Effects of pressure on structural, mechanical, and electronic properties of trigonal and monoclinic MgSiO₃

We perform the first-principles calculation based on density-functional theory (DFT) to study the structural, electronic and mechanical properties of two MgSiO₃ polymorphs (trigonal R‾3 and monoclinic P2₁/c) under pressure. The optimized lattice parameters of two MgSiO₃ polymorphs under pressure are obtained and analyzed. Comparing the enthalpy values of the two structures under pressure, we find that monoclinic MgSiO₃ will translate into trigonal structure in the range of 10 GPa–20 GPa. The relationship between the band structure and pressure is revealed. And we calculate the total density of state (TDOS) under different pressures and partial density of state (PDOS) under 0 GPa. The electron populations are obtained for trigonal and monoclinic MgSiO₃. We systemically studied the mechanical properties of trigonal and monoclinic MgSiO₃ under pressure including elastic constants, bulk modulus, shear modulus, Lamé's constants, Young's modulus, Poisson's ratio as well as B/G values. According to the modified Born stability conditions under pressure, trigonal MgSiO₃ is stable from 0 GPa to 150 GPa, but monoclinic MgSiO₃ is unstable from 80 GPa to 100 GPa.