Phase transition and elasticity of enstatite under pressure from experiments and first-principles studies

We have investigated the thermodynamic stability, crystal structure, elastic constants, and sound velocities of MgSiO₃-enstatite using data from X-ray diffraction and ultrasonic measurements up to 16.8GPa and first-principles calculations up to 30GPa. The calculated enthalpies provide theoretical support for the phase transition from Pbca to P2₁/c between 9 and 14GPa previously observed in natural orthoensatite and MgSiO₃ enstatite. A density increase of 1.4-1.5% for the Pbca→P2₁/c transition is obtained from both first-principles and experimental studies. Elastic constants of Pbca, P2₁/c, C2/c, P2₁ca and Pbca-II are all calculated, and a softening in the shear constant C₅₅ is predicted for Pbca and Pbca-II phases. C₅₅ of Pbca is found to be closely correlated with the A-site SiO₄ tetrahedra chain angle while C₄₄ and C₆₆ are correlated with the B-site chain angle. Pbca, P2₁/c and C2/c all exhibit similar volumetric compressibilities at all pressures. The calculated velocities of the P2₁/c phase at 12GPa are equal to those of Pbca for P and 1.3% higher for S waves. The experimentally observed P and S wave velocity anomalies can be qualitatively described by the transformation from Pbca to P2₁/c; however, the magnitudes of the velocity decreases between 10 and 14GPa remain to be verified by future single crystal data or polycrystalline measurements at high pressures. The predicted velocity jumps of 2.8% and 4.5% for P and S waves, respectively, between Pbca and C2/c in the pressure range of 5-12GPa are in excellent agreement with the values of ~3(1)% and ~5(1)% obtained from the directly measured data, thereby making it a plausible candidate for the seismic X-discontinuity at depths of 250-300km in the Earth's upper mantle.