Cation distributions on CoAl₂O₄ and Co₂AlO₄ spinels: pressure and temperature effects

Abstract

Structural, energetic and thermodynamic properties of CoAl₂O₄ and Co₂AlO₄ spinels are investigated using the density functional theory formalism. Thermal effects are incorporated by means of a non-empirical quasi-harmonic Debye-like model that allows us to study the influence of temperature on the relative stability of different cation distributions over tetrahedral and octahedral interstices of the oxygen sub-lattice. Our simulations are able to reproduce the experimentally observed trend of preferring a greater cationic disorder in CoAl₂O₄ than in Co₂AlO₄. This behavior can be explained as due to the higher average charge (smaller size) of the cobalt ion in Co₂AlO₄ (+2.5) than in CoAl₂O₄ (+2). According to our calculations, pressure and temperature enhance the preference for structures with Al³⁺ in tetrahedral sites (and Co²⁺ at octahedral sites).

Keywords:

• Co₂AlO₄
• CoAl₂O₄
• DFT calculations
• cation distribution

Acknowledgements

Financial support from Spanish MALTA-Consolider Ingenio 2010 Program (CSD-2007-0045) and Spanish MAT2006-13548-C02-02 is gratefully acknowledged. The authors thank IDRIS and CCRE (UPMC) for providing the computational facilities.