Prediction study on mechanical and thermodynamic properties of orthorhombic Mg₂SiO₄ under high temperature

In this work, based on density functional theory and quasi-harmonic Debye model, mechanical and thermodynamic properties of orthorhombic Mg₂SiO₄ under high temperature are predicted. We found out that α-Mg₂SiO₄ is mechanically stable under the condition from about 0 to 74 GPa. Results indicate that the main cause of mechanical instability is high pressure, and the effect caused by high temperature is small. C₁₁, C₂₂, C₃₃, B and v_p reduce with temperature just a little and increase with pressure obviously. Mg₂SiO₄ has excellent resistance to strong compression; however the resistance to shear is unsatisfactory. The C_v tends to the Petit and Dulong limit at high temperature under any pressure, and it is proportional to T³ at extremely low temperature. Pressure has an opposite effect on C_v than temperature. The suppressed effect on C_v caused by pressure is not obvious under low and very high temperature. Mg₂SiO₄ has three different thermal expansion coefficients (α) along a-, b- and c-axes, and α_a<α_c<α_b. α increases rapidly at low temperature (about <300 K), and slows down at high temperature. High pressure would greatly suppress expansion caused by temperature. Nevertheless, increasing tendency of α_b and α_c is still obvious under high pressure, especially α_b. All the properties are mainly due to Si–O covalent bonds and their directions