X-ray single-crystal structural characterization of MgCr2O4, a post-spinel phase synthesized at 23 GPa and 1600 °C

The crystal structure and chemical composition of a crystal of MgCr2O4 post-spinel phase synthesized in the model system Mg3Cr2Si3O12-Mg4Si4O12 at 23 GPa and 1600 °C have been investigated. Electron microprobe analysis confirmed the MgCr2O4 stochiometry of the studied phase. The compound was found to crystallize with the orthorhombic calcium-titanate (CaTi2O4) structure type, space group Bbmm, with lattice parameters a=9.468(1), b=9.670(1), c=2.845(1) Å, V=260.5(1) Å3, and Z=4. The structure was refined to R1=0.046 using 286 independent reflections. Magnesium was found to fully occupy the eightfold-coordinated A site (with a mean bond distance of 2.289 Å) and Cr the octahedral B site (mean: 1.986 Å). The successful synthesis of MgCr2O4 with (CaTi2O4)-type structure and its structural characterization demonstrate the stability of the new post-spinel phase. The absence of MgCr2O4 compounds with spinel structure coexisting with the post-spinel phase in the investigated run is discussed.