Structural and oxygen content changes of hexagonal HoMnO_3+δ manganite at the stability boundary in the perovskite phase have been studied by X-ray diffraction and thermogravimetry using in situ oxidation and reduction processes at elevated temperatures in oxygen and air. The oxygen storage properties during structural transformation between stoichiometric Hex0 and oxygen-loaded Hex1 phases, transition temperatures and kinetics of the oxygen incorporation and release are reported for materials prepared by the solid-state synthesis and high-impact mechanical milling. Long-term annealing experiments have shown that the Hex0 (δ = 0) → Hex1 (δ ≈ 0.28) phase transition is limited by the surface reaction and nucleation of the new phase for HoMnO_3+δ 15MM. The temperatures of Hex0 ↔ Hex1 transitions have been established at 290 °C and 250 °C upon heating and cooling, respectively, at a rate of 0.1° min^-1, also indicating that the temperature hysteresis of the transition could possibly be as small as 10 °C in the equilibrium. Ball-milling of HoMnO_3+δ has only a small effect on improving the speed of the reduction/oxidation processes in oxygen, but importantly, allowed for considerable oxygen incorporation in air at a temperature range of 220-255 °C after prolonged heating. The Mn 2p XAS results of the Mn valence in oxygen loaded samples support the oxygen content determined by the TG method. The magnetic susceptibility data of the effective Mn valence gave inconclusive results due to dominating magnetism of the Ho³⁺ ions. Comparison of HoMnO_3+δ with previously studied DyMnO_3+δ indicates that a tiny increase in the ionic size of lanthanide has a huge effect on the redox properties of hexagonal manganites and that practical properties could be significantly improved by synthesizing the larger average size (Y,Ln)MnO_3+δ manganites.

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