High-Pressure Phase Relations and Crystal Structures of Postspinel Phases in MgV 2 O 4 , FeV 2 O 4 , and MnCr 2 O 4 : Crystal Chemistry of AB 2 O 4 Postspinel Compounds.

We have investigated high-pressure, high-temperature phase transitions of spinel (Sp)-type MgV2 O4 , FeV2 O4 , and MnCr2 O4 . At 1200-1800 °C, MgV2 O4 Sp decomposes at 4-7 GPa into a phase assemblage of MgO periclase + corundum (Cor)-type V2 O3 , and they react at 10-15 GPa to form a phase with a calcium titanite (CT)-type structure. FeV2 O4 Sp transforms to CT-type FeV2 O4 at 12 GPa via decomposition phases of FeO wüstite + Cor-type V2 O3 . MnCr2 O4 Sp directly transforms to the calcium ferrite (CF)-structured phase at 10 GPa and 1000-1400 °C. Rietveld refinements of CT-type MgV2 O4 and FeV2 O4 and CF-type MnCr2 O4 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 MgV2 O4 and FeV2 O4 and CF-type MnCr2 O4 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 VIII A2+ and VI B3+ and crystal structures (CF- and CT-types) of A2+ B2 3+ O4 is discussed using the above new data and available previous data of the postspinel phases. We found that CF-type A2+ B2 3+ O4 crystallize in wide ionic radius ranges of 0.9-1.4 Å for VIII A2+ and 0.55-1.1 Å for VI B3+ , whereas CT-type phases crystallize in very narrow ionic radius ranges of ∼0.9 Å for VIII A2+ and 0.6-0.65 Å for VI B3+ . 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.