Particular rock-salt structure, oxides

Cadmium is a member of Group 12 (Zn, Cd, Hg) of the Periodic Table, having a filled d shell of electrons 4valence state of +2. In rare instances the +1 oxidation state may be produced in the form of dimeric Cd2+2 species [59458-73-0], eg, as dark red melts of Cd° dissolved in molten cadmium halides or as diamagnetic yellow solids such as (Cd2)2+ (AlCl [79110-87-5] (2). The Cd + species is unstable in water or other donor solvents, immediately disproportionating to Cd2+ and Cd. In general, cadmium compounds exhibit properties similar to the corresponding zinc compounds. Compounds and properties are listed in Table 1. Cadmium(TT) [22537 48-0] tends to favor tetrahedral coordination in its compounds, particularly in solution as complexes, eg, tetraamminecadmium(II) [18373-05-2], Cd(NH3)2+4. However, solid-state cadmium-containing oxide or halide materials frequently exhibit octahedral coordination at the Cd2+ ion, eg, the rock-salt structure found for CdO. 

The extensive NMR work on the rare-earth pnictides (RN, RP, etc.) was reviewed in section 2.2.3 and 2.2.3.2. Various of the rare-earth chal-cogenides (oxides, sulfides, etc.) also form in the cubic rock-salt structure, in particular those of Eu and Sm. The europium monochalcogenides are semiconducting magnetic materials whose properties have been reviewed by Wachter (1972) also see ch. 19. Both NGR and NMR methods afford opportunity to study these compounds, and the work on oxides has been reviewed in the fore-going section (section 2.3.3). Table 18.24 provides a brief summary listing of experimental work. 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

More recently efforts have been directed towards the development of perovskite related cathode materials, particularly those of the K2Nip4 structure type [2,3], Fig. 1. These oxides consist of alternating layers of ABO3 and AO (rock salt) and have been demonstrated to accommodate a significant oxygen non-stoichiometry. 

---