Understanding and predicting lattice dynamics in strongly anharmonic crystals is one of the long-standing challenges in condensed matter physics. Here, we propose a first-principles method that gives accurate quasiparticle (QP) peaks of the phonon spectrum with strong anharmonic broadening. On top of the conventional first-order self-consistent phonon (SC1) dynamical matrix, the proposed method incorporates frequency renormalization effects by the bubble self-energy within the QP approximation. We apply the developed methodology to the strongly anharmonic $\alpha \text{−}{\text{CsPbBr}}_3$ that displays phonon instability within the harmonic approximation in the whole Brillouin zone. While the SC1 theory significantly underestimates the cubic-to-tetragonal phase transition temperature ($T_c$) by more than 50%, we show that our approach yields $T_c=404–423\mathrm{K}$, in excellent agreement with the experimental value of 403 K. We also demonstrate that an accurate determination of QP peaks is paramount for quantitative prediction and elucidation of the phonon linewidth.

• Received 18 April 2022
• Accepted 19 September 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.185901