Electronic structure of crystalline uranium nitride: LCAO DFT calculations

Abstract

The results of the first LCAO DFT calculations of cohesive energy, band structure and charge distribution in uranium nitride (UN) crystal are presented and discussed. The calculations are made with the uranium atom relativistic effective core potentials, including 60, 78 and 81 electrons in the core. It is demonstrated that the chemical bonding in UN crystal has a metallic–covalent nature. Three 5f‐electrons are localized on the U atom and occupy the states near the Fermi level. The metallic nature of the crystal is due to the f‐character of both the valence‐band top and the conduction‐band bottom. The covalent bonds are formed by the interaction of 7s‐ and 6d‐states of the uranium atom with the 2p‐states of the nitrogen atom. It is shown that the inclusion of 5f‐electrons in the atomic core introduces small changes in the calculated cohesive energy of UN crystal and electron charge distribution. However, the inclusion of 5s‐, 5p‐, 5d‐electrons in the valence shell allows the better agreement with the calculated and experimental cohesive‐energy value. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A first‐principles DFT study of UN bulk and (001) surface: Comparative LCAO and PW calculations

“…Lastly, the nondegenerate U5f a 2u state is empty; it forms the bottom of the conduction band (CB). In other words, both bottom of the CB and the top of the valence band (VB) are formed by U5f states which leads to the metallic nature of this compound 13 (unlike UO 2 which is a semiconductor 16 ). The Mulliken effective atomic charges of 6(1.5-2.0) e calculated using the GAUSSIAN-03 code confirm the mixed nature of the UN chemical bonding and are in agreement with the Bader topological charges of 61.6 e obtained in PW calculations.…”

“…However, the first LCAO electronic structure calculations of crystalline UN have been only recently performed. 13 In that article, several different Relativistic Effective Core Potentials (RECP) containing 60, 78, and 81 electrons in a U ion core, which are discussed in more detail in the next RECP Formalism, were used and compared. In particular, for the RECP78, there exist 14 outermost uranium atom electrons ).…”

“…22 In the present calculations we have used for U atom the only radially local AREP version by the Mosyagin-Titov LC (MT78) and SC (MT60) pseudopotentials. 13 The radially local AREP form used in LCAO calculations is a sum of a Coulomb term C, a local term V loc and a semilocal term V sl usually presented analytically as…”

“…Recent LCAO calculations on UN bulk 13 have been performed using the GAUSSIAN-03 computer code with the PW91 exchange-correlation functional and periodic boundary conditions. Unlike previous PW calculations, the LCAO (RECP78) suggests the ground state with the three unpaired electrons (S z 5 3/2, i.e., SD % 3 l B ) whereas the S z 5 1/2 state lies slightly higher in energy ($0.5 eV).…”

Structural, electronic, and thermodynamic properties of UN: Systematic density functional calculations

“…On account of these obvious importances, several studies, such as electronic structure optimization [3,4], magnetic properties [5], point defects [6], and elastic constants [7], have already been conducted for uranium nitride. However, conventional density functional theory (DFT) that apply the LDA or GGA underestimates the strong on-site Coulomb repulsion of the 5f -electron and, consequently, describes UN as incorrect FM conductor instead of the experimentally observed AFM type-I structure [8] at the Néel temperature T N =53 K. Similar problems have been confirmed in studying other electronically correlated materials within the pure LDA/GGA scheme.…”