Polytypic structures

GIAB studies of sputtered thin films of different composition for tribological applications have been reported [4.157-4.159]. The technique has been used to study the structure of very thin GdS layers (deposited by chemical bath deposition) for photovoltaic applications in combination with 6-26 diffraction it enabled identification of their polytype structure [4.160]. Glancing angle diffraction in the GIAB geometry... 

The Laves phases form a homeotect structure type set (a family of polytypic structures). In all of them, described in terms of a hexagonal cell, three closely spaced 36 nets of atoms are followed (in the z direction of the same cell) by a 3636 net. The 36 nets are stacked on the same site as the kagome 3636 nets which they surround. For instance (3-BAC- -CAB in the two slabs MgZn2-type (h) structure, (3-BAC- -CBA-a-ACB in the three slabs MgCu2-type (c) structure,... 

The aforementioned papers (Guinier et al. 1984, Zvyagin 1987) also contain suggestions and recommendations on the nomenclature and symbolism for use in the general case of either simple or complex polytypic structures. 

Another method for describing polytypic structures has been suggested by Bokii and Laptev (1994). The polytypic structures, described by means of special unit... 

The various possible stacking sequences generate a family of polytype structures, a few examples of which are listed here ... 

Krishna, P. (ed.) (1983) Crystal Growth and Characterization of Polytype Structures, Pergamon Press, Oxford. 

Electronic properties of the disulphides of the fourth, fifth and sixth group transiton metals crystallizing in the Cdl2(lT), MoS2(2H, 3R) and related polytypic structures have been rationalized by similar considerations. Covalent interaction between quadrivalent cations and sulphide ions is so strong that collective d states are formed in all the cases. The properties crucially depend on the electronic configuration and the... 

Finally, the TEA+ and Na+ cations probably also play their complementary stabilizing role in the case of the FAU-polytype structure, as suggested by the chemical analysis (Table IV). The A1 framework negative charges are also nearly completely co-neutralized by both Na+ and TEA+ ions. This is not surprising if one considers that the FAU-polytype structure only differs from that of ZSM-20 by the close packing of the Faujasite truncated octahedra, that probably necessitate similar stabilization by Na+ and TEA+. 

Kalman, A., Parkanyi, L. and Argay, Gy. (1993 ) Classification of the isostructurality of organic molecules in the crystalline state. Acfa Crystallogr. B, 49,1039 9. [271] Kaneko, E, Sakashita, H., Kobayashi, M., Kitagawa, Y. Matsuura, Y. and Suzuki, M. (1994a). Double-layered polytypic structure of the B form of octadecanoic acid, C18H36O2. Acfa Crystallogr. C, 50, 245-7. [129]... 

Kaneko, K., Shirai, O., Miyamoto, H., Kobayashi, M. and Suzuki, M. (1994c). Oblique infrared transmission spectroscopic study on the E C and B C phase transitions of stearic acid effects on polytypic structure. J. Phys. Chem., 98, 2185-91. [129]... 

Nomenclature of Polytype Structures, A. Guinier, G.B. Bokij, K. Boll-Dornberger, J.M. Cowley, S. Durovic, H. Jagodzinski, P. Krishna, P.M. de Wolff, B.B. Zvyagin,... 

Polytype Structures. Report of the International Union of crystallography Ad-Hoc Committee on the Nomenclature of Disordered, Modrrlated and Polyt>T)e Structures Acta Crystallogr A40 399-404 GtrvenN (1971) Stmctural Factors Controlling Stacking Sequences in Dioctahedral Micas. Clays Clay Miner 19 159-165... 

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 / Franldurt, p 111-162... 

RS studies to other polytypes and verified the concept of a common phonon spectrum (within 2%) for all polytypes [13]. They also were able to construct the phonon dispersion curves for SiC from their first-order RS data from various polytypes (except 3C-SiC, since this polytype is isotropic), namely 4H, 6H, 15R and 21R. The phonon dispersion curves show discontinuities in the LZ representation (observed as doublet peaks in the Raman spectra), which result from the dependence of the force constant on the types of layers (hexagonal or cubic) in the polytype structure [17]. Discontinuities in the phonon dispersion curves are the main effect of layered structures (for a detailed discussion of mini-gaps , see [14] p.98). Recently Nakashima et al [18] used Raman measurements from 8H and 27R polytypes to fill out the phonon dispersion curve reported by Feldman et al [16]. 

The 4H polytype has a wider gap compared to 6H and a much higher electron mobility, making the growth of the 4H polytype of significant practical interest. Favourable properties of this polytype could not be employed in the early studies since the abundance of the 4H polytype in crystals grown by the Lely process is below 1 %. Currently methods of governing the polytype structure permit the intentional growth of this polytype. 

Figure 2. Schematic representation of principal surface sorption processes, (a) Projection of the CoOOH structure in the ab plane. MSC, multinuclear surface complexation represented by an epitaxy of a-FeOOH (left) ISC, mononuclear monodentate (middle right), mononuclear bidentate (top right) and binuclear bidendate (lower right) inner-sphere complexation OSC, outer-sphere surface complexation (top) LD, lattice diffusion (center), (b) Example of epitaxy without sharing of oxygens (Van der Waals forces). The. ..AB-AB... close-packed anionic layer sequence of Co(OH)2(s) is coherently stacked on the. ..AB-BC-CA... layer sequence of CoOOH. Co(OH)2(s) has a 1H polytypic structure, and CoOOH a 3R. Small circles are...

Figures 2 and 3 report the projection along [110] and [100] of the two polytype structures. The main results of the structure refinements are the following a) regular T-O distances and partial Si/Al ordering in both frameworks b) identification of two Ca sites in the monoclinic structure c) identification of two Ca sites in comparable positions in the tetragonal structure, but with lower occupancy d) a further cation site probably occupied by Mg in tetragonal-dominant crystals e) the presence of many other extraframework sites characterized by low electron densities and large distances from the framework oxygens.

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