International
Tables for Crystallography Volume A Space-group symmetry Edited by Th. Hahn © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. A. ch. 14.2, pp. 848-872
https://doi.org/10.1107/97809553602060000531 Chapter 14.2. Symbols and properties of lattice complexes
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Institut für Mineralogie, Petrologie und Kristallographie, Philipps-Universität, D-35032 Marburg, Germany The reference symbols of lattice complexes are described and the terms `degrees of freedom' of a lattice complex, `limiting complex', `comprehensive complex' and `Weissenberg complex' are defined and illustrated by examples. Then the descriptive symbols of lattice complexes are introduced, their properties are described and their interpretation is demonstrated by numerous examples. Tables 14.2.3.1 Keywords: lattice complexes; Wyckoff positions; Wyckoff sets; degrees of freedom; limiting lattice complexes; comprehensive lattice complexes; Weissenberg complexes; invariant lattice complexes. |
If a lattice complex can be generated in different space-group types, one of them stands out because its corresponding Wyckoff positions show the highest site symmetry. This is called the characteristic space-group type of the lattice complex. The space groups of all the other types in which the lattice complex may be generated are subgroups of the space groups of the characteristic type.
Different lattice complexes may have the same characteristic space-group type but in that case they differ in the oriented site symmetry of their Wyckoff positions within the space groups of that type.
The characteristic space-group type and the corresponding oriented site symmetry express the common symmetry properties of all point configurations of a lattice complex. Therefore, they can be used to identify each lattice complex. Within the reference symbols of lattice complexes, however, instead of the site symmetry the Wyckoff letter of one of the Wyckoff positions with that site symmetry is given, as was first done by Hermann (1935). This Wyckoff position is called the characteristic Wyckoff position of the lattice complex.
Examples
In Tables 14.2.3.1 and 14.2.3.2
, the reference symbols denote the lattice complex of each Wyckoff position. The reference symbols of characteristic Wyckoff positions are marked by asterisks (e.g. 2e in
). If in a particular space group several Wyckoff positions belong to the same Wyckoff set (cf. Koch & Fischer, 1975
), the reference symbol is given only once (e.g. Wyckoff positions 4l to 4o in
). To enable this, the usual sequence of Wyckoff positions had to be changed in a few cases (e.g. in
). For Wyckoff positions assigned to the same lattice complex but belonging to different Wyckoff sets, the reference symbol is repeated. In
, for example, Wyckoff positions 4c and 4d are both assigned to the lattice complex
. They do not belong, however, to the same Wyckoff set because the site-symmetry groups
.. of 4c and
.. of 4d are different.
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The number of coordinate parameters that can be varied independently within a Wyckoff position is called its number of degrees of freedom. For most lattice complexes, the number of degrees of freedom is the same as for any of its Wyckoff positions. The lattice complex with characteristic Wyckoff position , for instance, has two degrees of freedom. If, however, the variation of a coordinate corresponds to a shift of the point configuration as a whole, one degree of freedom is lost. Therefore,
is the characteristic Wyckoff position of a lattice complex with only two degrees of freedom, although position 8b itself has three degrees of freedom. Another example is given by
and P4 4d 1 xyz. Both Wyckoff positions belong to lattice complex
with two degrees of freedom.
According to its number of degrees of freedom, a lattice complex is called invariant, univariant, bivariant or trivariant. In total, there exist 402 lattice complexes, 36 of which are invariant, 106 univariant, 105 bivariant and 155 trivariant. The 30 plane lattice complexes are made up of 7 invariant, 10 univariant and 13 bivariant ones.
Most of the invariant and univariant lattice complexes correspond to several types of Wyckoff set. In contrast to that, only one type of Wyckoff set belongs to each trivariant lattice complex. A bivariant lattice complex may either correspond to one type of Wyckoff set (e.g. ) or to two types (P4 d, for example, belongs to the lattice complex with the characteristic position
).
For point groups, the occurrence of limiting crystal forms is well known. In , for instance, any tetragonal prism
is a special crystal form with face symmetry m.. . In point group 4, on the other hand, the tetragonal prisms
belong, as special cases, to the set of general crystal forms
, the tetragonal pyramids, and there is no difference between
and
in either the number or the symmetry of their faces. Therefore, the tetragonal prism is called a `limiting form' of the tetragonal pyramid. In a case like this, all possible sets of equivalent faces belonging to a special type of crystal form (the tetragonal prism) may also be generated as a subset of another more comprehensive type of crystal form (the tetragonal pyramid). Of course, it is not possible, by considering a tetragonal prism by itself, to decide whether it has been generated by point group
or by point group 4. This distinction can be made, however, if the tetragonal prism shows the right striations or occurs in combination with other appropriate crystal forms. Low quartz (oriented point group 321) gives a well known example: the hexagonal prism
has the same site symmetry 1 as any trigonal trapezohedron
. Therefore,
may be recognized as a limiting form only if the crystal shows in addition at least one trigonal trapezohedron.
A similar relation may exist between two lattice complexes. Let L be a lattice complex generated by a Wyckoff position of a space group (e.g. by
). An appropriate Wyckoff position of a subgroup
of
(e.g.
) may produce not only all point configurations of L but other point configurations in addition (with different orientations of the squares in the example). The complete set then forms a second lattice complex M. Such relationships led to the following definition (Fischer & Koch, 1974,
1978
):
If a lattice complex L forms a true subset of another lattice complex M, the lattice complex L is called a limiting complex of M and the lattice complex M a comprehensive complex of L.
The point configurations of the limiting complex L are generated within M by restrictions imposed on the coordinate or/and the metrical parameters.
In the above example, such a restriction holds for the y coordinate: the condition for Wyckoff position 4j of
filters out exactly those point configurations that constitute the lattice complex
. The latter complex is, therefore, a limiting complex of the lattice complex
. In the present case of restricted coordinates, both complexes belong to the same crystal family and L has fewer degrees of freedom than M.
Another kind of limiting-complex relation is connected with restrictions for metrical parameters. All point configurations of the lattice complex are also generated by
under the restriction
, i.e. in special space groups of type
. Here L and M have the same number of degrees of freedom, but belong to different crystal families.
Finally, the two types of parameter restrictions for limiting complexes may also occur in combination. The trivariant lattice complex with characteristic Wyckoff position , for example, contains the invariant cubic lattice complex
as a limiting complex. The parameter restrictions necessary are
.
As for a limiting form in crystal morphology, it is often impossible to decide by which symmetry (space group and Wyckoff set) a particular point configuration, regarded by itself, has been generated. If a point configuration belongs to a lattice complex that is part of a comprehensive complex, this point configuration is a member of both complexes. As a consequence, the lattice complexes do not form equivalence classes of point configurations. Only if a point configuration is inspected in combination with a sufficient number of other point configurations – like sets of symmetrically equivalent atoms in a crystal structure – does it make sense to assign this point configuration to a particular lattice complex. An example is found in the crystal structures of the spinel type. Here, the oxygen atoms occupy Wyckoff position 32e xxx in with
(referred to origin choice 1). If x is restricted to
, the point configurations generated are those of the lattice complex
(formed by all face-centred cubic point lattices). If for a spinel-type structure this restriction holds exactly, the point configurations of the cations would, nevertheless, reveal the true generating symmetry of the oxygen point configuration. It has, therefore, to be considered a member of the comprehensive complex
rather than a member of the lattice complex
(which includes among others the point configuration of the copper atoms in the crystal structure of copper). For practical applications, a point configuration contained in several lattice complexes may be investigated within the complex that is the least comprehensive but still allows the physical behaviour under discussion. This corresponds to the definition of the symmetry of a crystal generally used in crystallography: the highest symmetry that can be assigned to a crystal as a whole is that of its least symmetrical property known to date.
Even though limiting-complex relations are very useful for establishing crystallochemical relationships between different crystal structures, a complete study has not yet been carried out. Apart from isolated examples in the literature, systematic treatments have been given only for special aspects: plane lattice complexes (Burzlaff et al., 1968); cubic lattice complexes (Koch, 1974)
; point complexes, rod complexes and layer complexes (Fischer & Koch, 1978)
; extraordinary orbits for plane groups (Lawrenson & Wondratschek, 1976)
; noncharacteristic orbits of space groups except those that are due to metrical specialization (Engel et al., 1984)
. The closely related concepts of limiting complexes and noncharacteristic orbits have been compared by Koch & Fischer (1985)
.
Depending on their site-symmetry groups, two kinds of Wyckoff position may be distinguished:
A lattice complex contains either Wyckoff positions exclusively of the first or exclusively of the second kind. Most lattice complexes are made up from Wyckoff positions of the first kind.
There exist, however, 67 lattice complexes that do not contain point configurations with infinitesimal short distances between symmetry-related points [cf. Hauptgitter (Weissenberg, 1925)]. These lattice complexes have been called Weissenberg complexes by Fischer et al. (1973)
. The 36 invariant lattice complexes are trivial examples of Weissenberg complexes. In addition, there exist 24 univariant (monoclinic 2, orthorhombic 5, tetragonal 7, hexagonal 5, cubic 5) and 6 bivariant Weissenberg complexes (monoclinic 1, orthorhombic 2, tetragonal 1, hexagonal 2). The only trivariant Weissenberg complex is
. All Weissenberg complexes with degrees of freedom have the following common property: each Weissenberg complex contains at least two invariant limiting complexes belonging to the same crystal family.
Example
Pmma e
is a comprehensive complex of Pmmm a and of Cmmm a. Within the characteristic Wyckoff position, refers to Pmmm a and
to Cmmm a.
Except for the seven invariant plane lattice complexes, there exists only one further Weissenberg complex within the plane groups, namely the univariant rectangular complex p2mg c.
For the study of relations between crystal structures, lattice-complex symbols are desirable that show as many relations between point configurations as possible. To this end, Hermann (1960) derived descriptive lattice-complex symbols that were further developed by Donnay et al. (1966
) and completed by Fischer et al. (1973)
. These symbols describe the arrangements of the points in the point configurations and refer directly to the coordinate descriptions of the Wyckoff positions. Since a lattice complex, in general, contains Wyckoff positions with different coordinate descriptions, it may be represented by several different descriptive symbols. The symbols are further affected by the settings of the space group. The present section is restricted to the fundamental features of the descriptive symbols. Details have been described by Fischer et al. (1973)
. Tables 14.2.3.1
and 14.2.3.2
give for each Wyckoff position of a plane group or a space group, respectively, the multiplicity, the Wyckoff letter, the oriented site symmetry, the reference symbol of the corresponding lattice complex and the descriptive symbol.1 The comparatively short descriptive symbols condense complicated verbal descriptions of the point configurations of lattice complexes.
Invariant lattice complexes in their characteristic Wyckoff position are represented by a capital letter eventually in combination with some superscript. The first column of Table 14.2.3.3 gives a complete list of these symbols in alphabetical order. The characteristic Wyckoff positions are shown in column 3. Lattice complexes from different crystal families but with the same coordinate description for their characteristic Wyckoff positions receive the same descriptive symbol. If necessary, the crystal family may be stated explicitly by a small letter (column 2) preceding the lattice-complex symbol: c cubic, t tetragonal, h hexagonal, o orthorhombic, m monoclinic, a anorthic (triclinic).
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Example
D
is the descriptive symbol of the invariant cubic lattice complex a as well as of the orthorhombic lattice complex Fddd a. The cubic lattice complex cD contains – among others – the point configurations corresponding to the arrangement of carbon atoms in diamond and of silicon atoms in β-cristobalite. The orthorhombic complex oD is a comprehensive complex of cD. It consists of all those point configurations that may be produced by orthorhombic deformations of the point configurations of cD.
The descriptive symbol of a noncharacteristic Wyckoff position depends on the difference between the coordinate descriptions of the respective characteristic Wyckoff position and the position under consideration. Three cases may be distinguished, which may also occur in combinations.
In noncharacteristic Wyckoff positions, the descriptive symbol P may be replaced by C, I by F (tetragonal system), C by A or B (orthorhombic system), and C by A, B, I or F (monoclinic system). If the lattice complexes of rhombohedral space groups are described in rhombohedral coordinate systems, the symbols R, , M and
of the hexagonal description are replaced by P, I, J and
, respectively (preceded by the letter r, if necessary, to distinguish them from the analogous cubic invariant lattice complexes).
The descriptive symbols of lattice complexes with degrees of freedom consist, in general, of four parts: shift vector, distribution symmetry, central part and site-set symbol. Either of the first two parts may be absent.
Example
..2 C4xxz is the descriptive symbol of the lattice complex
in its characteristic position:
is the shift vector, ..2 the distribution symmetry, C the central part and 4xxz the site-set symbol.
Normally, the central part is the symbol of an invariant lattice complex. Shift vector and central part together should be interpreted as described in Section 14.2.3.2. The point configurations of the regarded Wyckoff position can be derived from that described by the central part by replacing each point by a finite set of points, the site set. All points of a site set are symmetrically equivalent under the site-symmetry group of the point that they replace. A site set is symbolized by a string of numbers and letters. The product of the numbers gives the number of points in the site set, whereas the letters supply information on the pattern formed by these points. Site sets replacing different points may be differently oriented. In this case, the distribution-symmetry part of the reference symbol shows symmetry operations that relate such site sets to one another. The orientation of the corresponding symmetry elements is indicated as in the oriented site-symmetry symbols (cf. Section 2.2.12
). If all site sets have the same orientation, no distribution symmetry is given.
Examples
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In the case of a Weissenberg complex, the central part of the descriptive symbol always consists of two (or more) symbols of invariant lattice complexes belonging to the same crystal family and forming limiting complexes of the regarded Weissenberg complex. The shift vector then refers to the first limiting complex. The corresponding site-set symbols are distinguished by containing the number 1 as the only number, i.e. each site set consists of only one point.
Example
In , each of the two points
and
, represented by
, is replaced by a site set containing only one point 1z, i.e. the points are shifted along the z axis. The shifts of the two points are related by a twofold rotation .2., i.e. are running in opposite directions. The point configurations of the two limiting complexes
and B refer to the special parameter values
and
, respectively.
The central parts of some lattice complexes with two or three degrees of freedom are formed by the descriptive symbol of a univariant Weissenberg complex instead of that of an invariant lattice complex. This is the case only if the corresponding characteristic space-group type does not refer to a suitable invariant lattice complex.
Example
In , each of the two points
and
, represented by
, is replaced by a site set 2y of two points forming a dumb-bell. These dumb-bells are oriented parallel to the y axis.
The symbol of a noncharacteristic Wyckoff position is deduced from that of the characteristic position. The four parts of the descriptive symbol are subjected to the transformation necessary to map the characteristic Wyckoff position onto the Wyckoff position under consideration.
Example
The lattice complex with characteristic Wyckoff position Imma 8h 0yz has the descriptive symbol for this position. Another Wyckoff position of this lattice complex is
. The corresponding point configurations are mapped onto each other by interchanging positive x and negative y directions and shifting by
. Therefore, the descriptive symbol for Wyckoff position Imma i is
.
In some cases, the Wyckoff position described by a lattice-complex symbol has more degrees of freedom than the lattice complex (see Section 14.2.2.1). In such a case, a letter (or a string of letters) in brackets is added to the symbol.
Different kinds of relations between lattice complexes are brought out.
In many cases, limiting-complex relations can be deduced from the symbols. This applies to limiting complexes due either to special metrical parameters (e.g. etc.) or to special values of coordinates (e.g. both P4x and P4xx are limiting complexes of P4xy). If the site set consists of only one point, the central part of the symbol specifies all corresponding limiting complexes without degrees of freedom that are due to special values of the coordinates (e.g.
.
for the general position of
).
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