Rhombohedral stacking sequence

There are three possible arrangements of atoms in a layer of SiC crystal, and each type has the same layers but a different stacking sequence (29). Designation (30) is by the number of layers in the sequence, followed by H, R, or C to indicate whether the type belongs to the hexagonal, rhombohedral, or cubic class. 

Cl 6c +(u,u,u +m, + m, + m + w, + w, +m) w= This structure has a rhombohedral symmetry but is usually described in terms of a triple-volume hexagonal cell, which makes comparison with the idealized Cdl2 structure simpler. The idealized unit cell is adequate for the purposes of this book. In this representation the anion layers are in cubic closest packing. .. ABC ABC ABC... The metal and nonmetal stacking sequence is... 

The atomic layers from c-BN (ABCABC) have to rearrange into an ABAB stacking sequence of h-BN during the solid state phase conversion. A possible mechanism would be the intermediate formation of the rhombohedral BN phase (r-BN) with ABCABC stacking. The r-BN phase is structurally related to the hexagonal phase, but only differs in the d-values (h-BN d = 6.66 A r-BN d = 10.0 A) of the layers (Fig. 5b). Subsequently the rhombohedral phase is transformed into the hexagonal modification at the reaction temperatures [10]. 

Another two-dimensional C60 polymer phase is the rhombohedral phase, in which the (111) plane of the original fee C60 lattice is the polymerized plane (Fig. 11). If the unit in the rhombohedral cell is spherical or, at least, has a six-fold symmetry along the axis perpendicular to the plane, the stacking sequence is unique. However, each polymerized C60 unit only has a three-fold symmetry along the axis, and there can be two different ways of stacking, ABC and ACB , where B and C correspond to the polymerized plane with the C60 positions above the B and C sites in Fig. Irrespectively [39]. 

This stacking, however, looks rather artificial and seems to violate the condition of uniformity (482). Two 6 R polytypes can be created from the structures of 2H—M0S2 and 2H—TaSa by rhombohedral stacking of the double-sandwich units, namely AbA BaB CaC AcA BcB CbC and AcA BcB CbC AbA BaB CaC. The former stacking sequence was reported for 6R—Tai+xS2 (67). 

Silicon carbide is remarkable for its unusually large variety of different morphologies, which differ in their stacking sequences of hexagonal and rhombohedral layers. All hexagonal and rhombohedral forms are often simply described as a-SiC. The commercially available SiC produced by the Acheson process is a-SiC. 

Figure 2.2 Graphite structures simple hexagonal, Bernal hexagonal, and rhombohedral a is the distance between nearest-neighbours, and c is the interlayer distance. A, B, C denote the stacking sequence of graphenes. (Reprinted from Ref. [25] with permission from Elsevier.)...

Fig. 47. Schematic diagram of molybdenite structure. Mo atoms (solid circles) are located at B sites, and the S atoms (open circles) at A sites which are above and below the plane in which the B sites lie. In hexagonal molybdenite the stacking sequence is AbBaAb. (In the rhombohedral variety the C sites assume importance.) (8 7)...

Zhadanov [7] visualised the SiC stacking sequences by considering the sequence of non-basal tetrahedral planes. The observed zig-zag of layers ([111] direction for cubic, and [0001] direction for hexagonal and rhombohedral) was represented by a notation which denotes the number of consecutive layers without rotation, the total stacking sequence, and a subscript indicating the sequence repetition. A detailed discussion of polytypism, polytypic transformation and notations has been reported by Jepps and Page [8]. 

Since in M0S2 and graphite the interlayer, van der Waals, bonding is very weak, the structures can also exist in a rhombohedral form with a stacking sequence AbA BaB CcC other layer materials naturally adopt this structure. 

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