Finite-Temperature Dynamics in Cesium Lead Iodide Halide Perovskite

Lattice dynamics are often regarded as signatures of the underlying crystal structure. Here, a first-principle-based effective Hamiltonian method combined with molecular dynamics simulations is used to study dynamical behaviors of CsPbI3 perovskite across temperature and structural phase transitions. A single (short-range tilting) parameter in this effective Hamiltonian is varied in order to make the temperature range of the intermediate tetragonal P4/mbm phase, existing in-between the cubic Pm3 over bar m and orthorhombic Pnma phases, either broader than observed or completely disappearing. Comparing the dynamics of these different cases allows one to conclude that real CsPbI3 perovskite should have i) two iodine-octahedral-tilt related modes that differ in frequency but both significantly soften as the temperature decreases within the cubic phase toward the Pm3 over bar m-to-P4/mbm transition; and ii) one mode that maintains a very low frequency (of the order of 1.0 cm(-1)) in the entire region of P4/mbm stability, as a result of the temporal exploration of various structural states. Such latter sub-THz mode mixes fluctuations of antiphase iodine tiltings and Cs antipolar motions because of a trilinear energetic coupling.