Metal halide perovskites (MHPs) have attracted a great deal of scholarly attention in the last few decades due to their unique properties and potential for photovoltaic and optoelectronic applications. The mechanical and electrical properties of MHPs are substantially affected by altering pressure and/or temperature. Near the phase transition pressure, these properties alter dramatically and a discontinuity in the Raman frequencies is observed. In this study, the Raman frequencies of the modes were calculated as a function of pressure (at room temperature) through the isothermal mode Grüneisen parameter for these modes by using the experimental volume data from the literature for CsPbI3. Moreover, I calculated the isothermal compressibility and the bulk modulus through the calculated Raman frequency shifts and isothermal Grüneisen parameter as a function of pressure for the orthorhombic and monoclinic phases of CsPbI3 at room temperature. The predicted Raman frequencies, isothermal Grüneisen parameter, isothermal compressibility, and the bulk modulus for the studied modes were compared with the experimental measurements below and above the orthorhombic and monoclinic phases.
Temperature dependence of the IR frequency and the damping constant (FWHM) of the ν 4 (1440 cm -1 ) NH 4 + and ν 4 (1084 cm -1 ) ND 4 + modes, are calculated for the metal formate frameworks (MOFs) of NH 4 Zn(HCOO) 3 and ND 4 Zn(DCOO) 3 , respectively, by using the experimental data from the literature. By assuming the IR frequency of the ν 4 mode as an order parameter, its temperature dependence is calculated close to the phase transition (T C =191 K) in the NH 4 Zn(HCOO) 3 and ND 4 Zn(DCOO) 3 by the molecular field theory. The temperature dependence of the damping constant of the ν 4 (NH 4 + ) IR mode is also calculated by means of the pseudospin-phonon coupled (PS) and the energy fluctuation (EF) models for these metal formate frameworks. The damping constant due to both models (PS and EF) is fitted to the observed FWHM data of the ν 4 (NH 4 + ) IR mode from the literature at various temperatures close to T C in the MOFs studied.Our results show that the molecular field theory is adequate for the temperature dependence of the IR frequency and also the PS model explains the observed behaviour of the FWHM for the ν 4 (NH 4 + ) mode in NH 4 Zn(HCOO) 3 and ND 4 Zn(DCOO) 3 .
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