The energetics of high temperature oxidation and formation mechanisms in binary lanthanide nitrides (LnN)

Nitride-based materials present exceptional thermal and structural properties for advanced material applications, however accurate determination of their thermodynamic properties is critical to their deployment. Herein, the standard enthalpies of formation, ΔH_f^°, for all LnN compounds (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu; excluding Pm) are determined via high temperature oxide melt calorimetry. These measurements are supported by high temperature oxidation studies of La, Ce, Sm, Eu, Er, Yb bearing LnN compounds using X-ray diffraction and thermogravimetric analysis, and are cross-examined against recent high temperature measurements on other LnN compounds¹. It was determined that ΔH_f^° systematically correlates with the ionic radius of Ln³⁺ cations, with smaller Ln bearing nitrides having more negative values. A discontinuity in ΔH_f^° was further observed between larger and smaller Ln bearing LnN compounds. Using the determined ΔH_f^° values with literature references, Gibbs energies of formation (ΔG_f^°), oxidation (ΔG_ox^°) and hydrolysis (ΔG_hyd^°) are calculated and compared against uranium nitride (UN) in the context of UN based nuclear fuel and related spent nuclear fuel (SNF) management. The calculations indicate that the occurrence of LnN phases within UN based SNF should lead to stabilisation of the structure and inhibit its oxidation.