Low lattice thermal conductivities and good thermoelectric performance of hexagonal antiperovskites X(Ba & Sr)3BiN with quartic anharmonicity

Antiperovskites are a burgeoning class of semiconducting materials that showcase remarkable optoelectronic properties and catalytic properties. However, there has been limited research on their thermoelectric properties. Combining first-principles calculations, self-consistent phonon theory and the Boltzmann transport equation, we have discovered that the hexagonal antiperovskites X(Ba & Sr)3BiN exhibit strong quartic lattice anharmonicity, where the anharmonic vibrations of the light N atoms primarily affect the lattice thermal conductivity (κL) along the c-axis direction. As a result, the lattice thermal conductivities along the a(b)-axis direction are low. At 300 K, the κL values of Ba3BiN and Sr3BiN are only 1.27 W m-1 K-1 and 2.24 W m-1 K-1, respectively. Moreover, near the valence band maximum, the orbitals of the N atoms dominate. This dominance allows Sr3BiN to achieve high power factor under p-type doping, resulting in an impressive thermoelectric figure of merit (ZT) of 0.94 along the c-axis direction at 800 K. In the a(b)-axis direction, at 800 K, n-type doped Ba3BiN exhibits a ZT value of 1.47, surpassing that of traditional thermoelectric materials. Our research elucidates that the hexagonal antiperovskites X(Ba & Sr)3BiN represent a category of potential thermoelectric materials with pronounced anisotropy, low thermal conductivity, and high thermoelectric performance.