Valence-electron contributions to the electric-field gradient in hcp metals and at Gd nuclei in intermetallic compounds with theThCr2Si2structure

Self-consistent band-structure calculations have been used to calculate valence-electron contributions to the electric-field gradient (EFG) in all elemental hcp metals, except those with nonspherical 4f shells, and at Gd nuclei in numerous intermetallic compounds of the type Gd${\mathit{T}}_2$ ${\mathrm{Si}}_2$ (T=Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, and Ag) and ${\mathrm{GdNi}}_2$ ${\mathit{X}}_2$ (X=Ge and Sn). Experimental determinations of the EFG in the latter two compounds were performed by a study of the ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}$ Mössbauer effect. The calculated EFG of hcp metals agrees well with the experimental values. The variation of the EFG with the T component across a given ${\mathit{RT}}_2$ ${\mathrm{Si}}_2$ series (R=rare earth, T=3d, 4d, 5d) is shown to be mainly due to a change of the asphericity of the Gd 6p charge distribution. The trends in this asphericity have also been explained qualitatively from a more simple treatment of the electronic structure, based on Miedema’s ‘‘macroscopic-atom’’ model for cohesion in metals. It is shown that as a consequence of the large valence-electron contribution to the EFG, the commonly used proportionality relation between the EFG and the second-order crystal-field parameter ${\mathit{A}}_2^0$ lacks a true physical basis, and may be invalid.