Low-temperature phase transition and transport properties of β-Cu_2-xSe fabricated using hydrothermal synthesis and evacuating-and-encapsulating sintering

A series of phonon-liquid electron-crystal materials of β-Cu_2-xSe (0 ≤ x ≤ 0.04) is fabricated using hydrothermal synthesis followed by evacuating-and-encapsulating sintering. These materials are characterized using powder X-ray diffraction, field-emission scanning electron micrograph, electronic and thermal transport. Low-temperature X-ray diffraction patterns along with the electronic and thermal transport data reveal that β-Cu₂Se exhibit a phase transition at around 160 K. We find that all the electrical conductivity, carrier concentration and thermopower of β-Cu_2-xSe increase with copper deficiency content x. A knee behavior is observed in the temperature dependence of both electrical resistivity and thermopower on cooling the samples from room temperature to around 160 K, which could be associated with the structural transition at the corresponding temperature. The thermal conductivity of β-Cu_2-xSe at room temperature is found to be larger than that of α-Cu_2-xSe.