Finite‐Temperature Dynamics in Cesium Lead Iodide Halide Perovskite

Abstract

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 Pmtrue3¯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 Pmtrue3¯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.

Mixed-Halide Perovskite Alloys CsPb(I1–xBrx)3 and CsPb(Br1–xClx)3: New Insight of Configurational Entropy Effect from First-Principles and Phase Diagrams

“…(b) The model system is not too small to exhibit disorder. (c) There are several previous DFT-based studies of these alloy series that can serve as references to this work. ,, Previous DFT-based studies of Cs-based pure perovskites (such as CsPbI 3 and CsPbBr 3 ) indicated that the α and β phases are essentially dynamical mixtures of disordered structures. − In this paper, we tackle this issue beyond pure compounds by constructing the phase diagrams for alloys. Contribution from the lattice (phonon) vibrations was taken into account for this purpose.…”

“…Actually, the nature of the cubic phase of Cs-based perovskites is unclear. Previous computational studies indicate that it is a dynamical mixture of highly disordered tilted structures , but not the untilted ones as used in this work. In addition, we admit that our phase diagram can only give quantitative information, mostly because the phonon calculations with harmonic approximations are not precise enough.…”