Nuclear Magnetic Resonance in Copper Alloys. Electron Distribution Around Solute Atoms

Abstract

The effects of the addition to copper of a wide variety of B subgroup elements on the nuclear magnetic resonance absorption of copper are described. The resonance amplitude, which undergoes a sharp reduction upon alloying, is of special interest; its dependence upon solute valence and size argues decisively in favor of conduction electron charge redistribution (valence effects) as the dominant source of the electric field gradients surrounding these solutes. Furthermore, these gradients are shown to decrease only about as $\frac{1}{r^3}$ rather than exponentially as had been supposed. Using the proportional change in the lattice parameter of the solid solution as a measure of the local strains surrounding a solute atom, only slight correlations between local strains and resonance amplitude were found. It is concluded that the origin of electric gradients around multivalent solutes in copper is almost purely an effect of conduction electron distribution and that this distribution is not of the exponentially screened Coulomb charge type. The spatially oscillating charge distribution derived and recognized by Friedel and recently elaborated by Kohn and Vosko and Friedel and co-workers satisfactorily explains the observations.