Leveraging crystal symmetry for thermoelectric performance optimization in cubic GeSe

In thermoelectrics, the manipulation of crystal symmetry is instrumental in optimizing the electrical and thermal transport parameters. Within this context, the present study explored the largely overlooked high-symmetry cubic GeSe, which presented larger band degeneracy than its widely studied medium-symmetry rhombohedral counterpart. We have successfully stabilized cubic GeSe at ambient conditions through co-alloying with AgSnTe₂ and Bi. The incorporation of AgSnTe₂ initiates the transition of GeSe from a low-symmetry orthorhombic to a medium-symmetry rhombohedral phase, culminating in a high-symmetry cubic structure, underpinned by variation in chemical bonding mechanisms. Notwithstanding this, the persistence of Ag₂Te precipitates impedes the total elimination of the residual orthorhombic phase due to the disparate chemical bonding mechanism between Ag₂Te and GeSe. Introducing Bi into the rhombohedral-dominated (GeSe)_0.7(AgSnTe₂)_0.3 matrix leads to the dissolution of Ag₂Te precipitates, elimination of the residual orthorhombic phase, and the subsequent stabilization of the exclusive cubic phase. Compared to its orthorhombic counterpart, the cubic GeSe exhibits diminished bandgap and Ge vacancy formation energy, amplified band degeneracy, reduced sound velocity, intensified lattice anharmonicity and multiple phonon scattering centres, engendering elevated carrier concentration and density-of-states effective mass, alongside restrained lattice thermal conductivity. Consequently, a peak zT of 0.46 at 573 K is attained for cubic (Ge_0.7Bi_0.3Se)_0.7(AgSnTe₂)_0.3, signifying a ninefold increase relative to the initial orthorhombic GeSe. These results illuminate the critical role of crystal symmetry manipulation in advancing the thermoelectric performance.