Crystal structure manipulation to achieve better thermoelectric performance in Te-substituted GeSe

GeSe has recently gained attention for its structural similarity to SnSe, an excellent thermoelectric material. However, for the orthorhombic GeSe, the maximum zT is limited to ∼0.2 at 700 K. A significant improvement in the thermoelectric performance is observed when GeSe is stabilized in a rhombohedral or cubic structure; thus, the crystal structure plays an important role in GeSe for improved zT. In this study, we investigated the structural transitions and thermoelectric properties of Te-substituted GeSe. Increasing Te substitution in GeSe1-xTex (x = 0.00–0.50) induces a transition from orthorhombic to rhombohedral crystal structure at ambient conditions with the maximum zT ∼ 0.58 observed in rhombohedral GeSe0.6Te0.4 at 573 K. The improved thermoelectric performance in the rhombohedral phase is due to a concurrent increase in the power factor and a decrease in lattice thermal conductivity. The phonon dispersion calculation tells that the high-frequency optical phonon modes significantly increase the phonon–phonon scattering for the rhombohedral phase, enhancing the lattice anharmonicity and reducing the lattice thermal conductivity. This behavior aligns with the presence of metavalent bonding in rhombohedral GeSe. Additionally, peak broadening observed in the Raman spectra of the rhombohedral phase indicates pronounced lattice anharmonicity and phonon modes softening due to the metavalent bond character.