First-order valence phase transition in CeNi1-xCoxSn alloys

Alloys in the pseudobinary series CeNi1-xCoxSn (x=0-0.5) have been investigated using a wide variety of experimental techniques. The gap in the electronic density of states, found in the Kondo compound CeNiSn at low temperatures, was found to decrease rapidly with Co concentration. The temperature dependence of the susceptibility shows a dramatic change with Co concentration. In alloys with 0≤x≤0.34, χ(T) has a form which shows an increasing trend towards intermediate valence behavior with increasing x: for x=0.34 χ(T) exhibits a broad maximum at 95 K. For x in the range 0.35-0.4 the susceptibility exhibits a sharp drop at temperatures between 40 and 75 K (dependent on composition). The unit-cell volume for the x=0.38 alloy, measured by neutron diffraction and linear thermal expansion, shows a 0.3% contraction, and the volume and anisotropic magnetostrictions exhibit large anomalies at the same temperature. We attribute these anomalies to a first-order valence phase transition. Inelastic-neutron-scattering measurements on the x=0.38 alloy above the phase transition temperature Tv show a quasielastic line and two crystal-field excitations at 26 and 42 meV, while below Tv the quasielastic peak disappears, and a gap opens up the response, extending up to 20 meV. This type of response is characteristic of intermediate valence systems. All the measurements are consistent with a first-order valence transition, like that observed in the compound YbInCu4.