Metavalent Bonding in Cubic SnSe Alloys Improves Thermoelectric Properties over a Broad Temperature Range

Monocrystalline SnSe is one of the most promising thermoelectric materials with outstanding performance and a high abundance of constituting elements. However, polycrystalline SnSe, which is more robust for applications, only shows large figure-of-merit (zT) values in its high-symmetry phase. Stabilizing the high-symmetry phase at low temperatures can thus enhance the average zT value over a broad temperature range. In this work, the high-symmetry rock-salt SnSe phase is successfully obtained by alloying SnSe with AgVVI2 compounds (V = Sb, Bi; VI = Se, Te). These cubic SnSe phases show a unique portfolio of properties including a high optical dielectric constant, a large maximum of optical absorption, a large Born effective charge, and abnormal bond-breaking behavior in laser-assisted atom probe tomography. All of these characteristics are indicative of metavalent bonding. In contrast, the Pnma phase of SnSe employs covalent bonding. The enhanced symmetry at low temperatures is realized by tailoring chemical bonding. Concomitantly, zT near room temperature is increased by a factor of more than 10 from the pristine Pnma SnSe to Fm3 over bar ${{\bar{3}}}$m SnSe alloys. This provides insights into the enhancement of the thermoelectric performance of SnSe and other chalcogenides over a broad temperature range by manipulating the chemical bonds. The phase transition of SnSe from Pnma to Fm-3m is realized by tailoring the chemical bonds. All the dopants that drive the phase transition prevail in a well-defined region of a chemical bonding map. While the Pnma SnSe utilizes covalent bonding, the Fm-3m SnSe employs metavalent bonding. This metavalent bonding results in superior thermoelectric performance over a wide temperature range. image