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

Abstract

Using soft x-ray absorption spectroscopy we were able to determine unambiguously the charge and spin states of the transition metal ions in stoichiometric $\mathrm{Y}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ and $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$. The trivalent and low-spin nature of both the Cu and Co ions in $\mathrm{Y}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ 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 $\mathrm{Y}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ remain low spin upon the Y-Ca substitution, very much unlike the ${\mathrm{La}}_{1\text{\ensuremath{-}}x}{\mathrm{Sr}}_{x}\mathrm{Co}{\mathrm{O}}_{3}$ system. The tetravalent Co ions in $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ are interestingly also in the low-spin state, which then explains the good electrical conductivity of $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{12}$ since charge exchange between neighboring ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions will not be hampered by the spin-blockade mechanism that otherwise would be in effect if the ${\mathrm{Co}}^{4+}$ and ${\mathrm{Co}}^{3+}$ 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.