Fig. 3: Lattice dynamics at lower temperatures.

a IR-reflectivity spectra (dashed curves) and fitted imaginary part of the dielectric function (solid curves, see Supplementary Note 4 for details) of CsPbBr3 and CsSrBr3 measured at room temperature. b DFT-calculated IR-absorption spectra within the harmonic approximation for the orthorhombic phases. c Raman spectra of orthorhombic CsPbBr3 and CsSrBr3 measured at 80 K. d DFT-calculated Raman spectra of both compounds within the harmonic approximation for the orthorhombic phases.