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

We have investigated high-pressure, high-temperature phase transitions of spinel (Sp)-type MgV2O4, FeV2O4, and MnCr(2)O(4)w. At 1200-1800 degrees 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 wustite + Cor-type V2O3. MnCr2O4 Sp directly transforms to the calcium ferrite (CF)-structured phase at 10 GPa and 1000-1400 degrees 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 A(2+) cation (A(2+) = 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 A(2+) 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 A(2+) cations, respectively. A relationship between cation sizes of (VIII)A(2+) and B-VI(3+) and crystal structures (CF- and CT-types) of A(2+)B(2)(3+)O(4) is discussed using the above new data and available previous data of the postspinel phases. We found that CF-type A(2)B(2)(3+)O(4) crystallize in wide ionic radius ranges of 0.9-1.4 angstrom for (VIII)A(2+) and 0.55-1.1 angstrom for B-VI(3+), whereas CT -type phases crystallize in very narrow ionic radius ranges of similar to 0.9 angstrom for (VIII)A(2+) and 0.6-0.65 angstrom for B-VI(3+). 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 A(2+) cation than that of CF-type.