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

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