Crystal Structure and Charge-Ordering in La_<1.5>Ca_<0.5>CoO_4 Studied by Neutron and Resonant X-ray Scattering(Condensed matter: structure and mechanical and thermal properties)

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Using neutron and resonant X-ray scattering, we studied crystal structure and charge-ordering pattern in La_<1.5>Ca_<0.5>CoO_4. Taking A2mm with a-b type twin structure into account, Co^<2+>/Co^<3+> checkerboard charge ordering pattern was suggested in La_<1.5>Ca_<0.5>CoO_4 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 Co^<2+>/Co〜<3+> 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=7.710 and 7.713keV, 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 La_<1.5>Ca_<0.5>CoO_4.
社団法人日本物理学会の論文
2008-04-15