Thermoelectric and thermal properties of the superionic AgxCu2-xSe (x=0.01, 0.02, 0.03, 0.04, 0.25) compounds

The results of investigations of the thermoelectric and thermal properties of AgxCu2-xSe (x = 0.01, 0.02, 0.03, 0.04, 0.25) alloys are presented in the paper. The AgxCu2-xSe samples with low silver content were prepared by solid state reaction of the pure elements in argon atmosphere at 773 K temperature. The Ag0.25Cu1.75Se sample was sintered by solid phase reaction of Cu2Se and Ag2Se mixture in argon atmosphere at 673 K. At room temperature X-ray diffraction study revealed the presence of three phases in the samples: the Cu2Se orthorhombic phase, the Cu1.8Se cubic phase and the AgCuSe orthorhombic phase. The heat of the superionic phase transition in Ag0.01Cu1.99Se was measured equal to (3.5 ± 0.3) kJ / mol. For Ag0.25Cu1.75Se sample the heat of the superionic phase transition was found to be (3.1 ± 0.3) kJ / mol. In addition to intense peak of the superionic phase transition occupying the 373-423 K temperature range, the weak thermal effects for Ag0.01Cu1.99Se at 317 K, and ones for Ag0.25Cu1.75Se at 316 K and 437 K were observed too. In the investigated temperature range of 290 - 770 K the electronic conductivity σ decreases, and Zeebeck coefficient α increases with silver content in the compounds. The thermal conductivity of Ag0.03Cu1.97Se compound decreases monotonically from 0.54 to 0.34 W m-1 K-1 in the range 420 - 650 K after the superionic phase transition, resulting in thermoelectric efficiency ZT = σα2T/λ increases monotonically, reaching a value ZT = 1 at 650 K.