Thermoelectric Properties And Phase Transition Of Doped Single Crystals And Polycrystals Of Bi2te3

The temperature dependences of the electrical conductivity sigma(T), Seebeck coefficient sigma(T), and heat capacity C-p(T) of polycrystalline samples of Bi2Te3, Bi2Te3+1%CuI, and Bi2Te3+1%(CuI+1/2Pb) are investigated in the temperature range below room temperature. Based on the temperature dependences of all investigated physical properties, it is discovered that phase transition occurs at 120-200 K. Investigation of single crystals shows that anomalies in the electrical resistivity (rho(T)=1/sigma(T)) occur only across the crystal growth axis (across the well-conducting Bi-Te plane). Investigation of the low-temperature dependence of electrical conductivity shows that all polycrystalline samples exhibit quasi-two-dimensional electron transport. Additionally, quasi-two-dimensional transport is detected in single crystals based on anisotropy analysis rho perpendicular to(T)/rho||(T):10 (where rho||(T) is the resistivity along the crystal growth axis, and rho perpendicular to(T) is resistivity across the crystal growth axis) and temperature dependence rho(T):T2 below 50 K. The Fermi energy EF is estimated using the temperature dependence of S(T). It is discovered that an increase in EF at T > 200 K is associated with the phase transition. For single-crystal samples, the maximum thermoelectric figure of merit ZT, as observed along the crystal growth axis, increases with doping. A maximum ZT value of similar to 1.1 is observed for the Bi2Te3+1%(CuI+1/2Pb) sample at room temperature (T=300K).