Pressure-driven crystal symmetry and carrier concentration optimization for superior thermoelectric transport properties in layered AgCrSe2

Phase engineering has proven to be an effective strategy for achieving superior thermoelectric performance, while pressure is an excellent means of expanding the phase space of a material. In this paper, the effect of pressure-induced phase transition on improving the crystal symmetry and enhancing the thermoelectric properties of AgCrSe2 under high pressure and high temperature are reported. A structural phase transition from the low-symmetry R3m phase to the high-symmetry P3̄m1 phase is discovered below 1 GPa, which increases band degeneracy and contributes to a high electrical conductivity. For the metallic P3̄m1 phase, the electrons surrounding the Se2− anion gradually transfer to the Ag+ and Cr3+ cations as the pressure increases, decreasing the density of states around the Fermi level and thus optimizing the carrier concentration, thereby increasing the Seebeck coefficient while maintaining a high electrical conductivity. Consequently, an ultrahigh power factor of 864 μW·m−1·K−2 is achieved at 5 GPa and 297 K. This study provides new insights into improving thermoelectric transport properties by applying physical pressure to enhance crystal symmetry and optimize thermoelectric parameters, and also indicates that phase engineering is a compelling strategy to discover or design novel high-performance thermoelectric materials starting from low-symmetry compounds.