Bonding Hierarchy Gives Rise to High Thermoelectric Performance in Layered Zintl Compound BaAu₂P₄

The search for new thermoelectric materials has gained rapid progress in recent years as thermoelectric technology offers the potential for environmentally friendly and sustainable energy conversion methods from waste heat to electricity. In this work, we use first-principles calculations based on density functional theory to predict high thermoelectric performance in BaAu₂P₄, a layered Zintl compound with a small band gap. BaAu₂P₄ exhibits crystallographic heterogeneity in which rigid [Au₂P₄]^2- units are separated by layers of Ba²⁺ cations, which are bonded relatively weakly to the lattice through electrostatic interactions. The phosphorus atoms are covalently bonded to each other and form infinite chains within the crystal. While the phosphorus chains facilitate large electrical conductivity, the presence of multiple bands near the Fermi level gives rise to an enhanced Seebeck coefficient. On the other hand, the loosely bound Ba along with Au strongly scatter the heat carrying acoustic phonons, significantly reducing the lattice thermal conduction along the stacking direction. As a consequence of this bonding hierarchy (i.e., coexisting rigid and fluctuating sublattices), BaAu₂P₄ exhibits a large power factor and low lattice thermal conductivity, which results in a high thermoelectric figure of merit (zT). Thus, our findings should encourage the exploration of new thermoelectric materials in the family of layered compounds with small band gaps and crystallographic heterogeneity.