High pressure equation of state and ideal compressive and tensile strength of MgO single crystal: Ab-initio calculations

Abstract

Ab-initio total energy calculations have been carried out on MgO single crystal as a function of hydrostatic compression to determine the 0 K isotherm, which is then utilized to derive the 300 K isotherm, isentrope, and Hugoniot of MgO. The theoretical isotherm, isentrope, and Hugoniot compare well with the experimental data. The shock parameters C0 and s obtained from theoretical Hugoniot are 6.74 km/s and 1.23 as compared to the experimental values of 6.87 km/s and 1.24, respectively. Having theoretically reproduced the various experimentally measured physical quantities at ambient conditions, the calculations have been extended to determine the ideal compressive and tensile strength of MgO single crystal subjected to uniaxial compressive and tensile loading along [001] direction under two deformation conditions, namely, “uniaxial strain condition” and “uniaxial stress condition.” Examination of elastic stability conditions suggests that for [001] compressive loading, the MgO single crystal will fail mechanically due to shear instability, whereas for expansion it will fail due to vanishing of tensile modulus. The ideal compressive strength under “uniaxial strain condition” and “uniaxial stress condition” is determined to be −283 GPa and −115 GPa, respectively. However, ideal tensile strength is evaluated to be 20 GPa and 11 GPa, respectively, for two loading conditions. Our results suggest that MgO single crystal will offer higher resistance against failure for compression as well as expansion along [001] direction under “uniaxial strain condition” than that for the “uniaxial stress condition.”