Correlated dynamic disorder, octahedral tilts, and acoustic phonon softening in CsSnBr3 and CsPbBr3

This work reports a systematic comparative study of structural fluctuations and atomic dynamics in CsSnBr${}_{3}$ and CsPbBr${}_{3}$. The study combines inelastic neutron scattering on single crystals as a function of temperature with extensive atomistic simulations based on first-principles methods and machine-learned interatomic potentials. The results reveal the existence of two-dimensional nanodomains of correlated tilts of Br octahedra in CsSnBr${}_{3}$, akin to monolayers of the tetragonal phase, similar to those previously reported in CsPbBr${}_{3}$. The correlated disorder results in diffuse rods in reciprocal space, particularly prominent in the cubic phase. The correlations are dynamic with a characteristic lifetime of a few picoseconds, and result from an overdamped phonon branch along the edge of the Brillouin zone. The Sn${}^{2+}$ or Pb${}^{2+}$ ions do not exhibit displacements from the center of the Br octahedron beyond those expected from phonon oscillations.