Oxidation states and thermoelectric properties of BiCuSeO bulks fabricated under Bi or Se deficiencies in the nominal composition

We have fabricated the BiCuSeO bulks using raw materials with Bi or Se deficiencies in the nominal composition and investigated crystallographic, chemical compositional, and thermoelectric properties. Owing to the Bi or Se deficiencies in the starting composition, excessive elements and related compounds were deposited as impurity phases and the matrix phase is nearly the stoichiometric BiCuSeO phase. The electrical resistivity, ρ(T), of the bulks decreases and thermoelectric power, S(T), also decreases with increasing the contents of Bi or Se deficiencies in the starting composition in spite of the stoichiometric matrix phase. These results strongly suggest that, from the X-ray photoelectron spectroscopy measurements, the actual oxidation states of Bi and Cu deviate from the formal valences of stoichiometric Bi3+Cu1+Se2−O2−. The introduction of a small amount of Bi and Se vacancies is also suggested. As a result, mobile carriers are introduced and the ρ and S values are changed. The maximum thermoelectric dimensionless figure of merit of ZT = 0.60 was achieved at 773 K for the Bi1-xCuSeO samples (x = 0.025 and 0.05) in the starting composition. These results are in clear contrast with the reported results for the Cu deficiency bulks. Using these results, we propose charge valence equations and the origin of the carriers in the present BiCuSeO bulks and discuss the influence of created carriers on the thermoelectric properties.