Excellent thermoelectric performance in alkali metal phosphides M3P (M = Na and K) with phonon-glass electron-crystal like behaviour

Abstract

Identifying ideal thermoelectric materials presents a formidable challenge due to the intricate coupling relationship between thermal conductivity and power factor. Here, based on first-principles calculations, along with self-consistent phonon theory and the Boltzmann transport equation, we theoretically investigate the thermoelectric properties of alkali metal phosphides M₃P (M = Na and K). The evident ‘avoided crossing’ phenomenon indicates the phonon glass behavior of M₃P (M = Na and K). Due to the strong lattice anharmonicity induced by alkali metal elements, accounting for quartic anharmonic corrections, the lattice thermal conductivities of Na₃P and K₃P at room temperature are merely 0.25 and 0.12 W m⁻¹ K⁻¹, respectively. Furthermore, the high degeneracy and ‘pudding-mold-type’ band structure lead to high p-type PF in M₃P (M = Na and K). At 300 K, the p-type power factors (PF) of Na₃P and K₃P can reach 3.90 and 0.80 mW mK⁻², respectively. The combination of ultralow κ_L and high PF leads to excellent thermoelectric figure of merit (ZT) values of 1.70 (3.38) and 1.18 (2.13) for p-type Na₃P and K₃P under optimal doping concentration at 300 K (500 K), respectively, surpassing traditional thermoelectric materials. These findings demonstrate that M₃P (M = Na and K) exhibits behavior similar to phonon-glass electron crystals, thereby indicating a direction for the search for high-performance thermoelectric materials.