Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

Abstract

Thermoelectric materials have attracted significant research interest in recent decades due to their promising application potential in interconverting heat and electricity. Unfortunately, the strong coupling between the material parameters that determine thermoelectric efficiency, i.e., the Seebeck coefficient, electrical conductivity, and thermal conductivity, complicates the optimization of thermoelectric energy converters. Main‐group chalcogenides provide a rich playground to alleviate the interdependence of these parameters. Interestingly, only a subgroup of octahedrally coordinated chalcogenides possesses good thermoelectric properties. This subgroup is also characterized by other outstanding characteristics suggestive of an exceptional bonding mechanism, which has been coined metavalent bonding. This conclusion is further supported by a map that separates different bonding mechanisms. In this map, all octahedrally coordinated chalcogenides with good performance as thermoelectrics are located in a well‐defined region, implying that the map can be utilized to identify novel thermoelectrics. To unravel the correlation between chemical bonding mechanism and good thermoelectric properties, the consequences of this unusual bonding mechanism on the band structure are analyzed. It is shown that features such as band degeneracy and band anisotropy are typical for this bonding mechanism, as is the low lattice thermal conductivity. This fundamental understanding, in turn, guides the rational materials design for improved thermoelectric performance by tailoring the chemical bonding mechanism.

Employing Interfaces with Metavalently Bonded Materials for Phonon Scattering and Control of the Thermal Conductivity in TAGS‐x Thermoelectric Materials

“…The high PME value observed systematically for chalcogenide thermoelectric materials is attributed to MVB. [ 8 ] The existence of MVB in TAGS is further verified by the optical dielectric constant ε ∞ given in Figure . One of the established property‐based fingerprints for MVB is the optical dielectric constant ε ∞ , which is an optical identifier and can be measured by Fourier‐transform infrared spectroscopy (FTIR).…”

“…These results are in‐line with recent work where it had been proven that main‐group chalcogenide thermoelectric materials exhibit systematically a high PME of ≥ 60%. [ 8,29 ]…”

“…Yet, many chalcogenides exhibit a significantly larger transverse optical Grüneisen parameter γ TO , which has been associated with lone pairs first [ 6 ] and subsequently with metavalent bonding (MVB). [ 7,8 ] Further scattering channels for the phonons can be added by introducing lattice defects such as point defects, dislocations, and interfaces generated by grain boundaries or precipitates. [ 5,9–11 ]…”

“…The work by Lee et al [ 7 ] has discussed the lattice thermal conductivity of metavalently bonded materials. [ 24,25 ] Materials with MVB show inherently a low thermal conductivity [ 8,24 ] because of the pronounced anharmonicity of the bonds and the small bond stiffness, which in turn leads to a large Grüneisen parameter and low phonon group velocity, respectively, both resulting in a low thermal conductivity.…”

Cu Intercalation and Br Doping to Thermoelectric SnSe2 Lead to Ultrahigh Electron Mobility and Temperature‐Independent Power Factor

“…The former directly depends on a figure of merit ZT = S 2 σT/κ tot , where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ tot is the total thermal conductivity from the electrical (κ ele ) and lattice vibration contribution (κ lat ). [10][11][12] When materials are subject to a large temperature difference between the hot and cold ends, engineering ZT (ZT eng ) may better predict their efficiency by considering a cumulative temperature dependence of S, σ, and κ tot defined by Equation (1)…”