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 vp 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 Cv 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 Cv than temperature. The suppressed effect on Cv 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 αa<αc<αb. 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 αb and αc is still obvious under high pressure, especially αb. All the properties are mainly due to Si–O covalent bonds and their directions.