Study of Temperature and Pressure Effect on Thermodynamic Properties of Thorium Phosphide Compound

In this paper, we use density functional theory and first-principles calculations to evaluate the structural, dynamic, and thermophysical properties of thorium phosphide. The structural properties including lattice constant (a0), bulk modulus (B0), and first-order derivative of bulk modulus (B0') are calculated by fitting the Brich-Murnaghan third-order equation of state and compared with other experimental data. This comparison shows a satisfactory agreement between the calculated and experimental lattice constants. The phonon dispersion diagram is calculated by the linear response approach along the high symmetry points. The results indicate the absence of negative modes in the phonon spectrum, which shows that the structure is dynamically stable. We observe a good agreement with the comparison of the obtained optical frequency with the experimental data from the inelastic neutron measurement. The analysis of the phonon density of the states diagram shows a phonon gap in the distance from 115 to 262 cm-1 for this material. Thermodynamic properties including Debye temperature, vibrational entropy, isothermal bulk modulus, isochoric heat capacity, thermal expansion, and Grüneisen parameter are evaluated by the quasi-harmonic Debye method at high pressures and temperatures. It is observed that the Debye temperature of thorium phosphide decreases with increasing temperature at a constant pressure and increases with increasing pressure at a constant temperature. The reduction of the Grüneisen parameter due to the application of pressure shows the changes in the phonon frequencies with the changes in the volume of the unit cell. Also, the increase of the Groningen parameter due to the increase in temperature can be the result of changing the dynamics of the network.