Limits of the phonon quasi-particle picture at the cubic-to-tetragonal phase transition in halide perovskites

• Published in: Communications physics Vol. 6; no. 1; pp. 173 - 7
• Main Authors: Fransson, Erik, Rosander, Petter, Eriksson, Fredrik, Rahm, J. Magnus, Tadano, Terumasa, Erhart, Paul
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 12.07.2023; Nature Publishing Group; Nature Portfolio
• Subjects: 639/301/1034/1035; 639/301/1034/1037; 639/301/119/2795; Anharmonicity; Damping; Elementary excitations; Molecular dynamics; Perovskites; Phase transitions; Phonons; Physics; Physics and Astronomy; Relaxation time; Transition temperature
• ISSN: 2399-3650; 2399-3650
• DOI: 10.1038/s42005-023-01297-8

The soft modes associated with continuous-order phase transitions are associated with strong anharmonicity. This leads to the overdamped limit where the phonon quasi-particle picture can break down. However, this limit is commonly restricted to a narrow temperature range, making it difficult to observe its signature feature, namely the breakdown of the inverse relationship between the relaxation time and damping. Here we present a physically intuitive picture based on the relaxation times of the mode coordinate and its conjugate momentum, which at the instability approach infinity and the inverse damping factor, respectively. We demonstrate this behavior for the cubic-to-tetragonal phase transition of the inorganic halide perovskite CsPbBr 3 via molecular dynamics simulations, and show that the overdamped region extends almost 200 K above the transition temperature. Further, we investigate how the dynamics of these soft phonon modes change when crossing the phase transition. The overdamped limit around the structural phase transitions in halide perovskites causes the phonon quasi-particle description to fail. The authors discuss these limits and propose an alternative picture, revealing that the overdamped limit extends to way broader temperature ranges, contrarily to what is commonly assumed.