Chemical, magnetic and orbital order in the substituted double perovskite Sr$_{(1{\-}x)}$Ca$_{(x)}$Mn$_{0.5}$Ru$_{0.5}$O$_{3}$

The structural and magnetic properties of Sr$_{(1{\-}x)}$Ca$_{(x)}$Mn$_{0.5}$Ru$_{0.5}$O$_{3}$ have been investigated. SrMn$_{0.5}$Ru$_{0.5}$O$_{3}$ exhibits antiferromagnetic ordering, T$_{N}$~$\approx $~200~K. Neutron powder diffraction of this perovskite indicates the presence of orbital ordering of the occupied Mn$^{+3}$ d$_{z}$2 orbitals, stabilizing the spin ordering corresponding to the AFM C-type structure. The substitution of smaller Ca$^{+2}$ for Sr$^{+2}$ on the A-site induces a change in the octahedral tilt system, (a$^{0}$a$^{0}$c$^{-})$ to (a$^{-}$b$^{+}$a$^{-})$ forcing a loss of this type of orbital ordering for x~$\ge $~0.2. This is accompanied by a crossover to a ferromagnetic ground state with 200K $\le $ T$_{C }\le $ 300K even in the absence of long range chemical order. Magnetic data and neutron powder diffraction of CaMn$_{0.5}$Ru$_{0.5}$O$_{3}$ and Sr$_{0.5}$Ca$_{0.5}$Mn$_{0.5}$Ru$_{0.5}$O$_{3}$ reveal reduced magnetic moments and imply a complex magnetic behavior as well as a complete loss of orbital ordering for CaMn$_{0.5}$Ru$_{0.5}$O$_{3}$. X-ray absorption studies show increased electron transfer from Mn to Ru.