Thermoelectric properties and phase transition of doped single crystals and polycrystals of Bi2Te3.

The temperature dependences of the electrical conductivity σ(T), Seebeck coefficient σ(T), and heat capacity Cp(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 (ρ(T)=1/σ(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 ρ⊥(T)/ρ‖(T):10 (where ρ‖(T) is the resistivity along the crystal growth axis, and ρ⊥(T) is resistivity across the crystal growth axis) and temperature dependence ρ(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 ∼1.1 is observed for the Bi2Te3+1%(CuI+1/2Pb) sample at room temperature (T=300K). [ABSTRACT FROM AUTHOR]