Hexagonal/rhombohedral polytypes

The identity of the polytype present in a given LDH sample may, in principle at least, be determined from the powder XRD pattern, although as we shall see for many LDHs this is not possible, as the amoimt of useful information therein is limited. By convention, the indexing of powder patterns for rhombohedral polytypes is based on a triple hexagonal unit cell (see Fig. 3). 

TABLE 1 Principal phonon energies (meV) in six polytypes derived from the luminescence spectrum (from [26]). The notation (TA) for transverse acoustic, (LA) for longitudinal acoustic, (TO) for transverse optical, and (LO) for longitudinal optical phonons is accurate for the cubic polytype, but not for the hexagonal and rhombohedral polytypes. For details see Datareview 2.2. 

Note The principal axis of the g-tensors of the hexagonal and rhombohedral polytypes is the c-axis, with the parallel component listed first and the perpendicular component second. In some cases, indicated by n.r., hyperfine interaction was not resolved. 

Alpha SiC is the high temperature form of SiC. Unlike pSiC which is a single compound, aSiC has a large number of polytypes, approximately 250 having been identified so far.OiHiz] These polytypes have either a rhombohedral or a hexagonal structure. Polytypes, unlike polymorphs, are the same thermodynamic phase, are formed under the same conditions of temperature and pressure, and have similar properties and structure. Their close-packed layers ( 0001 for hexagonal) are identical but have a... 

There are, in addition, numerous polymorphs or polytypes of many MX2 structures due to the possibility of different layer-stacking orientations. These include the hexagonal polytype with two layers per imit cell (2H), rhombohedral polytype with three layers per unit cell (3R), and the trigonal polytype (IT) with one layer pa- unit cell. Figure 3 illustrates the diffo-ence in layer stacking between the 2H and 3R polytypes of M0S2. 

Crystal Structure. Sihcon carbide may crystalline in the cubic, hexagonal, or rhombohedral stmcture. There is a broad temperature range where these stmctures may form. The hexagonal and rhombohedral stmcture designated as the a-form (noncubic) may crystalline in a large number of polytypes. 

Fig. 1.5 The (rhombohedral) 3R and (hexagonal) 2H stacking polytypes of M0S2. Adjacent layers in the 2H polytype are rotated by 60°, whereas those in the 3R polytype can be superimposed with a translation only. (Reproduced with permission from [77], Copyright 2009, American Chemical...

The structure of presolar silicon carbide grains can provide information about the conditions of formation. Crystalline silicon carbide is known to form about 100 different polytypes, including cubic, hexagonal, and rhombohedral structures. Presolar silicon carbide exists in only two of these, a cubic (fi-SiC) polytype and a hexagonal (a-SiC) polytype (Daulton et al.,... 

Silicon carbide is covalently bonded with a structure similar to that of diamond. There are two basic structures. One is a cubic form, /i-SiC which transforms irreversibly at about 2000 °C to one of a large number of hexagonal polytypes, and the other is a rhombohedral form also with many polytypes. Both the hexagonal and rhombohedral forms are commonly referred to as a-SiC. 

In order to distinguish the polytypes a prefix is added to the formula. The integer corresponds to half the number of X layers within the unit cell, which is equal to the number of sandwiches, XMX. The letter of the prefix designates the symmetry of the unit cell, e. g. T trigonal, H hexagonal or R rhombohedral. 

It is interesting that most of the NbSe2 nanotubes contain more than 10 layers with one or two containing a smaller number of layers. Some of the nanotubes exhibit a different type of stacking due to the presence of different polytypes just as in BN nanotubes where the local rhombohedral stacking occurs within the hexagonal phase.44... 

Class b polytypes. Successive lattice planes parallel to (001) are shifted by 1/3 of the short Class a) or the long Class b) diagonal of the two-dimensional pseudo-hexagonal mesh built on A, A2) axes. For Class b polytypes a pseudo-rhombohedral primitive cell can be chosen, having (almost) the same volume of the reduced cell (Fig. 19). The primitive cell is closer to rhombohedral when the layers are closer to... 

Figure 19. Projection onto the (001) plane of the primitive, conventional (double, monoclinic), pseudo-hexagonal (triple), Ci (sextuple, pseudo-orthohexagonal) and pseudoto c axis of the orthogonal cell-rhombohedral (primitive) cells of Class b polytypes.

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]. 

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