Crystal structure and charge-ordering in La1.5Ca0.5CoO4 studied by neutron and resonant X-ray scattering

Using neutron and resonant X-ray scattering, we studied crystal structure and charge-ordering pattern in La1.5Ca0.5CoO4. Taking A2mm with a-b type twin structure into account, Co2+/Co3+ checkerboard charge ordering pattern was suggested in La1.5Ca0.5CoO4 by neutron scattering structure analysis. In order to clarify the charge ordering pattern and the valences of the two Co sites, we performed a resonant X-ray scattering (RXS) measurement near the Co K-edge. We found a resonant signal at (3, 0, 0) reflection which corresponds to the charge ordering with a Co2+/Co3+ checkerboard pattern, and the Co valences of charge ordering were fully charge disproportionated. From the X-ray fluorescence spectra, we found the pre-edge peak structures at E = 7.7 10 and 7.713 keV, corresponding to 1S --> t(2g) and 1s ---> e(g) transitions, respectively. The polarization dependence of the latter one suggests that the anisotropy e(g) orbital is realized in La1.5Ca0.5CoO4.