Poly type

Figure C2.16.2. The sequence of atoms in tlie two poly types of SiC, zincblende and wurtzite, along tlie c-direction. The zincblende lattice has perfect tetraliedral angles.

This first example of the utility of quadrupolar nuclei as sources of information about nano-semiconductors showed that MAS-NMR could identify a poly type that is unstable in the bulk. A second example will show that information about changes in the electronic structure of nanocrystals as a function of synthesis and treatment conditions, as well as information about different polytypes, can be obtained via measurement of Knight shifts and chemical shifts. In this extensive study 69Ga... 

The ability of NMR to distinguish poly types will be greatly aided, as it was in the recent case discussed of SiC, by theoretical calculations of chemical shifts and CS A values (and for quadrupolar nuclei, NQCC values). Also, MAS-NMR may succeed in identifying low levels of polytypes or new ones that have not been seen by diffraction methods. 

Note that in the case of the 2Hi polytype, the cations all occupy b positions, i.e. are aligned on a normal to the layers (this is also the case for the IH polytype), whereas for the other two polytypes the cations alternate between b and c sites. Note also that the interlayers in the 2Hi poly type are all prismatic and those in the 2H2 polytype are all octahedral, whilst in the 2H3 polytype both types of interlayers are present. 

Bookin and Drits [98-100] have also described the large number of possible six-layer poly types, some of which have rhombohedral symmetry (6R) and the remainder hexagonal symmetry (6H). 

The degree of freedom in the translational properties of the various sheets gives the polytypism of the phase. Polytypes are structures with layers of essentially identical composition but differing in translational sequence. Poly types usually have different symmetry and periodicity along axis Z). Smith and Yoder... 

The relative abundance of large Fe ions in the octahedral sheet apparently determines whether a lMd goes ultimately to a 1M or 2M polytype. Although both types of clays are dioctahedral, in the IM iron-rich variety celadonite (Zvyagin, 1957) all three octahedral positions are of the same size (which may be true for the glauconites) whereas, in the Al-rich clays the two filled positions are smaller than the vacant position. This, in part, probably determines whether the 1M or 2M poly type is the stable phase. 

The lib chlorite is the stable poly type in normal chlorite-grade metamorphism and in medium and high-temperature ore deposits. Brown and Bailey suggest that when sufficient energy is available, the most stable polytype (lib) will form. They found that the orthohexagonal and monoclinic lb types were the ones most likely to be considered diagenetic chlorites. These are the chlorites with the lowest amount of tetrahedral Al and these polytypes are most apt to be stable with a small amount of... 

In a study of chlorite in sedimentary rocks, Hayes (1970) concluded that type-I chlorite most likely represents authigenic chlorite (because of its relative instability) the lib stable poly type, in most cases, would indicate that the chlorite is detrital and reflects formation by igneous or metamorphic processes. Hayes points out that a few of his samples of lib chlorite appear to be authigenic, probably formed in a higher-temperature environment caused by deep-burial or hydrothermal activity. 

Fig. 6.33. Polarized x-ray emission spectra (S Ap, and Mo LPj) obtained from a single crystal of MoS, (2H poly type) as a function of angle of incidence 0 (= 20-80 ) (after Yarmoshenko et al., 1983 reproduced with the publisher s permission).

The structure of Hi -TisS 12 has been refined to an R index of 2.8 %, and the structures of a new poly-type 24R of TiSj go o.o2 TiSj have been determined. ... 

The primitive cells of the n H and 3n R SiC poly types contain n formula (Si-C) units, and the unit cell of the polytypes along the c-axis is n times larger than that of the basic 3 C SiC polytype. The BZ of the corresponding polytype is thus reduced in the T — L direction by a factor f/n [70]. One then speaks of folded BZ and some of the folded acoustical phonons with non-zero frequencies at the zone centre are IR- and Raman-active. Their absorptions, with lines as sharp as 0.03 cm-1 at LHeT have been reported for the 6H and 15R SiC polytypes ([77], and references therein). 

Figure 11. Adjacent basal Ob oxygen atoms in the case of M and 20 polytypes, that show 0° and 180° rotations betweeiadjacent M layers (left), and (right) of 2Mi, 37, 2A/2 and 6H poly types where instead the rotation between adjacent M layers is +120° (2ff, 37) or +60° (M2 and 6H). The tetrahedral tilting Az (Fig. 10) is exaggerated.

Ivaldi G, Ferraris G, Curetti N, Compagnoni R (2001b) Coexisting 3T and 2A/i poly types in a phengite from Citna Pal (Val Savenca, western Alps) Chemical and polytypic zoning and structural characterisation. Eur J Mineral 13 1025-1034... 

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