The contribution of nitrogen Frenkel-pair formation to the high-temperature heat capacity of uranium mononitride

Abstract

The high-temperature heat capacity of uranium mononitride (UN) remains uncertain due to conflicting measurements and models above ~1700 K. To assess whether intrinsic defect formation contributes to the observed superlinear behavior of $C_P(T)$, we perform large-scale molecular dynamics simulations using two interatomic potentials to quantify nitrogen diffusion and Frenkel-pair populations from 1800--2600 K. Both models show increasing anion mobility, but the Tseplyaev potential yields substantially larger Frenkel concentrations, producing a defect heat-capacity contribution of up to ~10 J/(mol-K). This defect-driven term is consistent with the curvature seen in historical correlations and recent ab initio results, suggesting that nitrogen sublattice disorder provides a plausible intrinsic mechanism for the high-temperature heat capacity of UN.

Figures (2)

• Figure 1: (Color online) (a) Nitrogen self-diffusion coefficients obtained from MSD analysis. (b) Nitrogen Frenkel-pair concentrations extracted using Wigner–Seitz defect analysis over 200 ps trajectories. Data points denote temporal averages over the post-equilibration sampling window, and error bars represent the standard deviation obtained from time averaging. The equilibrium concentration is based on Frenkel-pair formation energy of 4.5 eV calculated elsewhere using DFT and 0 K MD Yang2021Kocevski2022IAbdulHameed2024.
• Figure 2: Defect-induced heat-capacity contribution $C_{\mathrm{def}}(T)$ computed from Wigner-Seitz Frenkel-pair statistics using both the Tseplyaev and Kocevski interatomic potentials.