Crystallochemical characterization of Ce/RE substituted phases of T′ structure type: Quantitative analysis of Cu valence (V) and its content (1 − y) in superconducting sample of nominal composition Nd1.95−xCexCuO4 (x = 0.15)

► A hypothesis is post on deficient nature of superconducting R2−xCexCuO4 phases. ► Cu evaporation caused deficiency in R2−xCexCuO4 phases as determinant of their superconductivity. ► The restricted solubility of Ce in 1 N HCl and its effect on valence of Cu ions in Nd2−xCexCuO4 phase. ► Oxygen reduction of Nd2−xCexCuO4 phase does not lead to a decrease of Cu valence to the level V = (2 − x). ► Evidence on absence of Cu ion electron doping in superconducting Nd1.95−xCexCu1−yO4−Δ (x = 0.15).