Rocksalt structure layers

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 rocksalt structure consists in two interpenetrating fee lattices of anions and cations, in which all atoms are in an octahedral environment. It is met in alkaline-earth oxides (MgO, CaO, SrO, BaO) and in some transition metal oxides like TiO, VO, MnO, FeO, CoO, NiO, etc, with cations in a 4-2 oxidation state. The non-polar surfaces of lowest Miller indices are the (100) and (110) surfaces they have neutral layers, with as many cations as oxygen ions, and their outermost atoms are 5- and 4-fold coordinated, respectively. Actually, planar surfaces can only be produced along the (100) orientation. The polar direction of lowest indices is (111) it has an hexagonal 2D unit cell, three-fold coordinated surface atoms and equidistant layers of either metal or oxygen composition. 

Air-sensitive Ca3CrN3 can be made from reaction of the binary nitrides at 1350 C in sealed Mo tubes [131], The structure contains planar CrN, groups and CaNj square pyramids N atoms center Ca5Cr octahedra. The anion-centered description of the structure begins from the rocksalt structure viewed down a 4-fold axis. If one removes 1 /4 of the octahedra in a layer and then shears the layers, one obtains a layer of the Ca3CrN, structure. The layers are joined by edge-sharing as shown in Fig. 17. 

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. 

CuBiSc2 has a rocksalt structure whereas for CuSbTe2 and CuBiTe2 a layer structure related to that of B12X03 has been reported [1]. 

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. 

Figure 6.5 The bandgap dependence of Mg. Zni .,0 over the entire compositional range. Energies of the fundamental band-to-band transitions E of wurtzitic ZnO and those of rocksalt structure. It should be kept in mind that the layers in the region corresponding to wurtzite-cubic transitions might be of mixed structure, (a) Refs [37-39] (spectroscopic ellipsomet ), (b) Ref. [40] (transmission),...

As above-mentioned, the oxidation of ZrC is diffusion controlled. The oxygen diffusion should be prompted in presence of carbon vacancies, especially ordered carbon vacancies. It has been reported that the starting temperature of oxidation should be at 300 °C for stoichiometric or nearly stoichiometric ZrC, independent of oxygen pressures (Shimada Ishii, 1990). At the initial stage of oxidation, a thin layer of oxycarbide can be observed to form. At an elevated temperature, the oxycarbide layer can be transformed to an amorphous one, from which the cubic/tetragonal zirconia nanocrystals can be developed. To date this process is still not clear. Though the formation of the oxycarbide layer can be proposed by the composition analysis, the structure is not completely identified. Many authors proposed that the structure of the formed oxycarbide should be similar to rocksalt structured ZrC with a small... 

Figure 10.1 Schematic layers of Ti and C atoms in the rocksalt crystal structure of TiC.

Figure 1.9 (a) The perovskite structure. Without the large A atom at the body centre position, the structure becomes that of cubic ReOj (b) The K2Nip4 structure consisting of rocksalt (KF) and perovskite (KNiFj) layers. The NiFg octahedra share equatorial corners restricting the Ni-F-Ni interaction to the xy-plane. 

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... 

MgO (100) (rocksalt) (lxl) Terminated bulk structure, no change from bulk layer spacing of 2.10 A. LEED/12/... 

The SrTiOa (111) [211-214] and (110) polar surfaces [215-217] and the BaTi03(lll) surface [218] have been produced and studied. At variance with rocksalt polar surfaces, many of these investigations suggest that one can obtain non-reconstructed quasi-planar polar surfaces. It should be realized that the perovskite structure is such that there exist ordered configurations of vacancies in the surface layers compatible with (1x1) diffraction patterns. In addition, SrTiO3(110) displays a variety of reconstructions, such as c(2 x 6) [215,217], under reducing conditions. No precise determination of the layer stoichiometry has been performed. 

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