Polytype

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Polytype is a term from crystallography and describes the phenomenon that a substance is present in two or more different combinations of layer-like structural units.

Basics

Polytype using the example of the mica group : The capital letters A, B, C denote layer units of the mica structure with different orientations

The structures of polytypes differ only in the sequence and orientation of the individual layers, but not in their structure and composition. Deviations in the compositions of different polytypes of a compound must not exceed 0.25 apfu (atoms per formula unit). If there are greater differences, one speaks of polytypoids.

The structural units of the different polytypes of a compound are structurally and in their composition very similar, but do not need to be absolutely identical. They only differ in their alignment with the crystallographic axes.

In the direction of the stacking of the layered assemblies, polytypes have lattice constants that are integral multiples of the thickness of the individual units. The other unit cell edges of different polytypes are almost the same.

Since polytypes only differ in the sequence of differently oriented but otherwise almost identical assemblies, their thermodynamic properties are also almost the same. As a result, different polytypes of a compound can be formed side by side under the same conditions, e.g. B. 3T and 2M phengite or 2O and 1M phlogopite . However, slight differences in the compositions and processes in crystal growth (kinetics) probably also play a role here.

In contrast to this, different polymorphs of a compound do not have to have structurally similar building units. Polymorphs generally have clearly defined stability regions and their lattice constants are not necessarily in simple integer relationships to one another.

nomenclature

A commission of the International Mineralogical Association (IMA) and International Union of Crystallography (IUCr) developed a notation system for polytype compounds. Accordingly, it combines the name of a polytype compound from the mineral name followed by a hyphen and a suffix. The suffix is ​​made up of four parts:

  • Capital letters to denote the crystal system and
  • three lower case letters (a, b, c) for the unit cell axes, each supplemented by a number indicating the periodicity of the structural units relative to the unit cell edges. In cases in which this periodicity is missing (disordered sequence of the structural units), the axis concerned is marked with a subscript d for disordered.

The following capital letters are used for the crystal systems:

If the true symmetry is unknown or the pseudosymmetry is of particular interest, the crystal system name can be prefixed with the letter P for pseudo.

The full names of the polymorphic compound Al (OH) 3 were e.g. B .:

This detailed naming of the polytype is mostly abbreviated. If the periodicity is 1, the number is omitted. In the case of tetragonal and hexagonal connections, the first two axes are identical and can be omitted entirely. The identifier of the periodicity is then written in front of the symbol for the symmetry. Graphit-Haa2c can thus be abbreviated to the commonly used name Graphit-2H . The same applies to pseudo-hexagonal connections, e.g. B. Cordierite-1PH .

These abbreviated notations are also used for many connections with less symmetry, even if, strictly speaking, they are not correct in this system. Thus, muscovite Mab2c (muscovite PHab2c) to muscovite 2M (muscovite 2M still easily one be abbreviated), while the muscovite Mba2c the abbreviation for muscovite 2M 2 strictly speaking, already violated the rules of that system. However, these widely used names are still valid, as are different mineral names for different polytypes of a compound.

Web links

literature

  • A. Guinier et al .: Nomenclature of polytype structures. Report of the International Union of Crystallography Ad hoc Committee on the Nomenclature of Disordered, Modulated and Polytype Structures . In: Acta Crystallographica Section A . tape 40 , no. 4 , 1984, pp. 399-404 , doi : 10.1107 / S0108767384000842 .
  • Baily, SW et al. (1987): Report of the International Minaralogical Association (IMA) - International Union of Cristallography (IUCr) Joint Committee on Nomenclature. Canadian Mineralogist, 16, 113 - 117 (PDF; 745 kB)
  • G. Ferraris et al .: First structure determination of an MDO-2O mica polytype associated with a 1M polytype . In: European Journal of Mineralogy . tape 13 , no. 6 , 2001, p. 1013-1023 , doi : 10.1127 / 0935-1221 / 2001 / 0013-1013 .