Intermediate valence of Ce in ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{Ni}}_2$${\mathrm{B}}_2$C (x=0, 0.05, 0.1, 0.25, 0.5, and 1)

We report on the intermediate valence of Ce in ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{Ni}}_2$${\mathrm{B}}_2$C as studied by x-ray diffraction (lattice parameters), x-ray absorption spectroscopy (XAS), and magnetic measurements for samples with x=0, 0.05, 0.1, 0.25, 0.5, and 1. The average valence 〈v〉 of Ce, as determined from XAS, is much below the saturation value 3.3 in ${\mathrm{CeNi}}_2$${\mathrm{B}}_2$C; 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 ${\mathrm{CeNi}}_2$${\mathrm{B}}_2$C and absence in the dilute alloy ${\mathrm{Y}}_{0.95}$${\mathrm{Ce}}_{0.05}$${\mathrm{Ni}}_2$${\mathrm{B}}_2$C) may also be responsible for this difference in the values of 〈v〉. We also report on the depression of the superconducting temperature ${\mathit{T}}_{\mathit{c}}$ induced by Ce, as studied by ac-susceptibility, in the series.