Optimizing the thermoelectric performance of the earth-abundant layered phosphide BaCuP through defect engineering.

Thermoelectric materials offer a direct route for converting waste heat into electricity, but the discovery of efficient, sustainable, and non-toxic compounds remains a major challenge. Here, we investigate the thermoelectric potential of the layered 1-1-1 phosphide BaCuP using a combination of hybrid density functional theory, defect calculations, Boltzmann carrier transport theory, and machine-learning assisted lattice thermal conductivity simulations. The electronic structure reveals a small indirect band gap of 0.19 eV and strongly anisotropic valence-band dispersion, arising from Cu-P derived states, which favours in-plane hole transport. Defect calculations show that BaCuP is intrinsically strongly p-type due to the very low formation energy of copper vacancies under Cu-poor conditions, yielding high hole concentrations. We further assess K and La doping as routes to tune the carrier concentration, and find that La doping is particularly promising because it slightly reduces the intrinsic hole concentration toward the optimum range for thermoelectric performance. To accurately describe phonon transport, we train a machine-learning interatomic potential and use homogeneous non-equilibrium molecular dynamics, which captures higher-order anharmonic effects beyond conventional third-order approaches. BaCuP is predicted to exhibit a lower lattice thermal conductivity relative to CaCuP due to the heavier Ba. Combining the electronic and thermal transport properties, we predict a peak directionally averaged figure of merit zT of about 0.69 at 800 K in La-doped BaCuP, with in-plane values approaching 1 but hindered by the out-of-plane direction. We further show that nanostructuring does not improve the figure of merit in this material, because reducing the mean free path suppresses the carrier transport more strongly than the lattice thermal conductivity. BaCuP nonetheless stands out as a promising phosphide thermoelectric.