Influence of Bi Substitution with Rare-Earth Elements on the Transport Properties of BiCuSeO Oxyselenides

In this study, we demonstrate that introduction of rare-earth elements, R = La or Pr, into the Bi–O charge reservoir layer of BiCuSeO leads to an increase of both the charge carrier concentration and the effective mass. Although the charge carrier mobility slightly decreases upon Bi³⁺ to R³⁺ substitution, the electronic transport properties are significantly improved in a broad temperature range from 100 to 800 K. In particular, the electrical resistivity decreases by 2 times, while the Seebeck coefficient drops from 323 to 238 μV K^–1 at 800 K. Thus, a power factor of nearly 3 μW cm^–1 K^–2 is achieved for Bi_0.92R_0.08CuSeO samples at 800 K. Meanwhile, a noticeable decrease of the lattice thermal conductivity is observed for the substituted samples, which can be attributed to the enhanced point defect scattering mostly originating from atomic mass fluctuations between R and Bi. Ultimately, a maximum zT value of nearly 0.34 at 800 K is obtained for the Bi_0.92La_0.08CuSeO sample, which is ∼30% higher than that of pristine BiCuSeO.