Cation disorder and size effects in magnetoresistive manganese oxide perovskites

Large disorder effects due to size differences between $A$-site $R^{3+}$ ($R=\mathrm{L}\mathrm{a},\mathrm{P}\mathrm{r},\mathrm{N}\mathrm{d},\mathrm{S}\mathrm{m}$) and $M^{2+} (M=\mathrm{C}\mathrm{a},\mathrm{S}\mathrm{r},\mathrm{B}\mathrm{a})$ cations have been found in magnetoresistive $\left(R_{0.7}{M}_{0.3}\right)\mathrm{Mn}{\mathrm{O}}_3$ perovskites. The ferromagnetic-metal-paramagnetic-insulator transition temperature $T_m$ varies as $T_m=T_m\mathrm{}\left(0\right)-p{Q}^2$ due to strain fields resulting from ordered or disordered oxygen displacements $Q$ that are parametrized by the statistical mean and variance of the $A$ cation radius, respectively. The value of $p$ is related to the Mn-O force constant showing that ${\mathrm{Mn}}^{3+}$ Jahn-Teller distortions assist electron localization at $T_m$. The maximum possible $T_m$ is estimated to be ∼530 K although experimentally observable values are ≤360 K. A large suppression of magnetoresistance due to cation disorder is also evidenced.