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

Abstract 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, Δ H f ° , for all Ln N 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 Ln N compounds using X-ray diffraction and thermogravimetric analysis, and are cross-examined against recent high temperature measurements on other Ln N compounds 1 . It was determined that Δ H f ° systematically correlates with the ionic radius of Ln 3+ cations, with smaller Ln bearing nitrides having more negative values. A discontinuity in Δ H f ° was further observed between larger and smaller Ln bearing Ln N compounds. Using the determined Δ H f ° values with literature references, Gibbs energies of formation (Δ G f ° ), oxidation (Δ G ox ° ) and hydrolysis (Δ G 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 Ln N phases within UN based SNF should lead to stabilisation of the structure and inhibit its oxidation.