A Comprehensive Study on the Vibrational Spectra of Mixed-Halide Perovskites, MAPb(Br_1–<i>x</i>Cl_<i>x</i>)₃

Hybrid perovskites have shown immense potential for various energy-harvesting applications. There is great interest in the device applications of these systems, both in their pure as well as in derivative halide perovskite forms. At the same time, it becomes both crucial and important to study their local interactions, which further define their thermal and electrical properties, where lattice vibrations play a key role. The vibrational characteristics of atoms or molecules in a given configuration are significantly influenced by the nature of their bonding, which is determined by differences in atomic mass and ionic radii. Therefore, substituting halide anions offers a promising opportunity to investigate the thermal evolution of vibrational modes. Additionally, the interactions between halide anions and organic cations through hydrogen bonding are known to influence structural phase transitions. This highlights the potential for studies involving halide substitutions in these systems with the aim of tuning phase transition temperatures. In this context, in the present study, temperature-dependent Raman spectroscopy has been performed on the mixed-halide perovskites MAPb(Br1–xClx)3 (x = 0 to 1) and critically evaluated for the variation in local interactions for the different vibrational modes. Distinct signatures of structural phase transitions have been identified in the temperature-dependent Raman spectra of the derivative perovskites. A major finding of the work is that, along with the pure end compositions, MAPbBr3 and MAPbCl3, near-end compositions also showed features of structural phase transitions. These structural phase transitions also shift in temperature with halide tuning. Moreover, freezing of the high-temperature cubic phase is observed for all intermediate compositions, which is quite interesting. Low-frequency Raman modes showed the most prominent changes with halide substitution and temperature variation. Sharp changes in Raman frequency and FWHM values, indicative of structural phase transitions, were seen for some of the dominant modes around the transition temperature. Conclusively, we have critically studied the vibrational spectra of the mixed-halide perovskites, which indicate a strong dependence of mixed-halide compositions on structural phase transition behavior. This is an important aspect for tuning the structural phase transition temperature, which affects the device applications of these systems. In addition, our work provides descriptive insights into the lattice dynamics of the mixed-halide perovskite systems, contributing to the fundamental understanding of temperature and compositional dependence of different vibrational modes.