Remarkable thermoelectric performance in BaPdS2 via pudding-mold band structure, band convergence, and ultralow lattice thermal conductivity

Efficient thermoelectric materials require a rare and contraindicated combination of materials properties: large electrical conductivity, large Seebeck coefficient, and low thermal conductivity. One strategy to achieve the first two properties is via low-energy electronic bands containing both flat and dispersive parts in different regions of crystal momentum space, known as a pudding-mold band structure. Here, we illustrate that BaPdS2 successfully achieves the pudding-mold band structure for the valence band, contributing to a large p-type thermoelectric power factor, due to its anisotropic crystal structure containing zigzag chains of edge-sharing square planar PdS4 units; large power factor is achieved for n-type doping as well due to band convergence. In addition, BaPdS2 exhibits ultralow lattice thermal conductivity, and thus also achieves the third property, due to extremely soft and anharmonic interactions in its transverse acoustic phonon branch. We predict a remarkably large thermoelectric figure of merit, with peak values between 2 and 3 for two of the three crystallographic directions, suggesting BaPdS2 warrants experimental investigation.