No abstract

Simple algebraic equations show that the bulk compressibility in spinel-type compounds can be expressed by means of cation oxide polyhedral compressibilities and a term that accounts for the pressure effect on the internal oxygen position in the unit cell. The equations explain ͑i͒ the difference of compressibilities at octahedral and tetrahedral sites, ͑ii͒ why the macroscopic bulk modulus can be estimated as the average of these polyhedral bulk moduli, and ͑iii͒ the uniform behavior found in oxide spinels under hydrostatic pressure. Quantum-mechanical ab initio perturbed ion results on MgAl 2 O 4 , ZnAl 2 O 4 , ZnGa 2 O 4 , and MgGa 2 O 4 direct spinels and on MgGa 2 O 4 inverse spinel are reported to illustrate the interpretative capabilities of the proposed equations.

In this study, structural and electronic properties of CoAl2O4 spinel are investigated for the first time by means of quantum chemical computational tools. Coupling supercell periodic calculations under the density functional theory formalism with a nonempirical quasi-harmonic Debye model, we examine the influence of temperature on the relative stability of several cation distributions of Co2+ and Al3+ over tetrahedral and octahedral interstices of the oxygen sublattice. Our simulations are able to reproduce the experimentally observed trend: (i) the normal spinel is calculated to be the stable structure at static and low-temperature conditions, and (ii) as the temperature increases, the preference of structures with Al3+ at tetrahedral sites (and Co2+ at octahedral sites) is found to progress following an asymptotic conduct. The effects of the cation distributions on geometrical variations of electronic and magnetic properties of CoAl2O4 can be interpreted as dominated by the local behavior of Co2+ at octahedral sites.