Oxygen deficiency and electrical conductivity of Nd_1-xA_xNiO_3-y (A = alkaline earth) prepared by the low-temperature process

The structure and electrical conductivity of oxygen-deficient Nd_1-xA_xNiO_3-y (A = alkaline earth) have been investigated. The samples were prepared from aqueous solutions of metal acetates and EDTA by the low-temperature process. Single-phase NdNiO_3-y and alkaline earth-substituted Nd_0.95A_0.05NiO_3-y (A = Mg, Ca and Sr) were obtained by heating at 700 °C for 20 h in flowing oxygen. All the samples had large numbers of oxygen vacancies with y = 0.25-0.28, resulting in the mixed valence state of Ni²⁺/Ni³⁺, as confirmed by iodometric titration and XPS measurements. The lattice constants of the alkaline earth-substituted samples were larger than those of the non-substituted sample. These results were attributed to the difference in ionic radius. The substitution of alkaline earths for Nd decreased the decomposition temperature. All the samples showed resistivity on the order of 10^-1 Ω cm at room temperature, and the temperature dependence of the electrical resistance revealed the transition from metallic to semiconducting at lower temperatures. The transition temperature was dependent on the substituted alkaline earths. This behavior is discussed in terms of the changes in lattice constants as well as in the valence of Ni resulting from the oxygen deficiency.