Fig. 4: Lattice dynamics at higher temperature.

Raman spectra of CsPbBr₃ a and CsSrBr₃ b in the high-temperature cubic phase measured experimentally and calculated using DFT-MD. The central peak appears for both compounds in the experiments and computations despite significant differences in bonding: [PbBr₆]⁴⁻ is proximate to lone-pair formation (i.e., exhibits a weak PJT effect)²⁹, while PJT effects associated with [SrBr₆]⁴⁻ are negligible.