A study of the first-order valence transition in Ce(Ni1-xCox)Sn single crystals by magnetic susceptibility measurements

The first-order valence transition in CeNi1-xCoxSn has been investigated for single crystalline samples with x = 0.32 and x = 0.35 as well as for polycrystalline samples with 0.31 less than or equal to x less than or equal to 0.38 (x varied in Steps of 0.01) through the measurements of magnetic susceptibility, chi. For the single crystal with x = 0.32, chi(T) exhibits a strong anisotropic mixed-valence behaviour. On the other hand, chi (T) of the crystal with x = 0.35 exhibits a sharp first-order valence transition at 50 K. In both crystals, the strong anisotropy, chi(a) > chi(b) approximate to chi(c), is maintained as in the host compound CeNiSn. The are-melted polycrystalline alloys with a narrow range of Co composition, x = 0.33-0.37, show a sharp drop in K (T) below 70 K and large hysteresis. This transition becomes sharper by annealing at 1000 degrees C. For the optimum concentration x= 0.35, the drop in chi(T) at the transition becomes as high as 72% for the single crystal and 69% for the polycrystalline alloy for which a jump in ac hear capacity was observed at 55 K. Analysis of the above results reveals that the phase transition involves a large change in the Kondo temperature Delta T-K/T-K = 11.2 The small volume discontinuity Delta V/V = -0.003 at the transition suggests that the phase transition is not due to a Kondo volume collapse. A possible mechanism responsible for the phase transition in the present alloys is proposed.