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

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