Raman and X-ray diffraction study of pressure-induced phase transition in synthetic Mg2TiO4

Synthetic Mg2TiO4 qandilite was investigated to 50 and 40.4GPa at room temperature using Raman spectroscopy and X-ray diffraction, respectively. The Raman measurements showed that cubic Mg2TiO4 spinel transforms to a high pressure tetragonal (I4(1)/amd, No.141) phase at 14.7GPa. Owing to sluggish kinetics at room temperature, the spinel phase coexists with the tetragonal phase between 14.7 and 24.3GPa. In the X-ray diffraction experiment, transformation of the cubic Mg2TiO4 to the tetragonal structure was complete by 29.2GPa, similar to 5GPa higher than the transition pressure obtained by Raman measurements, owing to slow kinetics. The obtained isothermal bulk modulus of Mg2TiO4 spinel is K-T0=148(3) GPa when K-T0'=6.6, or K-T0=166(1) GPa when K-T0' is fixed at 4. The isothermal bulk modulus of the high-pressure tetragonal phase is calculated to be 209(2) GPa and V-0=270(2) angstrom(3) when K-T0' is fixed at 4, and the volume reduction on change from cubic to tetragonal phase is about 9%. The calculated thermal Gruneisen parameters (gamma(th)) of cubic and tetragonal Mg2TiO4 phases are 1.01 and 0.63. Based on the radii ratio of spinel cations, a simple model is proposed to predict post-spinel structures.