Comments on the crystal-field splitting of the localizedGd3+moments in Au single crystals

New ESR experiments on Au single crystals doped with Gd impurities yield a positive crystal-field splitting parameter $b_4=+30.5\mathrm{}\pm 1.0$ G corresponding to +28.5 × ${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. A fully resolved spectrum is observed. At higher concentrations an exchange-narrowed line appears, produced by interacting impurity spins. Previously published data showed a superposition of a single-ion spectrum and a "cluster" line. The positive sign is in agreement with a recent interpretation of lanthanide $S$ states splitting in metals.