Effects of internal structural parameters on the properties of Ba-substituted La$_{0.5}$Sr$_{0.5}$MnO$_{3}$*

Barium substituted La$_{0.5}$Sr$_{0.5-x}$Ba$_{x}$MnO$_{3}$ materials have been synthesized and investigated using neutron powder diffraction. We show that Ba substitution suppresses the low temperature orbital-ordering previously observed in La$_{0.5}$Sr$_{0.5}$MnO$_{3}$, and demonstrate the evolution of the magnetic and nuclear structures as a function of increasing Ba content. All samples exhibit paramagnetic and ferromagnetic properties near room temperature. The effects of A-site ionic size, size variance, and strains in the lattice on the ferromagnetic ordering temperature, T$_{C}$, are discussed and compared with other members of the general La$_{0.5}$(Ca,Sr,Ba)$_{0.5}$MnO$_{3}$ series. Depending on the substitution path, the relationship between T$_{C}$ and $<$r$_{A}>$ is either nearly constant or looks like an inverted parabola.