Seebeck Coefficients of Layered BiCuSeO Phases: Analysis of Their Hole-Density Dependence and Quantum Confinement Effect

Hole-doped layered BiCuSeO phases include substitutionally doped Bi 1– x A x CuSeO (A = alkali, alkaline earth) as well as vacancy-doped Bi 1−δ Cu 1-γ SeO and Bi 1−δ CuSeO. To probe how their Seebeck coefficients are related to their hole density p, we calculated the Seebeck coefficient for defect-free BiCuSeO as a function of the hole density, which is generated by lowering the Fermi level from the valence band maximum (VBM). In addition, we calculated the Seebeck coefficient for Bi 1−δ CuSeO (δ = 1/32, 1/16) with a large number of Bi vacancies. The Seebeck coefficients of the hole-doped BiCuSeO phases are governed by the electronic states lying within ∼0.5 eV from the VBM. These states are composed of largely Cu 3d xz/yz and Se 4p x/y states and possess the character of a uniform one-dimensional (1D) chain rather than a uniform two-dimensional (2D) lattice expected for a layered phase. The observed S -vs- p relationship for Bi 1– x A x CuSeO (A = alkali, alkaline earth) as well as Bi 1−δ Cu 1-γ SeO (δ = 0, 0.025; γ = 0, 0.025) and Bi 1−δ CuSeO (δ = 0.0, 0.025) is very well reproduced by the calculated relationship for defect-free BiCuSeO within the rigid band approximation. The observed S -vs- p relationship reflects the quantum confinement effect of uniform 1D chains, despite that the hole-doped BiCuSeO phases consist of 2D layers, (Cu 2 Se 2 ) 2– and (Bi 2 O 2 ) 2+ . The drastic decrease in the S values of Bi 1−δ CuSeO with large δ (= 0.05, 0.075, 0.10) arises from the loss of the quantum confinement effect in the (Cu 2 Se 2 ) 2– layers; that is, the uniform 1D chain character is lost because of their geometry distortion induced by a large number of Bi vacancies.