Energetics of oxidation of oxynitrides: Zr–N–O, Y–Zr–N–O, Ca–Zr–N–O, and Mg–Zr–N–O

The enthalpies of oxidation, ΔH_ox, of oxynitrides Y_yZr_1-yO_2x-0.5y-2/3xN_x (0.016 < x < 0.2), Ca_yZr_1-yO_2x-y-2/3xN_x (0.03 < x < 0.14), Mg_yZr_1-yO_2x-y-2/3xN_x (0.146 < x < 0.28), and Zr–O–N (β-type and γ phases) are measured using drop solution calorimetry in molten sodium molybdate (3Na₂O · 4MoO₃) at 973 K. Linear relations between the enthalpy ΔH_ox and nitrogen content were found in all oxynitrides. They indicate that, within the experimental range of nitrogen concentrations, sites occupied by nitrogen ions are energetically equivalent in a given substitutional series. The enthalpies normalized per mole of nitrogen, ΔHⁿ_ox, for compounds of Y–Zr–N–O, Ca–Zr–N–O, and Zr–N–O are similar, about −500 kJ/(mol of N). A more exothermic value of ΔHⁿ_ox, of about −950 kJ/(mol of N), is seen in Mg–Zr–N–O compounds. The energetics of vacancy formation in zirconium oxynitrides was determined and compared to the energetics of vacancy formation in yttria- and calcia-stabilized zirconia. The enthalpy of vacancy formation (enthalpy of formation relative to end members normalized per vacancy) in zirconium oxynitrides (−190.5 ± 27.0 kJ/mol of Voö) is more exothermic than that in yttria- and calcia-stabilized zirconia (-105 ± 7.2 and -91.4 ± 3.8 kJ/mol of Voö, respectively). This is consistent with the higher tendency for long-range ordering in zirconium oxynitrides compared to stabilized zirconia. Some technological implications of the results are briefly discussed.