Phonon anharmonicity and soft-phonon mediated structural phase transition in $Cs_3Bi_2Br_9$

TL;DR

This study investigates the structural phase transition in Cs3Bi2Br9 driven by phonon anharmonicity and soft-mode dynamics, combining experimental Raman and x-ray diffraction data with theoretical insights.

Contribution

It provides a detailed experimental and theoretical analysis of phonon anharmonicity and soft-mode behavior driving the phase transition in Cs3Bi2Br9.

Findings

Trigonal to monoclinic transition at ~95 K driven by soft mode

Raman modes show anomalous temperature dependence due to anharmonic coupling

Volume thermal expansion dominates Raman frequency shifts in trigonal phase

Abstract

We have carried out temperature-dependent x-ray diffraction and Raman scattering experiments on powder $Cs_3Bi_2Br_9$. Trigonal to monoclinic structural transition at around 95 K is discussed and shown to be driven by the softening of the soft mode. We propose a model to describe the dynamics of the incomplete soft-mode. Raman scattering experiments demonstrate the origin of the soft mode to the rocking motions of Br atoms that participate to form $BiBr_6$ octahedra, which correlates the reported theoretical calculations. Some of the Raman mode frequencies exhibit anomalous temperature dependence due to strong anharmonic phonon-phonon coupling. Temperature-dependent x-ray diffraction analysis estimate the volume thermal expansion coefficient in trigonal phase to be $13.54\times10^{-5} K^{-1}$. In the trigonal phase, the broadening of the full width at half maximum (FWHM) with increase in temperature for $E_g$ and $A_{1g}$ modes is accompanied by decaying of one optical phonon into two acoustic phonons. The volume thermal expansion rather than anharmonic phonon-phonon interaction dominates the frequency shift for the Raman modes in trigonal phase. In the monoclinic phase, the strength of four phonon processes to the frequency shift and linewidth broadening is much smaller than that for three phonon processes for some of the modes. The observed temperature dependence of FWHM of certain Raman modes in both phases suggests unusual electron-phonon coupling in the crystal.