Ce substitution and reduction annealing effects on electronic states in Pr_2−xCe_xCuO₄ studied by Cu K-edge X-ray absorption spectroscopy

We investigated the Ce substitution and reduction annealing effects on the electronic states at copper sites by Cu K-edge X-ray absorption near-edge structure measurements in Pr₂−_xCe_xCuO₄₊α−_δ (PCCO) with varying x and δ (amount of oxygen loss during annealing). Absorption near-edge spectra were modified by Ce substitution and reduction annealing similarly with increasing x and δ. Considering electron doping by Ce substitution, this similarity indicates an increase in electron number at the copper sites due to annealing (n_AN). Thus, the total number of electrons is determined by the amount of Ce and O ions. Furthermore, quantitative analyses of the spectra clarified that the number of Cu⁺ sites, corresponding to the induced electron number by Ce substitution (n_Ce) increases linearly with x in the as-sintered PCCO (δ = 0), whereas n_AN is not exactly equal to 2δ, which is expected from charge neutrality. For each x-fixed sample, n_AN tends to exceed 2δ with increasing δ, suggesting the emergence of two types of carrier due to annealing.