First principles investigation of thermodynamic property of diamond under high pressure and high temperature

The present study utilizes the density functional theory to obtain the diamonds' thermodynamic properties up to 2000 K and 100 GPa under the quasi-harmonic approximation. The results reveal that the variation of thermal expansion coefficient and heat capacity with temperature can be categorized into three distinct temperature regions within the range of 10-2000 K. These regions are accurately described by the Debye model, a combi-nation of Debye and Einstein models, and the Einstein model, respectively. Additionally, high pressures raise the upper temperature limit applicable to the Einstein model. And the study successfully explained the variation of Gruneisen coefficient under high-temperature and high-pressure conditions using the proposed model. In the temperature range where both Einstein and Debye models yield accurate results, the influence of pressure en-hances the contribution of the Debye model to the thermal expansion coefficient and heat capacity of diamond, resulting in a higher peak in the Gruneisen coefficient.