Theoretical Study of Temperature Dependence of Phonons in Orthorhombic SrZrO$_3$ Perovskite
We conduct first-principles theoretical studies to investigate the temperature-dependent phonon properties of orthorhombic SrZrO$_3$ (SZO) perovskite. Our calculations include the quasiharmonic approximation, in which we explored mode Grüneisen parameters, thermal expansion, and frequency shifts for several optical modes. For most modes these shifts exhibit a downward trend with increasing temperature, contributing to the normal behavior of frequencies with temperature. We also studied the temperature dependence of linewidths and lineshifts for several phonon modes within the third-order lattice anharmonic effect. The lineshifts for most modes also exhibit a downward trend with temperature, further supporting the normal temperature-dependent behavior of phonon modes. However, a few optical modes display an upward trend in lineshifts with increasing temperature, which in principle, could lead to anomalous temperature-dependent behavior in their frequencies. Nevertheless, when we incorporate all corrections, including quasiharmonic and third-order anharmonic shifts, to the frequencies obtained within the harmonic approximation, almost all modes exhibit normal behavior with temperature, displaying blue shifts with cooling.