Lattice dynamics, phonon vibrational spectra, and thermal properties of tetragonal SrPt3P: a first-principles study

The phonon vibrational spectra and thermal properties of the platinum-based superconductor SrPt3P are investigated by the generalized gradient approximation (GGA) plus the PBESOL exchange-correlation functional in the framework of density functional theory (DFT). The calculated structural parameters of SrPt3P at the ground state are in good agreement with the available experimental data and seem to be better than other calculated results. The finite displacement method is used to calculate the phonon vibrational spectra and phonon density of states of SrPt3P. The obtained phonon dispersion curves show that, no imaginary phonon frequency is observed in the whole Brillouin zone, while the imaginary frequencies exist at the gamma point as the pressure is greater than 100 GPa, indicating its structural instability at high pressure. The vibrational modes at the gamma point are analyzed from group theory, and the obtained phonon frequencies are in good agreement with other theoretical and experimental values. The temperature and pressure dependences of the thermal quantities of SrPt3P (such as the thermal expansion, the heat capacity at constant volume, the entropy, and the Gruneisen parameter) are calculated based on the quasi-harmonic approximation (QHA). Our results show that both the thermal expansion coefficient alpha and the heat capacity C-V increase with T-3 at low temperatures and gradually turn almost linear as the temperature increases. It is demonstrated that the entropy is sensitive to temperature while the Gruneisen parameter gamma is more sensitive to pressure.