Defect rock-salt structure

The disproportionation reaction destroys the layered structure and the two-dimensional pathways for lithium-ion transport. For >0.3, delithiated Li, AV02 has a defect rock salt structure without any well-defined pathways for lithium-ion diffusion. It is, therefore, not surprising that the kinetics of lithium-ion transport and overall electrochemical performance of Li, tV02 electrodes are significantly reduced by the transformation from a layered to a defect rock salt structure [76], This transformation is clearly evident from the... 

Complex lithium halide spinels (Kanno, Takeda and Yamamoto, 1982 Lutz, Schmidt and Haeuseler, 1981), based on Li2CdCl4 and Li2MgCl4 have remarkably high Li ion conductivity for close packed structures. Fig. 2.11. These are complicated materials however, they have essentially inverse spinel structures but may exist also in various distorted forms. Some of them undergo a phase transition to defect rock salt structures at high temperatures some are non-stoichiometric. 

M An 03, M3An 04 and MyAn Oe have been characterized for An = Pa Am. Compounds of the first of these types have the per-ovskite structure (p. 963), those of the second a defect-rock-salt structure (p. 242), and those of the third have structures based on hexagonally close-packed O atoms. In all cases, therefore, the actinide atom is octahedrally coordinated. It is also notable that magnetic and spectroscopic evidence shows that, for uranium, these compounds contain the usually unstable and not, as might have been supposed, a mixture of U and... 

Bismuth(V) oxide and bismuthates are even less well established though a recent important development has been the synthesis and structural characterization of LisBiOs, prepared by heating an intimate mixture of Li20 and Q -Bi203 at 650° for 24 h in dry O2. The structure is of the defect rock-salt type with an ordering of... 

In this large class of materials the blocks R = m (AX) with the rock-salt structure are made by two or more layers of type (AX) which may be identical to each other or have different chemical compositon. The blocks P = (BX2)oc (n-1) [(AX)c o(BX2)o c] with the perovskite structure may have different values of n, and the layers (AX), sandwiched between layers (BX2), may or may not be defective. The important homologous series with the rock salt-perovskite structure are listed in the scheme of Figure 9where they are compared with each other and with the basic structure of perovskite. 

In transition metal compounds where the transition metal is stable in more than one oxidation state, by nonstoichiometry. For example, in CaO and FeO (both of which possess the rock salt structure) Schottky defects are thermally formed. Additionally, however, FeO (wiistite) exhibits mixed valence and, accordingly, has Fe2+ and Fe3+ ions and vacancies distributed over the cation sublattice. 

The sodium tungsten bronzes already described are examples of incomplete lattice defect structures. Iron(II) oxide is rather similar it has a rock-salt structure but is always deficient in iron. Some Fe ions are always present to maintain electrical neutrality. Fe304 has the spinel structure which has the same arrangement of ions as FeO. Fe203. has also the same arrangement and oxidation of FeO to FcgOg consists of the replacement of Fe2+ ions by two thirds of their number of Fe ions. 

Solids in which different kinds of atoms occupy structurally equivalent sites have also defect structures. Thus mixed crystals, say of sodium and potassium chlorides (Fig. 99), are examples of defect lattices of this kind. Lithium titanate, LigTiOg, has a rock-salt structure in which cation sites are... 

For metal particles on ionic substrates one can differentiate between interactions with non-polar (stoichiometric) and polar surfaces. The former surfaces are represented by essentially unrelaxed (001) facets of oxides with rock-salt structure, such as MgO(OOl) which is widely utilized as metal support and was addressed in the previous subsection. Oxide supports exhibiting polar surfaces are also common. Clean polar surfaces are unstable and thus difficult to prepare unreconstructed, dehydroxylated, and free of defects [88]. Corundum, a-Al203, is a prototype of such metal oxides. Its most stable Al-terminated (0001) surface... 

With several other transition metal monoxides, covalent and special electronic effects that are not compatible with the ideal structure manifest themselves. As examples, PdO and PtO display the characteristic tendencies of heavier d ions for square-planar coordination by adopting different structures while a severe Jahn-Teller distortion yields essentially fourfold planar coordination in CuO. In the cases of TiO, VO, and NbO, formation of weak metal to metal bonds are seen as defect variants of the rock salt structure. 

Table9.4 Values ofm [the p02 exponent in the creep rate of Eq. (7)] for various oxygen defects in the transition metal oxides with the rock-salt structure [44].

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