VALENCE TRANSITION IN CENI(1-X)CO(X) SN ALLOYS

The temperature dependence of the susceptibility of CeNi1-xCoxSn alloys shows a dramatic change with Co concentration x. CeNiSn shows a Curie-Weiss behaviour for chi(T) between 50 and 300 K. For x = 0.34 chi(T) exhibits a broad maximum at 95 K, while for x = 0.35 it shows a sharp drop at 75 K. There is a marked contraction of the volume at the same temperature. On increasing x to 0.4 the anomaly moves to 40 K, but disappears for x greater-than-or-equal-to 0.42. We ascribe this behaviour to a first-order valence transition, like that observed in the compound YbInCu4.