IN SITU X-RAY OBSERVATIONS OF PHASE TRANSITIONS IN MGAL2O4 SPINEL TO 40 GPA USING MULTIANVIL APPARATUS WITH SINTERED DIAMOND ANVILS

Abstract

Phase transitions in MgAl2O4 spinel have been studied at pressures 22-38 GPa, and at temperatures up to 1600 °C, using a combination of synchrotron radiation and a multianvil apparatus with sintered diamond anvils. Spinel dissociated into a mixture of MgO plus Al2O3 at pressures to 25 GPa, while it transformed to the CaFe2O4 (calcium ferrite) structure at higher pressures via the metastably formed oxide mixture upon increasing temperature. Neither the e-phase nor the CaTi2O4-type MgAl2O4, which were reported in earlier studies using the diamond-anvil cell, were observed in the present pressure and temperature range. The zero-pressure bulk modulus of the calcium-ferrite-type MgAl2O4 was calculated as K=213 (3) GPa, which is significantly lower than that reported by Yutani et al. (1997), but is consistent with a more recent result by Funamori et al. (1998) and that estimated by an ab initio calculation by Catti (2001).