Layered-rocksalt compounds

Misfit layered compounds of the type, (RX) (TX2) , where R is a rare earth, Pb or Bi, T = Ti, V, Nb or Ta and X = S or Se, are typical incommensurate phases that have been investigated in detail (Rouxel Meerschaut, 1994). The incommensuration here arises from a structural misfit between the rocksalt-like RX and layered TXj dichalcogenide units which are stacked alternately. The interplay of structure and electron-transfer from the RX to TXj units gives rise to novel electronic properties. For... 

This increase in hardness has been associated with the formation of a rocksalt cubic (c) form of AIN, stabilized by the reduction of the interfacial energy with the rocksalt-structured TiN (Madan etal., 1997). As the thickness increases the effect is offset by the increase in the volume free energy, so that the cubic form is stable only at layer thicknesses of less than 2 nm, although some increase is possible by using rocksalt-structured compounds such as VN with a lower mismatch (Li et al., 2002, 2004). Stabilization with other materials such as W (Kim et al., 2001) or ZrN (Wong et al., 2000) is also possible, and other compounds such as CrN (Yashar et al., 1998) can also show stabilized cubic forms. 

The borderline which separates duster compounds from simple salts is well illustrated with the example of Gd20C. The structure is built from dose packed layers of Gd (abc), Q (ABQ and C atoms a,p,y) in a cubic sequence AqSaC-bacBayb. .. This arrangement bears a dose similarity to the structure of the monofluoride Ag2p [181] which contains layers of condensed but empty Ag octahedra. In the structure of GdaQC, all octahedral voids are occupied by interstitial C atoms which stabilize the dusters. If the distinction between the different kinds of anions is omitted (t. e. ABC afiy), then Gd20C has the rocksalt structure. 

If we neglect the H atom in LiOH then its structure corresponds to the anti-PbO type. Since the axial ratio and the free positional parameter of this tetragonal structure can vary in a certain range, different isopuntal structure [9] types are possible. Thus for c/a = V2 and z(anion) = j, a cubic close-packing of the anions results with the cations in tetrahedral holes. An axial ratio c/a = 1/V2 and an anion parameter z =, on the other hand, correspond to the CsCl type with coordination number 8. As follows from Table 56, LiOH approximates a cubic close-packing with Li in deformed tetrahedral coordination. The position of the lone electron pair of PbO is here taken by H (corrected O—H distance 0.98 A [325] similar to the lone pair-cation distance). The electron density corresponds to Li 0 ° H and one electron smeared between the layers [1012]. In Table 55, LiOH is compared with chemically related compounds. Lithium amide has a closely related structure in which the layers of tetrahedral cation sites are alternately I and i occupied (5T1 + IT2 and ti, respectively) instead of the completely occupied and completely empty layers of LiOH. This is obviously a consequence of the weaker dipole character of NHJ. LiF, with no dipole moment, crystallizes in the rocksalt structure. The structure of LiSH is similar to chalcopyrite whereas that of the hydrosulfides and hydroselenides of Na, K and Rb is a rhombohedrally deformed rocksalt type. 

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