Linking local structure and properties in perovskites containing equal concentrations of manganese and ruthenium

Abstract

The local structures of six perovskite compounds containing equal amounts of manganese and ruthenium on the $B$-site have been investigated by neutron and x-ray pair distribution function analysis. The compounds SrMn${}_{0.5}$Ru${}_{0.5}$O${}_{3}$, Sr${}_{0.5}$Ca${}_{0.5}$Mn${}_{0.5}$Ru${}_{0.5}$O${}_{3}$, and CaMn${}_{0.5}$Ru${}_{0.5}$O${}_{3}$ were studied to investigate the effects of pure chemical pressure on the local structure and valency ratio between Mn${}^{3+}$/Ru${}^{5+}$ and Mn${}^{4+}$/Ru${}^{4+}$. Reverse Monte Carlo simulations confirm that there is a shift in the $B$-site cation charge distribution from nearly equal amounts of Mn${}^{3+}$, Ru${}^{5+}$, Mn${}^{4+}$, and Ru${}^{4+}$ for SrMn${}_{0.5}$Ru${}_{0.5}$O${}_{3}$ to primarily Mn${}^{4+}$ and Ru${}^{4+}$ for CaMn${}_{0.5}$Ru${}_{0.5}$O${}_{3}$. The compounds Ba${}_{0.5}$La${}_{0.5}$Mn${}_{0.5}$Ru${}_{0.5}$O${}_{3}$, Ca${}_{0.5}$La${}_{0.5}$Mn${}_{0.5}$Ru${}_{0.5}$O${}_{3}$, and Sr${}_{0.5}$Ca${}_{0.25}$La${}_{0.25}$Mn${}_{0.5}$Ru${}_{0.5}$O${}_{3}$ were also investigated to study the effects of changing the charge of the $A$-site cation. Although substitution of La${}^{3+}$ for a divalent alkaline earth ion increases the Mn${}^{3+}$ content, this series of compounds also shows a relative increase in the concentration of Mn${}^{4+}$ as the average size of the $A$-site cation is decreased. In all compounds the octahedra containing Mn${}^{3+}$ are found to be Jahn-Teller distorted regardless of whether or not long-range orbital ordering is observed, while the Ru-centered octahedra are symmetric. No evidence for short-range cation ordering at either the $A$- or $B$-sites was found for any of the compositions. This study also reports that locally the $A$-site cations lie closer to the Mn ions than to the Ru ions and this asymmetry appears to be correlated to the degree of octahedral tilting.