Theoretical insights into lattice dynamics and thermal transport properties of lead-free quadruple halide per-ovskite Cs4CuSb2Cl12

Abstract

In spite of extensive studies of halide perovskites, little attention has been paid to lattice dynamics and thermal transport properties of halide perovskite Cs4CuSb2Cl12 so far. In this work, we systematically investigate structural, mechanical, lattice dynamics and thermal transport properties for lead-free quadruple halide perovskite Cs4CuSb2Cl12 in monoclinic phase using state-ofthe-art first-principles methods. Scrutiny of crystal structure reveals that Cs atoms are located inside over-sized cage-like structure CsCl12, while Cu (Sb) atoms are tightly filled into octahedral Cu(Sb)Cl6 , expecting a role of atomic rattler for the Cs atoms. We carry out anharmonic lattice dynamics calculations at finite temperature based on self-consistent phonon theory, finding that monoclinic Cs4CuSb2Cl12 is dynamically stable at elevated temperature. Moreover, we find that the Cs atomic rattlers result in lattice anharmonicity and severely scatter heat-carrying acoustic and low-energy optical phonons, consequently leading to low phonon group velocity and extremely short phonon lifetime. We then calculate temperature-dependent lattice thermal conductivity κl of monoclinic Cs4CuSb2Cl12 using unified theory of thermal transport for both crystals and glasses, demonstrating an extremely low κl of 0.27, 0.36 and 0.14 W/m·K at 300 K along x-, yand z-axis. Moreover, it is found that nano-structuring can further suppress the κl by half. Strong lattice anharmonicity is again confirmed from Grüneisen parameter calculation and temperature dependence of κl ∝ T −0.72. In addition, elastic constant calculation demonstrates that monoclinic Cs4CuSb2Cl12 is mechanically stable with a brittle nature. Our work highlights theoretical insights into lattice dynamics and thermal transport properties of monoclinic Cs4CuSb2Cl12.