Anharmonicity and Ultra-Low Thermal Conductivity in Lead-Free Halide Double Perovskites

The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to the presence of toxic Pb and long-term stability of the very intensively studied class of single lead halide single perovskites.

The single lead halide perovskites are well known to be very anharmonic materials. In particular, the atomistic dynamics relating to distortions and tilting of octahedral units and movement of the A site is highly complex. Anharmonicity in the lead-free halide double perovskites are comparatively much less studied.

In this talk I present our first-principles study of the lattice vibrations in Cs2AgBiBr6, the prototypical lead-free halide double perovskite, and show that it is very soft and highly anharmonic, in particular in regards to tilting of AgBr₆ and BiBr₆ octahedra and for motion along the Ag-Br-Bi bond. Using an energy and temperature dependent phonon spectral function derived from ab inito molecular dynamics we then show how the experimentally observed cubic-to-tetragonal phase transformation can be related to the collapse of a soft phonon branch. We finally reveal that the softness and anharmonic features of Cs2AgBiBr6 yield an ultra-low thermal conductivity with a non standard ~T^-0.5 temperature dependence.