Structural stabilities and electrical properties of Ba₈Ga_16-xCu_xSn₃₀ single crystals under high temperatures

Single crystalline samples of type-I and type-VIII Ba₈Ga_16-xCu_xSn₃₀ (x = 0, 1) clathrates are prepared by the Sn-flux method. Effects of Cu-doping on stability and electrical properties of Ba₈Ga₁₆Sn₃₀ single crystal are explored by first-principle and experiment. All samples are heated to different high temperatures and maintained at these temperatures for 120 min and then cooled to room temperature to explore their structural stabilities. Results from DTA and powder x-ray diffraction analysis indicate that type-I Ba₈Ga₁₆Sn₃₀ structure is transformed into type-VIII phase after the sample has been heated to 185℃. Type-VIII BGS is stable during heating and cooling, but type-VIII Ba₈Ga₁₅CuSn₃₀ decomposes into Sn and Ba(Ga/Sn)₄ during cooling. Meanwhile, the electrical properties of type-I samples are measured, their electrical conductivities are enhanced, and the Seebeck efficient is reduced with Cu substitution. The type-I samples after phase transformations show the electrical characteristics of type-VIII samples.