Strictly speaking, by a domain one understands a connected part of the crystal, called the domain region, which is filled with a homogeneous low-symmetry crystal structure. Domain walls can be associated with the boundaries of domain regions. The interior homogeneous bulk structure within a domain region will be called a domain state. Equivalent terms are variant or structural variant (Van Tendeloo & Amelinckx, 1974). We shall use different adjectives to specify domain states. In the microscopic description, domain states associated with the primary order parameter will be referred to as primary (microscopic, basic) domain states. In the macroscopic description, the primary domain states will be called principal domain states, which correspond to Aizu's orientation states. (An exact definition of principal domain states is given below.)

Further useful division of domain states is possible (though not generally accepted): Domain states that are specified by a constant value of the spontaneous strain are called ferroelastic domain states; similarly, ferroelectric domain states exhibit constant spontaneous polarization etc. Domain states that differ in some tensor properties are called ferroic or tensorial domain states etc. If no specification is given, the statements will apply to any of these domain states.

A domain is specified by a domain state and by domain region : . Different domains may possess the same domain state but always differ in the domain region that specifies their shape and position in space.

The term `domain' has also often been used for a domain state. Clear distinction of these two notions is essential in further considerations and is illustrated in Fig. 3.4.2.1. A ferroelectric domain structure (Fig. 3.4.2.1a) consists of six ferroelectric domains , , , but contains only two domain states , characterized by opposite directions of the spontaneous polarization depicted in Fig. 3.4.2.1(d). Neighbouring domains have different domain states but non-neighbouring domains may possess the same domain state. Thus domains with odd serial number have the domain state (spontaneous polarization `down'), whereas domains with even number have domain state (spontaneous polarization `up').

Figure 3.4.2.1 | top | pdf | Hierarchy in domain-structure analysis. (a) Domain structure consisting of domains and domain walls and ; (b) domain twin and reversed twin (with reversed order of domain states); (c) domain pair consisting of two domain states and ; (d) domain states and .

A great diversity of observed domain structures are connected mainly with various dimensions and shapes of domain regions, whose shapes depend sensitively on many factors (kinetics of the phase transition, local stresses, defects etc.). It is, therefore, usually very difficult to interpret in detail a particular observed domain pattern. Domain states of domains are, on the other hand, governed by simple laws, as we shall now demonstrate.

We shall consider a ferroic phase transition with a symmetry lowering from a parent (prototypic, high-symmetry) phase with symmetry described by a point group G to a ferroic phase with the point-group symmetry , which is a subgroup of G. We shall denote this dissymmetrization by a group–subgroup symbol (or in Section 3.1.3 ) and call it a symmetry descent or dissymmetrization. Aizu (1970a) calls these symmetry descents species and uses the letter F instead of the symbol .

As an illustrative example, we choose a phase transition with parent symmetry and ferroic symmetry (see Fig. 3.4.2.2). Strontium bismuth tantalate (SBT) crystals, for instance, exhibit a phase transition with this symmetry descent (Chen et al., 2000). Symmetry elements in the symbols of G and are supplied with subscripts specifying the orientation of the symmetry elements with respect to the reference coordinate system. The necessity of this extended notation is exemplified by the fact that the group has six subgroups with the same `non-oriented' symbol : , , , , , . Lower indices thus specify these subgroups unequivocally and the example illustrates an important rule of domain-structure analysis: All symmetry operations, groups and tensor components must be related to a common reference coordinate system and their orientation in space must be clearly specified.

Figure 3.4.2.2 | top | pdf | Exploded view of single-domain states , , and (solid rectangles with arrows of spontaneous polarization) formed at a phase transition from a parent phase with symmetry to a ferroic phase with symmetry . The parent phase is represented by a dashed square in the centre with the symmetry elements of the parent group shown.

The physical properties of crystals in the continuum description are expressed by property tensors. As explained in Section 1.1.4 , the crystal symmetry reduces the number of independent components of these tensors. Consequently, for each property tensor the number of independent components in the low-symmetry ferroic phase is the same or higher than in the high-symmetry parent phase. Those tensor components or their linear combinations that are zero in the high-symmetry phase and nonzero in the low-symmetry phase are called morphic tensor components or tensor parameters and the quantities that appear only in the low-symmetry phase are called spontaneous quantities (see Section 3.1.3.2 ). The morphic tensor components and spontaneous quantities thus reveal the difference between the high- and low-symmetry phases. In our example, the symmetry allows a nonzero spontaneous polarization , which must be zero in the high-symmetry phase with .

We shall now demonstrate in our example that the symmetry lowering at the phase transition leads to the existence of several equivalent variants (domain states) of the low-symmetry phase. In Fig. 3.4.2.2, the parent high-symmetry phase is represented in the middle by a dashed square that is a projection of a square prism with symmetry . A possible variant of the low-symmetry phase can be represented by an oblong prism with a vector representing the spontaneous polarization. In Fig. 3.4.2.2, the projection of this oblong prism is drawn as a rectangle which is shifted out of the centre for better recognition. We denote by a homogeneous low-symmetry phase with spontaneous polarization and with symmetry F₁ = . Let us, mentally, increase the temperature to above the transition temperature and then apply to the high-symmetry phase an operation , which is a symmetry operation of this high-symmetry phase but not of the low-symmetry phase. Then decrease the temperature to below the transition temperature. The appearance of another variant of the low-symmetry phase with spontaneous polarization obviously has the same probability of appearing as had the variant . Thus the two variants of the low-symmetry phase and can appear with the same probability if they are related by a symmetry operation suppressed (lost) at the transition, i.e. an operation that was a symmetry operation of the high-symmetry phase but is not a symmetry operation of the low-symmetry phase . In the same way, the lost symmetry operations and generate from two other variants, and , with spontaneous polarizations and , respectively. Variants of the low-symmetry phase that are related by an operation of the high-symmetry group G are called crystallographically equivalent (in G) variants. Thus we conclude that crystallographically equivalent (in G) variants of the low-symmetry phase have the same chance of appearing.

We shall now make similar considerations for a general ferroic phase transition with a symmetry descent . By the state S of a crystal we shall understand, in the continuum description, the set of all its properties expressed by property (matter) tensors in the reference Cartesian crystallophysical coordinate system of the parent phase (see Example 3.2.3.9 in Section 3.2.3.3.1 ). A state defined in this way may change not only with temperature and external fields but also with the orientation of the crystal in space.

We denote by a state of a homogeneous ferroic phase. If we apply to a symmetry operation of the group G, then the ferroic phase in a new orientation will have the state , which may be identical with or different. Using the concept of group action (explained in detail in Section 3.2.3.3.1 ) we express this operation by a simple relation:

Let us first turn our attention to operations that do not change the state : The set of all operations of G that leave invariant form a group called a stabilizer (or isotropy group) of a state in the group G. This stabilizer, denoted by , can be expressed explicitly in the following way: where the right-hand part of the equation should be read as `a set of all operations of G that do not change the state ' (see Section 3.2.3.3.2 ).

Here we have to explain the difference between the concept of a stabilizer of an object and the symmetry of that object. By the symmetry group F of an object one understands the set of all operations (isometries) that leave this object invariant. The symmetry group F of an object is considered to be an inherent property that does not depend on the orientation and position of the object in space. (The term eigensymmetry is used in Chapter 3.3 for symmetry groups defined in this way.) In this case, the symmetry elements of F are `attached' to the object.

A stabilizer describes the symmetry properties of an object in another way, in which the object and the group of isometries are decoupled. One is given a group G, the symmetry elements of which have a defined orientation in a fixed reference system. The object can have any orientation in this reference system. Those operations of G that map the object in a given orientation onto itself form the stabilizer of in the group G. In this case, the stabilizer depends on the orientation of the object in space and is expressed by an `oriented' group symbol with subscripts defining the orientation of the symmetry elements of . Only for certain `prominent' orientations will the stabilizer acquire a symmetry group of the same crystal class (crystallographic point group) as the eigensymmetry of the object.

We shall define a single-domain orientation as a prominent orientation of the crystal in which the stabilizer of its state is equal to the symmetry group which is, after removing subscripts specifying the orientation, identical with the eigensymmetry of the ferroic phase: This equation thus declares that the crystal in the state has a prominent single-domain orientation.

The concept of the stabilizer allows us to identify the `eigensymmetry' of a domain state (or an object in general) with the crystallographic class (non-oriented point group) of the stabilizer of this state in the group of all rotations O(3), .

Since we shall further deal mainly with states of the ferroic phase in single-domain orientations, we shall use the term `state' for a `state of the crystal in a single-domain orientation', unless mentioned otherwise. Then the stabilizer will often be replaced by the group , although all statements have been derived and hold for stabilizers.

The difference between symmetry groups of a crystal and stabilizers will become more obvious in the treatment of secondary domain states in Section 3.4.2.2 and in discussing disoriented ferroelastic domain states (see Section 3.4.3.6.3).

As we have seen in our illustrative example, the suppressed operations generate from the first state other states. Let be such a suppressed operation, i.e. but . Since all operations that retain are collected in , the operation must transform into another state , and we say that the state is crystallographically equivalent (in G) with the state , .

We define principal domain states as crystallographically equivalent (in G) variants of the low-symmetry phase in single-domain orientations that can appear with the same probability in the ferroic phase. They represent possible macroscopic bulk structures of (1) ferroic single-domain crystals, (2) ferroic domains in non-ferroelastic domain structures (see Section 3.4.3.5), or (3) ferroic domains in any ferroic domain structure, if all spontaneous strains are suppressed [this is the so-called parent clamping approximation (PCA), see Section 3.4.2.5]. In what follows, any statement formulated for principal domain states or for single-domain states applies to any of these three situations. Principal domain states are identical with orientation states (Aizu, 1969) or orientation variants (Van Tendeloo & Amelinckx, 1974). The adjective `principal' distinguishes these domain states from primary (microscopic, basic – see Section 3.4.2.5) domain states and secondary domain states, defined in Section 3.4.2.2, and implies that any two of these domain states differ in principal tensor parameters (these are linear combinations of morphic tensor components that transform as the primary order parameter of an equitranslational phase transition with a point-group symmetry descent , see Sections 3.1.3.2 and 3.4.2.3). A simple criterion for a principal domain state is that its stabilizer in G is equal to the symmetry of the ferroic phase [see equation (3.4.2.4)].

When one applies to a principal domain state all operations of the group G, one gets all principal domain states that are crystallographically equivalent with . The set of all these states is denoted and is called an G-orbit of (see also Section 3.2.3.3.3 ), In our example, the G-orbit is .

Note that any operation g from the parent group G leaves the orbit invariant since its action results only in a permutation of all principal domain states. This change does not alter the orbit, since the orbit is a set in which the sequence (order) of objects is irrelevant. Therefore, the orbit is invariant under the action of the parent group G, .

A ferroic phase transition is thus a paradigmatic example of the law of symmetry compensation (see Section 3.2.2 ): The dissymmetrization of a high-symmetry parent phase into a low-symmetry ferroic phase produces variants of the low-symmetry ferroic phase (single-domain states). Any two single-domain states are related by some suppressed operations of the parent symmetry that are missing in the ferroic symmetry and the set of all single-domain states (G-orbit of domain states) recovers the symmetry of the parent phase. If the domain structure contains all domain states with equal partial volumes then the average symmetry of this polydomain structure is, in the first approximation, identical to the symmetry of the parent phase.

Now we find a simple formula for the number n of principal domain states in the orbit and a recipe for an efficient generation of all principal domain states in this orbit.

The fact that all operations of the group leave invariant can be expressed in an abbreviated form in the following way [see equation (3.2.3.70 )]: We shall use this relation to derive all operations that transform into : The second part of equation (3.4.2.8) shows that all lost operations that transform into are contained in the left coset (for left cosets see Section 3.2.3.2.3 ).

It is shown in group theory that two left cosets have no operation in common. Therefore, another left coset generates another principal domain state that is different from principal domain states and . Equation (3.4.2.8) defines, therefore, a one-to-one relation between principal domain states of the orbit and left cosets of [see equation (3.2.3.69 )], From this relation follow two conclusions:

This result can be illustrated in our example. Table 3.4.2.1 presents in the first column the four left cosets of the group . The corresponding principal domain states , and the values of spontaneous polarization in these principal domain states are given in the second and the third columns, respectively. It is easy to verify in Fig. 3.4.2.2 that all operations of each left coset transform the first principal domain state into one principal domain state ,

Table 3.4.2.1 | top | pdf | Left and double cosets, principal and secondary domain states and their tensor parameters for the phase transition with and Left cosets Principal domain states Secondary domain states

The left coset decompositions of all crystallographic point groups and their subgroup symmetry are available in the software GIKoBo-1, path: Subgroups\View\Twinning Group.

Let us turn briefly to the symmetries of the principal domain states. From Fig. 3.4.2.2 we deduce that two domain states and in our illustrative example have the same symmetry, , whereas two others and have another symmetry, . We see that symmetry does not specify the principal domain state in a unique way, although a principal domain state has a unique symmetry .

It turns out that if transforms into , then the symmetry group of is conjugate by to the symmetry group of [see Section 3.2.3.3 , Proposition 3.2.3.13 and equation (3.2.3.55 )]: One can easily check that in our example each operation of the second left coset of (second row in Table 3.4.2.1) transforms into itself, whereas operations from the third and fourth left cosets yield . We shall return to this issue again at the end of Section 3.4.2.2.3.

In this section we demonstrate that any morphic (spontaneous) property appears in the low-symmetry phase in several equivalent variants and find what determines their number and basic properties.

As we saw in Fig. 3.4.2.2, the spontaneous polarization – a principal tensor parameter of the phase transition – can appear in four different directions that define four principal domain states. Another morphic property is a spontaneous strain describing the change of unit-cell shape; it is depicted in Fig. 3.4.2.2 as a transformation of a square into a rectangle. This change can be expressed by a difference between two strain components , which is a morphic tensor parameter since it is zero in the parent phase and nonzero in the ferroic phase. The quantity is a secondary order parameter of the transition (for secondary order parameters see Section 3.1.3.2 ).

From Fig. 3.4.2.2, we see that two domain states and have the same spontaneous strain, whereas and exhibit another spontaneous strain . Thus we can infer that a property `to have the same value of spontaneous strain' divides the four principal domain states , , and into two classes: and with the same spontaneous strain and and with the same spontaneous strain . Spontaneous strain appears in two `variants': and .

We can define a ferroelastic domain state as a state of the crystal with a certain value of spontaneous strain , irrespective of the value of the principal order parameter. Values and thus specify two ferroelastic domain states and , respectively. The spontaneous strain in this example is a secondary order parameter and the ferroelastic domain states can therefore be called secondary domain states.

An algebraic version of the above consideration can be deduced from Table 3.4.2.1, where to each principal domain state (given in the second column) there corresponds a left coset of (presented in the first column). Thus to the partition of principal domain states into two subsets there corresponds, according to relation (3.4.2.9), a partition of left cosets where we use the fact that the union of the first two left cosets of is equal to the group . This group is the stabilizer of the first ferroelastic domain state , . Two left cosets of correspond to two ferroelastic domain states, and , respectively. Therefore, the number of ferroelastic domain states is equal to the number of left cosets of in , i.e. to the index of in , : : , and the number of principal domain states in one ferroelastic domain state is equal to the index of in , i.e. : : .

A generalization of these considerations, performed in Section 3.2.3.3.5 (see especially Proposition 3.2.3.30 and Examples 3.2.3.10 and 3.2.3.33 ), yields the following main results.

Assume that is a secondary order parameter of a transition with symmetry descent . Then the stabilizer of this parameter is an intermediate group, Lattices of subgroups in Figs. 3.1.3.1 and 3.1.3.2 are helpful in checking this condition.

The set of n principal domain states (the orbit ) splits into subsets

Each of these subsets consists of principal domain states,The number is called a degeneracy of secondary domain states.

The product of numbers and is equal to the number n of principal domain states [see equation (3.2.3.26 )]:

Principal domain states from each subset have the same value of the secondary order parameter and any two principal domain states from different subsets have different values of . A state of the crystal with a given value of the secondary order parameter will be called a secondary domain state . Equivalent terms are degenerate or compound domain state.

In a limiting case , the parameter is identical with the principal tensor parameter and there is no degeneracy, .

Secondary domain states are in a one-to-one correspondence with left cosets of in the decompositiontherefore

Principal domain states of the first secondary domain state can be determined from the first principal domain state : where is the representative of the kth left coset of of the decomposition

The partition of principal domain states according to a secondary order parameter offers a convenient labelling of principal domain states by two indices , where the first index j denotes the sequential number of the secondary domain state and the second index k gives the sequential number of the principal domain state within the jth secondary domain state [see equation (3.2.3.79 )]:where and are representatives of the decompositions (3.4.2.20) and (3.4.2.23), respectively.

The secondary order parameter can be identified with a principal order parameter of a phase transition with symmetry descent (see Section 3.4.2.3). The concept of secondary domain states enables one to define domain states that are characterized by a certain spontaneous property. We present the three most significant cases of such ferroic domain states.

3.4.2.2.1. Ferroelastic domain state

| top | pdf |

The distinction ferroelastic–non-ferroelastic is a basic division in domain structures. Ferroelastic transitions are ferroic transitions involving a spontaneous distortion of the crystal lattice that entails a change of shape of the crystallographic or conventional unit cell (Wadhawan, 2000). Such a transformation is accompanied by a change in the number of independent nonzero components of a symmetric second-rank tensor that describes spontaneous strain.

In discussing ferroelastic and non-ferroelastic domain structures, the concepts of crystal family and holohedry of a point group are useful (IT A , 2005). Crystallographic point groups (and space groups as well) can be divided into seven crystal systems and six crystal families (see Table 3.4.2.2). A symmetry descent within a crystal family does not entail a qualitative change of the spontaneous strain – the number of independent nonzero tensor components of the strain tensor u remains unchanged.

Table 3.4.2.2 | top | pdf | Crystal systems, holohedries, crystal families and number of spontaneous strain components Point group M Crystal system Holohedry HolM Spontaneous strain components Crystal family FamM Independent Nonzero , , , , Cubic 1 3 Cubic , , , , , , Hexagonal 2 3 Hexagonal , , , , Trigonal 2 3 , , , , , , Tetragonal 2 3 Tetragonal , , Orthorhombic 3 3 Orthorhombic , m, Monoclinic 4 4 Monoclinic , Triclinic 6 6 Triclinic

We shall denote the crystal family of a group M by the symbol FamM. Then a simple criterion for a ferroic phase transition with symmetry descent to be a non-ferroelastic phase transition is

A necessary and sufficient condition for a ferroelastic phase transition is

A ferroelastic domain state is defined as a state with a homogeneous spontaneous strain . [We drop the suffix `s' or `(s)' if the serial number of the domain state is given as the superscript . The definition of spontaneous strain is given in Section 3.4.3.6.1.] Different ferroelastic domain states differ in spontaneous strain. The symmetry of a ferroelastic domain state R_i is specified by the stabilizer of the spontaneous strain of the principal domain state [see (3.4.2.16)]. This stabilizer, which we shall denote by , can be expressed as an intersection of the parent group G and the holohedry of group , which we shall denote Hol (see Table 3.4.2.2):This equation indicates that the ferroelastic domain state R_i has a prominent single-domain orientation. Further on, the term `ferroelastic domain state' will mean a `ferroelastic domain state in single-domain orientation'.

In our illustrative example,

The number of ferroelastic domain states is given byIn our example, . In Table 3.4.2.7, last column, the number of ferroelastic domain states is given for all possible ferroic phase transitions.

The number of principal domain states compatible with one ferroelastic domain state (degeneracy of ferroelastic domain states) is given byIn our example, , i.e. two non-ferroelastic principal domain states are compatible with each of the two ferroelastic domain states (cf. Fig. 3.4.2.2).

The product of and is equal to the number n of all principal domain states [see equation (3.4.2.19)],The number of principal domain states in one ferroelastic domain state can be calculated for all ferroic phase transitions from the ratio of numbers n and that are given in Table 3.4.2.7.

According to Aizu (1969), we can recognize three possible cases:

(i) Full ferroelastics: All principal domain states differ in spontaneous strain. In this case, , i.e. , ferroelastic domain states are identical with principal domain states. (ii) Partial ferroelastics: Some but not all principal domain states differ in spontaneous strain. A necessary and sufficient condition is , or, equivalently, . In this case, ferroelastic domain states are degenerate secondary domain states with degeneracy . In this case, the phase transition can also be classified as an improper ferro­elastic one (see Section 3.1.3.2 ). (iii) Non-ferroelastics: All principal domain states have the same spontaneous strain. The criterion is , i.e. .

A similar classification for ferroelectric domain states is given below. Both classifications are summarized in Table 3.4.2.3.

Table 3.4.2.3 | top | pdf | Aizu's classification of ferroic phases is the number of ferroelastic domain states, is the number of ferroelectric domain states and is the number of ferroic domain states. Ferroelastic Ferroelectric Fully Partially Non-ferroelastic Fully Partially Non-ferroelectric , 1

Example 3.4.2.1. Domain states in leucite.  Leucite (KAlSi₂O₆) (see e.g. Hatch et al., 1990) undergoes at about 938 K a ferro­elastic phase transition from cubic symmetry to tetragonal symmetry . This phase can appear in single-domain states, which we denote , , . The symmetry group of the first domain state is . This group equals the stabilizer of the spontaneous strain of since Hol( (see Table 3.4.2.2), hence this phase is a full ferroelastic one.

At about 903 K, another phase transition reduces the symmetry to . Let us suppose that this transition has taken place in a domain state with symmetry ; then the room-temperature ferroic phase has symmetry . The phase transition is a non-ferroelastic one [ ] with non-ferro­elastic domain states, which we denote and . Similar considerations performed with initial domain states R₂ and R₃ generate another two couples of principal domain states , and , , respectively. Thus the room-temperature phase is a partially ferroelastic phase with three degenerate ferroelastic domain states, each of which can contain two principal domain states. Both ferroelastic domains and non-ferroelastic domains within each ferroelastic domain have been observed [see Fig. 3.3.10.13 in Chapter 3.3 , Palmer et al. (1988) and Putnis (1992)].

In the preceding section we derived relations for domain states without considering their specific physical properties. Basic formulae for the number of principal and secondary domain states [see equations (3.4.2.11) and (3.4.2.17), respectively] and the transformation properties of these domain states [equations (3.4.2.12) and (3.4.2.21), respectively] follow immediately from the symmetry groups G, of the parent and ferroic phases, respectively. Now we shall examine which components of property tensors specify principal and secondary domain states and how these tensor components change in different domain states.

A property tensor is specified by its components. The number of independent tensor components of a certain tensor depends on the point-group symmetry G of the crystal (see Chapter 1.1 ). The number of nonzero Cartesian (rectangular) components depends on the orientation of the crystal in the reference Cartesian coordinate system and is equal to, or greater than, the number of independent tensor components; this number is independent of orientation. Then there are linear relations between Cartesian tensor components. The difference is minimal for a `standard' orientation, in which symmetry axes of the crystal are, if possible, parallel to the axes of the reference coordinate system [for more on this choice, see Nye (1985) Appendix B, Sirotin & Shaskolskaya (1982), Shuvalov (1988) and IEEE Standards on Piezoelectricity (1987)]. Even in this standard orientation, only for point groups of triclinic, monoclinic and orthorhombic crystal systems is the number of nonzero Cartesian components of each property tensor equal to the number of independent tensor components, i.e. all Cartesian tensor components are independent. For all other point groups , i.e. there are always relations between some Cartesian tensor components. One can verify this statement for the strain tensor in Table 3.4.2.2.

The relations between Cartesian tensor components can be removed when one uses covariant tensor components. [Kopský (1979); see also the manual of the the software GIKoBo-1 and Kopský (2001). An analogous decomposition of Cartesian tensors into irreducible parts has been performed by Jerphagnon et al. (1978).] Covariant tensor components are linear combinations of Cartesian tensor components that transform according to irreducible matrix representations of the group G of the crystal (i.e. they form a basis of irreducible representations of G; for irreducible representations see Chapter 1.2 ). The number of covariant tensor components equals the number of independent components of the tensor .

The advantage of expressing property tensors by covariant tensor components becomes obvious when one considers a change of a property tensor at a ferroic phase transition. A symmetry descent is accompanied by the preservation of, or an increase of, the number of independent Cartesian tensor components. The latter possibility can manifest itself either by the appearance of morphic Cartesian tensor components in the low-symmetry phase or by such changes of nonzero Cartesian components that break some relations between tensor components in the high-symmetry phase. This is seen in our illustrative example of the strain tensor u. In the high-symmetry phase with , the strain tensor has two independent components and three nonzero components: . In the low-symmetry phase with , there are three independent and three nonzero components: , i.e. the equation does not hold in the parent phase. This change cannot be expressed by a single Cartesian morphic component.

Since there are no relations between covariant tensor components, any change of tensor components at a symmetry descent can be expressed by morphic covariant tensor components, which are zero in the parent phase and nonzero in the ferroic phase. In our example, the covariant tensor component of the spontaneous strain is , which is a morphic component since for the symmetry but for symmetry .

Tensorial covariants are defined in an exact way in the manual of the software GIKoBo-1 and in Kopský (2001). Here we give only a brief account of this notion. Consider a crystal with symmetry G and a property tensor with independent tensor components. Let be a -dimensional physically irreducible matrix representation of G. The covariant of consists of the following covariant tensor components: , where a = and numbers different -tuples formed from components of . These covariant tensor components are linear combinations of Cartesian components of that transform as so-called typical variables of the matrix representation , i.e. the transformation properties under operations of covariant tensor components are expressed by matrices .

The relation between two presentations of the tensor is provided by conversion equations, which express Cartesian tensor components as linear combinations of covariant tensor components and vice versa [for details see the manual and Appendix E of the software GIKoBo-1 and Kopský (2001)].

Tensorial covariants for all non-equivalent physically irreducible matrix representations of crystallographic point groups and all important property tensors up to rank four are listed in the software GIKoBo-1 and in Kopský (2001). Thus, for example, in Table D of the software GIKoBo-1, or in Kopský (2001) p. 5, one finds for the two-dimensional irreducible representation E of group 422 the following tensorial covariants: , , , , , .

Let us denote by a tensorial covariant of in the first single-domain state . A crucial role in the analysis is played by the stabilizer of these covariants, i.e. all operations of the parent group G that leave invariant. There are three possible cases:

Now we shall indicate how one can find particular property tensors that fulfil conditions (3.4.2.39) or (3.4.2.40). The solution of this group-theoretical task consists of three steps:

Phase transitions associated with reducible representations are treated in detail only in the software GIKoBo-1 and in Kopský (2001). Fortunately, these phase transitions occur rarely in nature.

A rich variety of observed structural phase transitions can be found in Tomaszewski (1992). This database lists 3446 phase transitions in 2242 crystalline materials.

The use of covariant tensor components has two practical advantages:

Firstly, the change of tensor components at a ferroic phase transition is completely described by the appearance of new nonzero covariant tensor components. If needed, Cartesian tensor components corresponding to covariant components can be calculated by means of conversion equations, which express Cartesian tensor components as linear combinations of covariant tensor components [for details on tensor covariants and conversion equations see the manual and Appendix E of the software GIKoBo-1 and Kopský (2001)].

Secondly, calculation of property tensors in various domain states is substantially simplified: transformations of Cartesian tensor components, which are rather involved for higher-rank tensors, are replaced by a simpler transformation of covariant tensor components by matrices of the matrix representation of , or of [see again the software GIKoBo-1 and Kopský (2001)]. The determination of the tensor properties of all domain states is discussed in full in the book by Kopský (1982).

The relations between morphic properties, tensor parameters, order parameters and names of domain states are compared in Table 3.4.2.4, from which it is seen that what matters in distinguishing different domain states is the stabilizer of the spontaneous (morphic) property, where physically different parameters may possess a common stabilizer. The latter thermodynamic division, based on conditions of the stability, is finer than the former division, which is based on symmetry only. This difference manifests itself, for example, in the fact that two physically different tensor parameters, such as the principal order parameter and a `similar' order parameter , transform according to different representations and but have the same stabilizer (such symmetry descents are listed in Table 3.1.3.2 ) and possess common domain states. This `degeneracy' of domain states can be even more pronounced in the microscopic description, where the same stabilizer and therefore a common basic domain state can be shared by three physically different order parameters: a primary order parameter (the order parameter, components of which form a quadratic invariant with a temperature-dependent coefficient in the free energy), a pseudoproper order parameter that transforms according to the same representation as the primary order parameter but has a temperature coefficient that is not as strongly temperature-dependent as the primary order parameter, and a `similar' order parameter with a representation different from .

Table 3.4.2.4 | top | pdf | Morphic properties, tensor parameters, order parameters, stabilizers and domain states Morphic property Tensor or order parameter Stabilizer of morphic property Domain states Principal tensor parameter Principal `Similar principal' tensor parameter Secondary tensor parameter Secondary ferroic Spontaneous polarization Ferroelectric Spontaneous strain Ferroelastic Primary order parameter Primary, basic, microscopic Pseudoproper order parameter `Similar' order parameter Secondary order parameter Secondary microscopic

The considerations of this and all following sections can be applied to any phase transition with point-group symmetry descent . All such non-magnetic crystallographically non-equivalent symmetry descents are listed in Table 3.4.2.7 together with some other data associated with symmetry reduction at a ferroic phase transition. These symmetry descents can also be traced in lattices of point groups, which are displayed in Figs. 3.1.3.1 and 3.1.3.2 .

The symmetry descents listed in Table 3.4.2.7 are analogous to Aizu's `species' (Aizu, 1970a), in which the symbol F stands for the symbol in our symmetry descent, and the orientation of symmetry elements of the group with respect to G is specified by letters p, s, ps, pp etc.

As we have already stated, any systematic analysis of domain structures requires an unambiguous specification of the orientation and location of symmetry elements in space. Moreover, in a continuum approach, the description of crystal properties is performed in a rectangular (Cartesian) coordinate system, which differs in hexagonal and trigonal crystals from the crystallographic coordinate system common in crystallography. Last but not least, a ready-to-use and user-friendly presentation calls for symbols that are explicit and concise.

To meet these requirements, we use in this chapter, in Section 3.1.3 and in the software GIKoBo-1 a symbolism in which the orientations of crystallographic elements and operations are expressed by means of suffixes related to a reference Cartesian coordinate system. The relation of this reference Cartesian coordinate system – called a crystallophysical coordinate system – to the usual crystallographic coordinate system is a matter of convention. We adhere to the generally accepted rules [see Nye (1985) Appendix B, Sirotin & Shaskolskaya (1982), Shuvalov (1988), and IEEE Standards on Piezoelectricity, 1987].

We list all symbols of crystallographic symmetry operations and a comparison of these symbols with other notations in Tables 3.4.2.5 and 3.4.2.6 and in Figs. 3.4.2.3 and 3.4.2.4.

Table 3.4.2.5 | top | pdf | Symbols of symmetry operations of the point group Standard: symbols used in Section 3.1.3 , in the present chapter and in the software; all symbols refer to the cubic crystallographic (Cartesian) basis, (all ositive), . BC: Bradley & Cracknell (1972). AH: Altmann & Herzig (1994). IT A: IT A (2005). Jones: Jones' faithful representation symbols express the action of a symmetry operation on a vector (see e.g. Bradley & Cracknell, 1972). Standard BC AH IT A Jones Standard BC AH IT A Jones 1 or e E E 1 or i I i     2    m     2    m     2    m     2    m     2    m     2    m     2    m     2    m         m

Table 3.4.2.6 | top | pdf | Symbols of symmetry operations of the point group Standard: symbols used in Section 3.1.3 , in the present chapter and in the software; suffixes (in italic) refer to the Cartesian crystallophysical coordinate system. BC: Bradley & Cracknell (1972). AH: Altmann & Herzig (1994). IT A: IT A (2005), coordinates (in Sans Serif) are expressed in a crystallographic hexagonal basis. Jones: Jones' faithful representation symbols express the action of a symmetry operation of a vector in a crystallographic basis (see e.g. Bradley & Cracknell, 1972). Standard BC AH IT A Jones Standard BC AH IT A Jones 1 or e E E or i I I

Figure 3.4.2.3 | top | pdf | Oriented symmetry operations of the cubic group and of its subgroups. The Cartesian (rectangular) coordinate system is identical with the crystallographic and crystallophysical coordinate systems. Correlation with other notations is given in Table 3.4.2.5.

Figure 3.4.2.4 | top | pdf | Oriented symmetry operations of the hexagonal group and of its hexagonal and trigonal subgroups. The coordinate system corresponds to the Cartesian crystallophysical coordinate system, the axes of the crystallographic coordinate system are parallel to the twofold rotation axes and to the sixfold rotation axis . Correlation with other notations is given in Table 3.4.2.6.

Now we can present the synoptic Table 3.4.2.7.