Ab initio studies of electric field gradients and magnetic properties of uranium dipnicties
In this paper, we explore the electric field gradients (EFGs) at 238U sites for antiferromagnetic UX2 (X = P, As, Sb, Bi) using LDA, LDA + U, GGA, GGA + U, and the exact exchange for correlated electrons schemes by considering the diagonalization of the spin–orbit coupling Hamiltonian in the space of the scalar relativistic eigenstates using the second-order variational procedure. The electronic structures and magnetic properties of the compounds are also investigated. It is found that the density functional theory approaches except exact exchange for correlated electrons are not successful in reproducing the experimental zero electric field gradient value in UBi2, even LDA + U and GGA + U within their default 4f density matrices by varying the U parameter in an energy interval of [0; 4 eV], though these techniques with no need to manually adopt their initial conditions (elements of the occupation matrix) are effective in the calculation of the nonzero electric field gradients for the other compounds. The exact exchange for correlated electrons has efficiently provided a null electric field gradient in UBi2 and nonzero electric field gradients for the other compounds by adjusting its dimensionless parameter α to 0.4. The physics of the null electric field gradient in UBi2 is revealed in this article and it is discussed that the source of the ignorable electric field gradient originates from the antiferromagnetic ordering of ↑↓ as compared to the long-range antiferromagnetic ordering of ↑↑↓↓ in the other compounds. Furthermore, our calculated magnetic moments for the uranium atoms in these compounds are consistent with the available experimentally measured values as compared to the severely underestimated theoretical results.