Alloy-induced reduction and anisotropy change of lattice thermal conductivity in Ruddlesden– Popper phase halide perovskites

The effective modulation of the thermal conductivity of halide perovskites is of great importance in optimizing their optoelectronic device performance. Based on first-principles lattice dynamics calculations, we found that alloying at the B and X sites can significantly modulate the thermal transport properties of 2D Ruddlesden?Popper （RP） phase halide perovskites, achieving a range of lattice thermal conductivity values from the lowest （κc = 0.05 W·m?1·K?1@Cs4AgBiI8） to the highest （κa/b = 0.95 W·m?1·K?1@Cs4NaBiCl4I4）. Compared with the pure RP-phase halide perovskites and three-dimensional halide perovskite alloys, the two-dimensional halide perovskite introduces more phonon branches through alloying, resulting in stronger phonon branch coupling, which effectively scatters phonons and reduces thermal conductivity. Alloying can also dramatically regulate the thermal transport anisotropy of RP-phase halide perovskites, with the anisotropy ratio ranging from 1.22 to 4.13. Subsequently, analysis of the phonon transport modes in these structures revealed that the lower phonon velocity and shorter phonon lifetime were the main reasons for their low thermal conductivity. This work further reduces the lattice thermal conductivity of 2D pure RP-phase halide perovskites by alloying methods and provides a strong support for theoretical guidance by gaining insight into the interesting phonon transport phenomena in these compounds.