  The binary compound SnSe exhibits record high thermoelectric performance,
largely because of its very low thermal conductivity. The origin of the strong
phonon anharmonicity leading to the low thermal conductivity of SnSe is
investigated through first-principles calculations of the electronic structure
and phonons. It is shown that a Jahn-Teller instability of the electronic
structure is responsible for the high-temperature lattice distortion between
the Cmcm and Pnma phases. The coupling of phonon modes and the phase transition
mechanism are elucidated, emphasizing the connection with hybrid improper
ferroelectrics. This coupled instability of electronic orbitals and lattice
dynamics is the origin of the strong anharmonicity causing the ultralow thermal
conductivity in SnSe. Exploiting such bonding instabilities to generate strong
anharmonicity may provide a new rational to design efficient thermoelectric
materials.
