Rock-salt type metal oxide

Surface Hydroxyl Croups of Rock-Salt-Type Metal Oxides... 

Table 3.12 Rock-salt type metal oxides...

The heavier metal tantalum is distinctly less inclined than niobium to form oxides in lower oxidation states. The rutile phase TaOz is known but has not been studied, and a cubic rock-salt-type phase TaO with a narrow homogeneity range has also been reported but not yet fully characterized. TazOs has two well-established polymorphs which have a reversible transition temperature at 1355°C but the detailed structure of these phases is too complex to be discussed here. 

This is the least known of the oxidation states. The black oxide, VO, has a rock-salt type lattice but it shows a marked tendency to non-stoichiometry, being obtainable with anywhere from 45 to 55 atom % oxygen. It has a metallic luster and rather good electrical conductivity of a metallic nature. There is probably considerable V—V bonding. The oxide is basic and dissolves in mineral acids, giving V1 solutions. 

X-ray diffraction patterns of NiMgAl oxides essentially only show the pattern of a rock salt-type structure common to periclase MgO and to NiO, in spite of the significant amount of alununa (33% metal molar ratio in the case of the sample in Fig. 18.1). 

Figure 9.1 Crystal structures of metal oxides, (a) MgO rock salt type. Mg coordination number (CN) 6 O CN 6. (b) Ce02 fluorite-type, Ce CN 8, O CN 4. (c) T1O2 rutile, Ti CN 6, O CN 3. (d) Si02 quartz, Si CN 4,0 CN 2. (e) WO3, Re03 type, acmally distorted, W CN 6, O CN 2. (f) M0O3 layered structure, the coordinations are questionable, (g) Cr03 linear polymer, Cr CN 4, O CN 1 and 2...

The fourth and final crystal structure type common in binary semiconductors is the rock salt structure, named after NaCl but occurring in many divalent metal oxides, sulfides, selenides, and tellurides. It consists of two atom types forming separate face-centered cubic lattices. The trend from WZ or ZB structures to the rock salt structure takes place as covalent bonds become increasingly ionic [24]. 

The simplest of structures is the rock salt structure, depicted in Figure 2.2a. Magnesium oxide is considered to be the simplest oxide for a number of reasons. It is an ionic oxide with a 6 6 octahedral coordination and it has a very simple structure — the cubic NaCl structure. The structure is generally described as a cubic close packing (ABC-type packing) of oxygen atoms in the (111) direction forming octahedral cavities. This structure is exhibited by other alkaline earth metal oxides such as BaO, CaO, and monoxides of 3d transition metals as well as lanthanides and actinides such as TiO, NiO, EuO, and NpO. 

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