Off-stoichiometry effects on the thermoelectric properties of Cu2+δSe (-0.1 ≤ δ ≤ 0.05) compounds synthesized by a high-pressure and high-temperature method

The chemical composition can directly tune the transport properties of Cu2Se liquid-like materials, including the carrier concentration, carrier mobility and superionic feature. In this work, the offstoichiometry effects of both the deficiency and excess of Cu on the phase transition and thermoelectric properties of Cu2+delta Se (-0.1 <= delta <= 0.05) compounds are investigated. The Cu2+delta Se samples were synthesized by a high-pressure and high-temperature (HPHT) technique in 30 minutes. Cu2+delta Se (-0.1 = delta < 0) existed as a mixture of the monoclinic alpha-Cu2Se phase and cubic beta Cu2Se phase at room temperature, while only the monoclinic a-Cu2Se phase was detected in the Cu2+delta Se (0 <= delta <= 0.05) compounds. Increasing the Cu content can effectively regulate the carrier concentration and reduce the thermal conductivity from 1.45 W m(-1) K-1 (delta = -0.1) to 0.47 W m(-1) K-1 (delta = 0.05) at 843 K. The ultralow lattice thermal conductivity of 0.3 W m(-1) K-1 was achieved for Cu1.97Se and Cu2.0Se by stoichiometry tuning. A maximum zT value of 1.46 at 843 K was obtained for Cu2.0Se, which is 100% higher than that of Cu1.9Se. This work demonstrates the potential of off-stoichiometry for tuning the TE properties of Ag+ and Cu+ based superionic materials.