Origins of three-dimensional charge and two-dimensional phonon transports in Pnma phase PbSnSe2 thermoelectric crystal

Recently, PbSnSe2 alloy was found to exhibit a large hysteresis effect on transport properties, demonstrating its significant potential for thermoelectric applications. Using ab initio approaches, we studied the carrier transport properties of PbSnSe2 crystal, which is a special case of the alloy with the shortest-range order. A peak power factor of 134.2 mu W cm(-1) K-2 was found along the cross-plane direction in the n-type PbSnSe2 at a doping concentration of 7 x 10(20) cm(-3) at 700 K. This high power factor originates from delocalized p electrons between intra-plane Pb-Se pairs and between cross-plane Sn-Se pairs that can build up transport channels for conducting electrons, leading to a high electrical conductivity of 5.9 x 10(5) S m(-1). Introducing Pb atoms into Pnma phase SnSe can decrease the phonon group velocities and enhance the phonon-phonon scatterings, leading to a low thermal conductivity of 0.53 W m(-1) K-1 at 700 K along the cross-plane direction. The calculated peak ZT of similar to 3 along the cross-plane direction at an n-type doping concentration of around 5 x 10(19) cm(-3), which represents a theoretical upper limit for an idealized PbSnSe2 crystal. This work interprets the origins of three-dimensional charge and two-dimensional phonon transport behavior in PbSnSe2 and demonstrates that such crystals are promising high-performance thermoelectric semiconductors.