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

Ching-Pao Wang,Abd-Erraouf Djirar,Sergey N Tkachev,Justin D Rumney,Dongzhou Zhang,Anthony C Withers,Sean R Shieh,Xi Liu,Tianqi Xie

doi:10.1038/s41598-020-63202-5

Abstract

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 KT0 = 148(3) GPa when KT0’ = 6.6, or KT0 = 166(1) GPa when KT0’ 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) Å3 when KT0’ is fixed at 4, and the volume reduction on change from cubic to tetragonal phase is about 9%. The calculated thermal Grüneisen parameters (γth) 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.

Highlights

Synthetic Mg2TiO4 qandilite was investigated to 50 and 40.4 GPa at room temperature using Raman spectroscopy and X-ray diffraction, respectively
In situ high- pressure Raman measurements and equations of states of both spinel and post-spinel structures of synthetic Mg2TiO4 qandilite were investigated at room temperature
The cubic Mg2TiO4 spinel fully transformed to a tetragonal phase above 24.3 GPa and the new phase persisted to 50 GPa

Summary

Introduction

Synthetic Mg2TiO4 qandilite was investigated to 50 and 40.4 GPa at room temperature using Raman spectroscopy and X-ray diffraction, respectively. The isothermal bulk modulus of the high-pressure tetragonal phase is calculated to be 209(2) GPa and V0 = 270(2) Å3 when KT0’ is fixed at 4, and the volume reduction on change from cubic to tetragonal phase is about 9%. In addition to the orthorhombic structures, a tetragonal structure (space group I41/amd) was reported for post-spinel phases at high pressure conditions as an intermediate phase[20,21,22,23,24,25]. In situ high- pressure Raman measurements and equations of states of both spinel and post-spinel structures of synthetic Mg2TiO4 qandilite were investigated at room temperature. The post-spinel phase was identified and a model for prediction of post-spinel structure is reported

Methods

Findings

Discussion

Conclusion