Thermodynamic evaluation of the Np–Zr system using CALPHAD and ab initio methods

A new thermodynamic description of Np–Zr alloys is developed using the CALPHAD (CALculation of PHAse Diagrams) method based on available experimental information on phase equilibria, supplemented by ab initio energetics for end members that are not available in the standard CALPHAD unary database. The present thermodynamic description shows improvements compared to previous models in the predicted phase diagram when comparing to assessed reliable experimental data. Ab initio Density Functional Theory (DFT) calculations are also performed on all known stable solid phases of Np–Zr alloys and the end member Np and Zr metals. Comparing to the formation energetics predicted from the CALPHAD models of both this work and a previous study we find that DFT with the Generalized Gradient Approximation (GGA) to the exchange-correlation potential overestimates the formation enthalpies of Np and Np–Zr by about 0.15 eV/atom, and the so-called DFT + U approach with a Ueff of near 0.65 eV can reduce this error by about 0.07–0.10 eV. Our comprehensive comparison between existing CALPHAD, ab initio and experimental results for Np–Zr indicates a need for further experiments on the phase equilibrium.