High-Pressure Phase Relations and Crystal Structures of Postspinel Phases in MgV₂O₄, FeV₂O₄, and MnCr₂O₄: Crystal Chemistry of AB₂O₄ Postspinel Compounds

Abstract

We have investigated high-pressure, high-temperature phase transitions of spinel (Sp)-type MgV₂O₄, FeV₂O₄, and MnCr₂O₄. At 1200-1800 °C, MgV₂O₄ Sp decomposes at 4-7 GPa into a phase assemblage of MgO periclase + corundum (Cor)-type V₂O₃, and they react at 10-15 GPa to form a phase with a calcium titanite (CT)-type structure. FeV₂O₄ Sp transforms to CT-type FeV₂O₄ at 12 GPa via decomposition phases of FeO wüstite + Cor-type V₂O₃. MnCr₂O₄ Sp directly transforms to the calcium ferrite (CF)-structured phase at 10 GPa and 1000-1400 °C. Rietveld refinements of CT-type MgV₂O₄ and FeV₂O₄ and CF-type MnCr₂O₄ confirm that both the CT- and CF-type structures have frameworks formed by double chains of edge-shared B³⁺O₆ octahedra (B³⁺ = V³⁺ and Cr³⁺) running parallel to one of orthorhombic cell axes. A relatively large A²⁺ cation (A²⁺ = Mg²⁺, Fe²⁺, and Mn²⁺) occupies a tunnel-shaped space formed by corner-sharing of four double chains. Effective coordination numbers calculated from eight neighboring oxygen-A²⁺ cation distances of CT-type MgV₂O₄ and FeV₂O₄ and CF-type MnCr₂O₄ are 5.50, 5.16, and 7.52, respectively. This implies that the CT- and CF-type structures practically have trigonal prism (six-coordinated) and bicapped trigonal prism (eight-coordinated) sites for the A²⁺ cations, respectively. A relationship between cation sizes of ^VIIIA²⁺ and ^VIB³⁺ and crystal structures (CF- and CT-types) of A²⁺B₂³⁺O₄ is discussed using the above new data and available previous data of the postspinel phases. We found that CF-type A²⁺B₂³⁺O₄ crystallize in wide ionic radius ranges of 0.9-1.4 Å for ^VIIIA²⁺ and 0.55-1.1 Å for ^VIB³⁺, whereas CT-type phases crystallize in very narrow ionic radius ranges of ∼0.9 Å for ^VIIIA²⁺ and 0.6-0.65 Å for ^VIB³⁺. This would be attributed to the fact that the tunnel space of CT-type structure is geometrically less flexible due to the smaller coordination number for A²⁺ cation than that of CF-type.