Spin-State Equilibria in Holmium Cobaltate

M\"ossbauer and magnetic susceptibility studies of HoCo${\mathrm{O}}_{3}$ have shown that there is coexistence of low-spin Co(III) ions and high-spin ${\mathrm{Co}}^{3+}$ ions; Co(III) being more predominant at low temperatures. The population of Co(III) and ${\mathrm{Co}}^{3+}$ equalizes above a particular temperature with these ions occupying alternate oxygen octahedra, leading to an ordered phase. The ordering transition is evidenced by the temperature variation of Lamb-M\"ossbauer factor, x-ray Debye-Waller factor, and inverse susceptibility. Electrical-conductivity data reflect these changes in the spin-state equilibria and show that at around 1080 K, HoCo${\mathrm{O}}_{3}$ becomes metallic. At this temperature, a first-order localized electron-collective electron transition seems to occur. Co(II) and ${\mathrm{Co}}^{4+}$ are not formed by electron transfer from ${\mathrm{Co}}^{3+}$ to Co(III) as in LaCo${\mathrm{O}}_{3}$. This behavior is correlated with the variation of covalency in the cobaltates.