Spin-State Equilibria in Holmium Cobaltate

M\"ossbauer and magnetic susceptibility studies of HoCoO₃ have shown that there is coexistence of low-spin Co(III) ions and high-spin Co³⁺ ions; Co(III) being more predominant at low temperatures. The population of Co(III) and Co³⁺ 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, HoCoO₃ becomes metallic. At this temperature, a first-order localized electron-collective electron transition seems to occur. Co(II) and Co⁴⁺ are not formed by electron transfer from Co³⁺ to Co(III) as in LaCoO₃. This behavior is correlated with the variation of covalency in the cobaltates.