Critical phonon frequency renormalization and dual phonon coexistence in layered Ruddlesden-Popper inorganic perovskites
Zezhu Zeng, Chen Chen, Cunzhi Zhang, Qian Zhang, Yue Chen
Abstract
Phonon-related behaviors such as hot-carrier bottleneck and thermooptic coefficients stimulate the study of lattice dynamics and thermal conductivity ($\ensuremath{\kappa}$) in perovskites. Herein, we report ultralow thermal conductivities of synthesized highly oriented samples of ${\mathrm{Cs}}_{2}{\mathrm{PbI}}_{2}{\mathrm{Cl}}_{2}$ ($\ensuremath{\sim}$ 0.45 W/mK at 300 K) and ${\mathrm{Cs}}_{2}{\mathrm{SnI}}_{2}{\mathrm{Cl}}_{2}$ ($\ensuremath{\sim}$ 0.60 W/mK at 300 K). Phonon frequency renormalization of the octahedral rotational soft modes in ${\mathrm{Cs}}_{2}{\mathrm{PbI}}_{2}{\mathrm{Cl}}_{2}$ triggered by the fourth-order anharmonicity considerably amends the three- and four-phonon linewidths, significantly altering the thermal transport property of ${\mathrm{Cs}}_{2}{\mathrm{PbI}}_{2}{\mathrm{Cl}}_{2}$. Furthermore, the phonon gas model partially breaks down, and both normal and diffuson-like phonons must be considered to capture the nearly temperature-independent $\ensuremath{\kappa}$ of ${\mathrm{Cs}}_{2}{\mathrm{PbI}}_{2}{\mathrm{Cl}}_{2}$. We also find these unusual phonon behaviors in other layered inorganic perovskites such as ${\mathrm{Cs}}_{2}{\mathrm{PbBr}}_{2}{\mathrm{Cl}}_{2}$ and ${\mathrm{Cs}}_{2}{\mathrm{SnI}}_{2}{\mathrm{Cl}}_{2}$. Our results rationalize the crucial high-order phonon interactions and two-channel lattice thermal transports in perovskites with ultralow $\ensuremath{\kappa}$.