Intermediate valence of Ce in Y_1-xCe_xNi₂B₂C (x=0, 0.05, 0.1, 0.25, 0.5, and 1)

Abstract

We report on the intermediate valence of Ce in Y_1-xCe_xNi₂B₂C as studied by x-ray diffraction (lattice parameters), x-ray absorption spectroscopy (XAS), and magnetic measurements for samples with x=0, 0.05, 0.1, 0.25, 0.5, and 1. The average valence <v> of Ce, as determined from XAS, is much below the saturation value 3.3 in CeNi₂B₂C; it increases from 3.15(2) at 300 K to 3.21(2) at 10 K. Ce is in mixed valence state [<v>=3.18(2)] in the dilute limit (x=0.05) in which case the temperature dependence of <v> is rather weak. Cooperative effects (their presence in the stoichiometric material CeNi₂B₂C and absence in the dilute alloy Y_0.95Ce_0.05Ni₂B₂C) may also be responsible for this difference in the values of <v>. We also report on the depression of the superconducting temperature T_c induced by Ce, as studied by ac-susceptibility, in the series.