Rockseline lead chalcogenides: An alternative class of high-performance thermoelectric materials

Lead chalcogenides crystallizing in NaCl-type structure are known as excellent thermoelectric materials. Yet, they have suffered the long-standing issue of performance imbalance between n-type and p-type thermoelectric devices. In this work, we have explored a highly stable low-energy allotrope of those lead chalcogenides, namely rockseline phase. This phase is energetically close to the stable NaCl-type phase with a hull distance of only 50 meV/atom. It can be potentially realized by applying elevated pressure on an existing experimental phase. Compared with the NaCl-type structure containing solely octahedron units with an isotropic bonding network, the rockseline structure has alternatively stacked octahedron and triangular prism units, thus exhibiting the appealing feature of hierarchical bonds. This causes the lattice thermal conductivity in the rockseline structure being few times smaller than its NaCl-type counterpart, thanks to the softened phonon modes and concurrently enhanced anharmonicity. In addition, the rockseline structure favors superior electrical conductivity at n-type doping and an appealing valence band convergence feature, being beneficial for the pronounced Seebeck coefficient at p-type doping, causing balanced thermoelectric performances under different doping conditions. As an outstanding representative of the rockseline family, rockseline PbTe exhibits an ultralow lattice thermal conductivity of 0.8 W/(m·K) at room temperature and an excellent yet balanced figure of merit approaching 2 at both n-type and p-type doping and high temperatures. This work highlights the important role of crystal structure engineering in the design of thermoelectric materials with superior thermoelectric performances.