Solute Knight Shifts in Noble Metals

Abstract

Knight shifts are reported for Cu, Cd, Al, In, Sn, and Ga as dilute solutes in Au; Hg, In, and Al as dilute solutes in Ag; In as a dilute solute in Cu; and Sn as a dilute solute in Ag-Au alloys. These data are considered together with all other Knight-shift data on dilute solutes in the three noble metals. A valence effect is found which is opposite in sense for Cu and Ag on the one hand, and Au on the other. Cd resonance linewidths for Au-Cd alloys show appreciable broadening with increase in alloy content or test frequency. New values of hyperfine fields for the free atom are obtained, together with several choices of values for the paramagnetic spin susceptibility of the hosts. Implications of the rigid-band theory for lattice volume effects are examined. The problem of exchange enhancement of the spin susceptibility is reviewed. Values for Knight's parameter $\xi$, usually considered as a measure of the amount of $s$ character in the metal, are tabulated and discussed. The valence trends found in the Knight shifts are also evident in the $\xi$ values, and these cannot be attributed to a phase-shift analysis common to the three noble metals. Varying band character and local effects appear important to the results. The origin of the solvent Knight shifts in noble metal alloys is examined in terms of the rigid-band theory; the results emphasize several shortcomings in charge screening theory as currently applied to solvent shifts.