First-principles LDA+U and GGA+U study of neptunium dioxide

TL;DR

This study uses first-principles calculations with LDA+U and GGA+U methods to analyze the electronic, mechanical, and vibrational properties of neptunium dioxide, highlighting the importance of 5f electron localization.

Contribution

It provides a systematic first-principles analysis of NpO₂ using LDA+U and GGA+U, aligning orbital occupancy with experiments and comparing covalency with ThO₂.

Findings

Good agreement of Np 5f and O 2p orbital characters with experiments

Stronger covalency in NpO₂ due to 5f electron localization

Comparison reveals localization trends in actinides

Abstract

We have performed a systematic first-principles investigation to calculate the electronic structures, mechanical properties, and phonon dispersion curves of NpO$_{2}$. The local density approximation$+U$ and the generalized gradient approximation$+U$ formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Np $5f$ electrons. By choosing the Hubbard \emph{U} parameter around 4 eV, the orbital occupancy characters of Np 5\emph{f} and O 2\emph{p} are in good agreement with recent experiments [J. Nucl. Mater. \textbf{389}, 470 (2009)]. Comparing with our previous study of ThO$_{2}$, we note that stronger covalency exists in NpO$_{2}$ due to the more localization behavior of 5\emph{f} electrons of Np in line with the localization-delocalization trend exhibited by the actinides series.