Orbital ordering and valence states in (La1+xCa1-x)CoRuO6 double perovskites

(La1+xCa1-x)CoRuO6 double perovskites have been studied by neutron diffraction and x-ray absorption spectroscopy. The thermal evolution of the (LaCa)CoRuO6 structure has been investigated between 4 and 1073 K using neutron powder diffraction. The cell b axis shows a crossover from negative to positive thermal expansion at T approximate to 425 K, which is accompanied by a discontinuity in the c axis. This is shown to result from a partial orbital ordering of the Co2+ t(2g) holes. Ru valence states of doped (La1+xCa1-x)CoRuO6 (-0.25 <= x <= 0.25) materials have been investigated using XANES spectroscopy. Electron-doping (x > 0) leads to reduction of Ru5+-> Ru4+ while hole-doped x <= 0 compositions have a constant Ru5+ state. These observations support a proposed asymmetric doping model.