We have carried out temperature-dependent x-ray diffraction and Raman scattering experiments on powder ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{Br}}_9$. Trigonal to monoclinic structural transition at around 95 K is discussed and shown to be driven by the incomplete soft mode. We propose that the anharmonic scattering of phonons due to the nucleation of the new phase leads to 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 $\mathrm{Bi}{\mathrm{Br}}_6$ octahedra, which correlates with 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 estimates the volume thermal expansion coefficient in the trigonal phase to be $13.4\times {10}^{-5}{\text{K}}^{-1}$. In the trigonal phase, the broadening of the full width at half maximum (FWHM) with the increase in temperature for $E_g$ and $A_{1g}$ modes are mostly accompanied by the 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 the 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.

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• Phonons
• Structural phase transition
• Perovskites
• Raman spectroscopy
• X-ray powder diffraction