A study of the first-order valence transition in Ce(Ni_1-xCo_x)Sn single crystals by magnetic susceptibility measurements

The first-order valence transition in CeN_1-xCo_xSn has been investigated for single crystalline samples with x = 0.32 and x = 0.35 as well as for polycrystalline samples with 0.31 ≤ x ≤ 0 38 (x varied in steps of 0.01) through the measurements of magnetic susceptibility, x. For the single crystal with x = 0.32, χ(T) exhibits a strong anisotropic mixed-valence behaviour. On the other hand, χ(T) of the crystal with x = 0.35 exhibits a sharp first-order valence transition at 50 K. In both crystals, the strong anisotropy, χ_a > χ_b ≈ χ_c. is maintained as in the host compound CeNiSn. The arc-melted polycrystalline alloys with a narrow range of Co composition, x = 0.33-0.37, show a sharp drop in χ(T) below 70 K and large hysteresis. This transition becomes sharper by annealing at 1000 °C. For the optimum concentration x = 0.35, the drop in χ(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 heat capacity was observed at 55 K Analysis of the above results reveals that the phase transition involves a large change in the Kondo temperature Δ T_K / T_K = 11.2. The small volume discontinuity Δ 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.