Crystal dynamics and thermal properties of neptunium dioxide

Abstract

We report an experimental and theoretical investigation of the lattice dynamics and thermal properties of the actinide dioxide NpO${}_2$. The energy-wavevector dispersion relation for normal modes of vibration propagating along the $[001]$, $[110]$, and $[111]$ high-symmetry lines in NpO${}_2$ at room temperature has been determined by measuring the coherent one-phonon scattering of X-rays from a $\sim$1.2 mg single-crystal specimen, the largest available single crystal for this compound. The results are compared against ab initio phonon dispersions computed within the first-principles density functional theory in the generalized gradient approximation plus Hubbard $U$ correlation (GGA+$U$) approach, taking into account third-order anharmonicity effects in the quasiharmonic approximation. Good agreement with the experiment is obtained for calculations with an on-site Coulomb parameter $U=4$ eV and Hund's exchange $J=0.6$ eV in line with previous electronic structure calculations. We further compute the thermal expansion, heat capacity, thermal conductivity, phonon linewidth, and thermal phonon softening, and compare with available experiments. The theoretical and measured heat capacities are in close agreement with another. About 27% of the calculated thermal conductivity is due to phonons with energy higher than 25 meV ($\sim$ 6 THz ), suggesting an important role of high-energy optical phonons in the heat transport. The simulated thermal expansion reproduces well the experimental data up to about 1000 K, indicating a failure of the quasiharmonic approximation above this limit.

arXiv comment12 pages, 10 figures

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Cited by in corpus (7)

• Thermal Energy Transport in Oxide Nuclear Fuel
• First-principles theory of magnetic multipoles in condensed matter systems
• Synchrotron Radiation Techniques and their Application to Actinide Materials
• Study of phonons in irradiated epitaxial thin films of UO${}_2$
• Introduction to High-Resolution Inelastic X-Ray Scattering
• Efficient Method for Prediction of Meta-stable/Ground Multipolar Ordered States and its Application in Monolayer $\alpha$-\ce{RuX3} (X=Cl,I)
• Parameterizing empirical interatomic potentials for predicting thermophysical properties via an irreducible derivative approach: the case of ThO${}_2$ and UO${}_2$