Atomic Off-Centering Driven Phonon-Glass Electron-Crystal-like Thermoelectric Transport in Entropy-Stabilized Quinary Telluride

Entropy engineering offers innovative design opportunities for synthesizing new thermoelectric materials by integrating conflicting physical parameters. Optimization of configurational entropy holds the potential to simultaneously reduce the thermal conductivity through inherent disorder and enhance the Seebeck coefficient by symmetrizing the crystal lattice, both of which are crucial to augmenting the thermoelectric performance of a crystalline solid. Here, we synthesized an entropy-stabilized quinary metal telluride single crystal, AgGeSnSbTe₄, exhibiting an intriguing phonon-glass electron-crystal (PGEC)-like thermoelectric transport. Synchrotron X-ray pair distribution function (X-PDF) analysis infers that entropy-driven stabilization generates a highly symmetric rock-salt average structure but is accompanied by cation distortion in the local structure, which further enhances with temperature, reminiscent of emphanisis. Local lattice distortion-induced anharmonicity with considerable atomic disorder leads to glass-like lattice thermal conductivity, where the phonon mean free path approaches the interatomic distance. Phonon dispersion analysis corroborates the presence of local symmetry breaking, primarily driven by the off-centering displacement of Ge atoms due to the stereochemical expression of the 4s² lone pair, which results in local ferroelectric lattice instability. Notably, the glassy thermal conductivity is complemented by good electrical conductivity and a high Seebeck coefficient, enabled through long-range atomic order within the average cubic framework. The realization of the PGEC paradigm results in a promising thermoelectric figure-of-merit (zT) of ∼1.2 at 670 K in the Bridgman-grown AgGeSnSbTe₄ crystal.