Anion for oxides of formula ab204 charge balance can

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: on. For oxides of formula AB204 charge balance can be achieved by three combinations of cation oxidation state: A"B?'04, A'VBf04, and AV'Bi04. The first combination is the most numerous and examples are known with A" = Mg, (Ca); Cr, Mn, Fe, Co, Ni, Cu; Zn, Cd, (Hg); Sn B"' = Al, Ga, In; Ti, V , Cr, Mn, Fe, Co, Ni; Rh The anion can be 0, S, Se or Te. Most of the A" cations have radii (6-coordinate) in the range 65-95 pm and larger cations such as Ca" (100pm) and Hg" (102pm) do not form oxide spinels. The radii of B"' fall predominantly in the range 60-70pm though Al"' (53pm) is smaller, and In"' (80pm) normally forms sulfide spinels only. Examples of spinels with other combinations of oxidation state are: A'VByX;" : TiMg,04, PbFezO4, SnCu2S4 AV'B;X;I1 : MoAg204, MoNazO4, WNa204 AUB;XT1 : NiLi2F4, ZnK2(CN)4, CdK2(CN)4 Many of the spinel-type compounds mentioned above do not have the normal structure in which A are in tetrahedral sites (t) and B are in octahedral sites (0);nstead they adopt the inverse spinel i structure in which half the B cations occupy the tetrahedral sites whilst the other half of the B cations and all the A cations are distributed on the octahedral sites, i.e. (B),[AB],04. The occupancy of the octahedral sites may be random or ordered. Several factors influence whether a given spinel will adopt the normal or inverse structure, including (a) the relative sizes of A and B, (b) the Madelung constants for the normal and inverse structures, (c) ligand-field stabilization energies (p. 1131) of cations on tetrahedral and octahedral sites, and (d) polarization or covalency effects.(56) Thus, if size alone were important it might be expected that the smaller cation would occupy the site of lower coordination number, i.e. Alt[MgA1],04; however, in spinel itself this is outweighed by the greater lattice energy achieved by having the cation of higher charge, (Al"') on the site of higher coordination and §7.3.4 Ternary and more complex oxide phases 249 It will also be recalled that y-A1203 (p. 243) the normal structure is adopted: (Mg),[A12],04. has a defect spinel structure in which not all of An additional factor must be considered in the cation sites are occupied, Le. Al"', 0 3 O32: * a spinel such as NiA1204 since the crystal 2 13 2 field stabilization energy of Ni" is greater in the relation to spinel (Mg: AlyiO,,) is obvious, octahedral than tetrahedral coordination; this the 8Mg" having been replaced by the redresses the balance, making the normal and isoelectronically equivalent 5 iA1"'. This explains inverse structures almost equal in energy and why MgA1204 can form a complete range of there is almost complete randomization of solid solutions with y-Al203: the oxygen builds all the cations on all the available sites: on to the complete fcc oxide ion lattice and the (A10.75Ni0.25> t rNi0.75A11.251004. Al"' gradually replaces Mg", electrical neutrality Inverse and disordered spinels are said to have being achieved simply by leaving 1 cation site a defect st...
View Full Document

Ask a homework question - tutors are online