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

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 MP (M = Na and K). The evident 'avoided crossing' phenomenon indicates the phonon glass behavior of MP (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 NaP and KP at room temperature are merely 0.25 and 0.12 W m K, respectively. Furthermore, the high degeneracy and 'pudding-mold-type' band structure lead to high p-type PF in MP (M = Na and K). At 300 K, the p-type power factors (PF) of NaP and KP can reach 3.90 and 0.80 mW mK, respectively. The combination of ultralow and high PF leads to excellent thermoelectric figure of merit () values of 1.70 (3.38) and 1.18 (2.13) for p-type NaP and KP under optimal doping concentration at 300 K (500 K), respectively, surpassing traditional thermoelectric materials. These findings demonstrate that MP (M = Na and K) exhibits behavior similar to phonon-glass electron crystals, thereby indicating a direction for the search for high-performance thermoelectric materials.