Crystal Structure of High Pressure Phase of MgAl2O4 synthesized at conditions of 2200 K and 41.8GPa

Graduate School of Science, Tohoku University* Geodynamics Research Center, Ehime University**
â—‹Yasuhiro Kudoh* Takahiro Kuribayashi* Yuichiro Sueda** Tetsuo Irifune**

The specimen used in this study was a single crystal of high pressure phase of MgAl2O4 synthesized using a multi-anvil apparatus at conditions of 2200 K and 41.8 GPa and quenched to ambient condition by Sueda et al. (2004). EDS analysis showed that the specimen has a chemical composition of ideal formula MgAl2O4 within the limit of erperimental error. Sets of X-ray diffraction intensities were measured with a single crystal of 47x47x24micron using synchrotron radiation at the beam line BL-10A, Photon Factory, High Energy Accelerator Reasearch Organization, Tukuba, Japan. The wave length, λ=0.7009 Å was calibrated by the unit cell constants of a ruby standard crystal (a=4.76099(6) Å, c=12.99625(35) Å). The crystallographic data obtained are: orthorhombic, a=2.781(3) Å, b=9.183(3) Å, c=9.383(3) Å, V=239.6(3) Å3. The systematic abscences and N(Z) test for a center of symmetry indicated the centrosymmetric space group Cmcm (No.63). From the total of 2043 reflections measured in the sphere of sinθ/λ=1.00Å-1, 1264 symmetry-independent reflections were obtained by averaging the symmetry equivalent intensities in Laue group mmm (Rint=11%). The crystal structure is isostructural with CaTi2O4 and refined to an R=8.7 % (Rw=8.3%) with anisotropic temperature factors. The calculated density value 3.94 g/cm3 is 10% larger than 3.578 g/cm3 value of spinel, MgAl2O4 which is the stable phase at ambient condition. The Mg atom is surrouded by 8 oxygen atoms with the average Mg-O distance being 2.246(4) Å. The Al atom is surrouded by 6 oxygen atoms with the average Al-O distance being 1.921(3) Å which is comparable to the 1.926 Å value of spinel.