Charge and spin degrees of freedom in A-site ordered YCu3Co4O12 and CaCu3Co4O12

Using soft x-ray absorption spectroscopy we were able to determine unambiguously the charge and spin states of the transition metal ions in stoichiometric YCu3Co4O12 and CaCu3Co4O12. The trivalent and low-spin nature of both the Cu and Co ions in YCu3Co4O12 makes this correlated system to be effectively a nonmagnetic band semiconductor. The substitution of Y by Ca produces formally tetravalent Co ions but the doped holes are primarily on the oxygen ligands. Concerning the spin degrees of freedom, the trivalent Co ions in YCu3Co4O12 remain low spin upon the Y-Ca substitution, very much unlike the La1-xSrxCoO(3) system. The tetravalent Co ions in CaCu3Co4O12 are interestingly also in the low-spin state, which then explains the good electrical conductivity of CaCu3Co4O12 since charge exchange between neighboring Co3+ and Co4+ ions will not be hampered by the spin-blockade mechanism that otherwise would be in effect if the Co4+ and Co3+ spin quantum numbers were to differ by more than one-half. We infer that the stability of the Co low-spin state is related to the very short Co-O bond lengths.

Automated summary

Using soft x-ray absorption spectroscopy, the charge and spin states of transition metal ions in YCu3Co4O12 and CaCu3Co4O12 were determined. The low-spin trivalent nature of Cu and Co ions in both materials make them nonmagnetic band semiconductors. The good electrical conductivity of CaCu3Co4O12 is attributed to the presence of low-spin Co ions, allowing for efficient charge exchange between Co3+ and Co4+.