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The term polytype implies that there is a family of structures to which the polytype belongs. The original idea of Baumhauer (1912, 1915), who introduced the term polytypism, was that the individual members of a family consist of identical layers and differ only in their stacking mode.

Since that time, different views concerning the notion of polytypism were expressed, but the present official definition recommended by the Ad-hoc Committee on the Nomenclature of Disordered, Modulated and Polytype Structures (Guinier et al, 1984) is very close to the original concept of Baumhauer. According to this definition,

"an element or compound is polytypic if it occurs in several structural modifications, each of which can be regarded as built up by stacking layers of (nearly) identical structure and composition, and if the modifications differ only in their stacking sequence. Polytypism is a special case of polymorphism: the two-dimensional translations within the layers are essentially preserved".

The Ad-hoc Committee, however, admitted that this definition is too wide because " except for the two-dimensional periodicity of layers" it imposes no restrictions on the sequence and stacking mode of layers.

The fact that the definition is not sufficiently geometric prompted Ďurovič (1999) to suggest that the layers and their stacking must be limited by the vicinity condition, and that a family can encompass only those polytypes which are built on the same structural and symmetry principle, i.e. only those which belong to the same OD groupoid family. This idea was in principle supported also by Makovicky (1997) who, at the same time, proposed to distinguish between proper polytypes, belonging to the same OD groupoid family, and improper polytypes, which cannot be interpreted as such. Recently, Christiansen et al. (1999) suggested a more detailed classification concept related to this subject. Makovicky also accepted the term polytypoids for polytypic substances in which more than 0.25 atoms per formula unit differ in at least one component as proposed by the IMA-IUCr Joint Committee on Nomenclature (Bailey et al, 1977).


Abstract polytypes

Sensu stricto, each real polytype is an individual polymorph with its own stability field, although the energy differences between polytypes of the same compounds are very small. This is caused by desymmetrization, i.e. by changes in the atomic coordinates of individual layers imposed by the influence of the neighboring layers and it is different for different stacking modes. Thus, even layers in different polytypes of the same substance are not identical.

Desymmetrization occurs even in such less pliable structures as SiC, as convincingly reported by researchers at the former Leningrad Electrotechnical Institute (Sorokin et al, 1982a,b; Tsvetkov, 1982; Tairov and Tsvetkov, 1983) who showed that also the chemical composition (the ratio of Si/C) varies from polytype to polytype grown under (nearly) the same conditions.

If these facts were taken absolutely at the face value, the notion of polytypism would loose its unifying significance. In order to overcome these difficulties, the concept of a polytype is often considered an abstract notion referring to a structural type with relevant geometric properties, belonging to an abstract family whose members consist of layers with identical structure and with identical bulk compositions. Such an abstract notion lies at the root of all systematization and classification schemes of polytypes.


HTREM observations and some implications

The application of the High Resolution Transmission Electron Microscopy (HRTEM) has made possible the observation of several stacking sequences that would not be revealed by other techniques. At the same time, HRTEM has raised the question of the limits within which an observed stacking sequence should be considered a polytype. Kogure and Nespolo (1999) stated that the stacking sequences revealed by HRTEM observation can be defined as a polytype only when they are repeated sufficiently to reveal the presence of a memory mechanism reproducing with regularity the stacking sequence; otherwise, they should rather be considered defects. In such cases it is recommended to speak of "a sequence corresponding to a certain polytype" The problem is similar to that of nanocrystals where it is also questionable how many unit cells are necessary to determine a phase.

Under construction

Last update: February 27, 2003



Bailey SW, Frank-Kamenetskii A, Goldsztaub S, Kato A, Pabst A, Schulz H, Taylor HFW, Fleischer M, Wilson AJC (1977) Report of the International Mineralogical Association (IMA) - International Union of Crystallography (IUCr) Joint Committee on Nomenclature. Acta Crystallogr A33, 681-684

Baumhauer H (1912) Über die Kristalle des Carborundums. Z Kristallogr 50, 33-39

Baumhauer H. (1915) ¨ber die verschiedenen Modifikationen des Carborundums und die Erscheinung der Polytypie. Z Kristallogr 55, 249-259

Christiansen CC, Makovicky E, Johnsen O (1999) Homology and typism in heterophyllosilicates: An alternative approach. N Jb Mineral Abh, 175, 153-189

Ďurovič S (1999) Layer stacking in general polytypic structures. Sect. 9.2.2 in International Tables for Crystallography, Vol. C (Ed. ACJ Wilson, E Prince) Dordrecht / Boston / London: Kluwer Academic Publishers, 752-765

Kogure T, Nespolo M (1999) A TEM study of long-period mica polytypes: determination of the stacking sequence of oxybiotite by means of atomic-resolution images and Periodic Intensity Distribution (PID) Acta Crystallogr B55, 507-516

Mackovicky E (1997) Modularity: different types and approaches. In Modular aspects of minerals / EMU Notes in Mineralogy, vol. 1. S Merlino (ed) Eötvös University press, Budapest, p 315-343

Sorokin ND, Tairov YuM, Tsvetkov VF, Chernov MA (1982a) The laws governing the changes of some properties of different silicon carbide polytypes. Dokl Akad Nauk SSSR 262, 1380-1383 (in Russian)

Sorokin ND, Tairov YuM, Tsvetkov VF, Chernov MA (1982b) Crystal-chemical properties of the polytypes of silicon carbide. Sov Phys Crystallogr 28, 539-542

Tairov YuM, Tsvetkov VF (1983) Progress in controlling the growth of polytypic crystals. In Crystal Growth and Characterization of Polytype Structures. P Krishna (ed) Pergamon Press, Oxford / New York / Toronto / Sydney / Paris / Frankfurt, p 111-162

Tsvetkov V F (1982) Problems and prospects of growing large silicon carbide crystals. Izv Leningr Elektrotekh Inst 302, 14-19 (in Russian)