Crossover from divalent to valence fluctuating state of Eu in EuCu2(Ge1−x Six)2 probed by 63,65Cu - NMR

63,65Cu field sweep NMR investigations on a series of EuCu2(Ge1-x Six)2 powder samples (x=0.3,0.7,1) are presented. EuCu2(Ge,Si)2 is unique among Eu-intermetallics because, upon tuning from a divalent antiferromagnetically ordered state for x=0 to a valence fluctuating state for x=1, Heavy Fermion behaviour was found around x{approximately equal to}0.7. The Knight shift 63,65K is negative for all stoichiometries. Hyperfine fields are largest for the antiferromagnetically ordered Ge-rich compound (x=0.3) and smallest for the valence fluctuating pure silicon compound (x=1). For the x=0.3 compound, above TN{approximately equal to}19K, 63K(T) exhibits a Curie-Weiss like behaviour ({approximately equal to} -10% at 20K) whereas for EuCu2Si2 a temperature-independent shift of 63K(T){approximately equal to}-1.5% is found. Our results are discussed in terms of formation of heavy quasiparticles and/or presence of valence fluctuations of Eu.