Isotope effect and cation disorder in manganites

The measurements of temperature dependence of electrical resistivity ρ(T)ρ(T) and magnetic susceptibility χ(T)χ(T) as well as neutron diffraction studies were performed for three groups of R1-xSrxMnO3R1-xSrxMnO3 manganites. Each group was characterized by the same average ionic radius 〈rA〉〈rA〉 of rare earth R, but by different degree of cation disorder σ2=xiri2-〈rA〉2. For each composition, the isotope-substituted samples (O16→O18) were also prepared. It was shown that the increase in σ2σ2 at fixed 〈rA〉〈rA〉 leads to appreciable changes in electrical and magnetic properties, as well as to a more pronounced isotope effect. Large values of σ2σ2 give rise to a significant scatter in the values of the electron hopping integral for neighboring Mn sites. This favors the electron localization and the tendency to antiferromagnetism in the phase-separated state, thus enhancing the effect of oxygen isotope substitution.