Stabilizing n‐Type Cubic GeSe by Entropy‐Driven Alloying of AgBiSe₂: Ultralow Thermal Conductivity and Promising Thermoelectric Performance

Abstract

The realization of n-type Ge chalcogenides is elusive owing to intrinsic Ge vacancies that make them p-type semiconductors. GeSe crystallizes into a layered orthorhombic structure similar to SnSe at ambient conditions. The high-symmetry cubic phase of GeSe is predicted to be stabilized by applying 7 GPa external pressure or by enhancing the entropy by increasing to temperature to 920 K. Stabilization of the n-type cubic phase of GeSe at ambient conditions was achieved by alloying with AgBiSe₂ (30-50 mol %), enhancing the entropy through solid solution mixing. The interplay of positive and negative chemical pressure anomalously changes the band gap of GeSe with increasing the AgBiSe₂ concentration. The band gap of n-type cubic (GeSe)_1-x (AgBiSe₂ )_x (0.30≤x≤0.50) has a value in the 0.3-0.4 eV range, which is significantly lower than orthorhombic GeSe (1.1 eV). Cubic (GeSe)_1-x (AgBiSe₂ )_x exhibits an ultralow lattice thermal conductivity (κ_L ≈0.43 W m^-1 K^-1 ) in the 300-723 K range. The low κ_L is attributed to significant phonon scattering by entropy-driven enhanced solid-solution point defects.