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

Synthetic Mg₂TiO₄ qandilite was investigated to 50 and 40.4 GPa 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 (I4₁/amd, No.141) phase at 14.7 GPa. Owing to sluggish kinetics at room temperature, the spinel phase coexists with the tetragonal phase between 14.7 and 24.3 GPa. In the X-ray diffraction experiment, transformation of the cubic Mg₂TiO₄ to the tetragonal structure was complete by 29.2 GPa, ~5 GPa 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 V₀ = 270(2) ų when K_T0' is fixed at 4, and the volume reduction on change from cubic to tetragonal phase is about 9%. The calculated thermal Grüneisen parameters (γ_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.