Observation of Extraordinary Vibration Scatterings Induced by Strong Anharmonicity in Lead-Free Halide Double Perovskites

Lead-free halide double perovskites provide a promising solution for the long-standing issues of lead-containing halide perovskites, i.e., the toxicity of Pb and the low stability under ambient conditions and high-intensity illumination. Their light-to-electricity or thermal-to-electricity conversion is strongly determined by the dynamics of the corresponding lattice vibrations. Here, the measurement of lattice dynamics is presented in a prototypical lead-free halide double perovskite(Cs₂NaInCl₆). The quantitative measurements and first-principles calculations show that the scatterings among lattice vibrations at room temperature are at the timescale of ≈1 ps, which stems from the extraordinarily strong anharmonicity in Cs₂NaInCl₆. Further the degree of anharmonicity of each type of atom is quantitatively characterized in the Cs₂NaInCl₆ single crystal, which stems from the interatomic forces, and demonstrate that this strong anharmonicity is synergistically contributed by the bond hierarchy, the tilting of the NaCl₆ and InCl₆ octahedral units, and the rattling of Cs⁺ ions. Consequently, the crystalline Cs₂NaInCl₆ possesses an ultralow thermal conductivity of ≈0.43 W mK^-1 at room temperature, and a weak temperature dependence of T ^-0.41. These findings uncovered the underlying mechanisms behind the dynamics of lattice vibrations in double perovskites, which can largely benefit the design of optoelectronics and thermoelectrics based on halide double perovskites.