Magnetic origin and oxydation states of cations in the TC∼440 K ferrimagnet Sr2Cu(Re0.69Ca0.31)O6

We have studied crystal structure, ionic-model energy calculation, and electron energy-loss spectroscopy (EELS) for a new magnetic ordered-perovskite Sr2Cu(Re0.69Ca0.31)O6 in order to determine oxidation states of the Re and Cu ions. All of the experimental results of the bond-valence sum, the static energies, and the energy-loss near-edge structure (ELNES) in EEL spectra consistently indicated that the Re7+, Cu2+ and Cu3+ valence states are realized in the compound. This is evident that the Cu spins are responsible for the magnetism in the phase. By considering the Cu 3d and O 2p orbital shapes, we concluded that the unusually high Curie temperature, TC∼440 K, originates in ferrimagnetism with strongly coupled Cu spins in a three dimensional (3-D) network of the Cu2+-O-Cu3+-O-Cu2+ bond in structure.