A recent reinvestigation of the alloy system CeIn_3-xSn_x between the antiferromagnetic (AF) Kondo-lattice CeIn₃ and the intermediate valent CeSn₃ has shown that the AF order disappears at a quantum critical point (QCP) at x_c\approx 0.65. Preliminary evidences for a first order transition below T_N at xc, indicating a complex magnetic phase diagram, have motivated the present detailed investigation of specific heat and resistivity on fine tuned samples in the 0.25 \leq x \leq x_c range. These results confirm the first order transition at T_I < T_N in a reduced concentration range (0.25 < x < 0.47) and suggest that the phase boundary T_I(x) merge with the T_N(x) near x\approx 0.37, leading to a tetracritical point. A similar first order transition was previously observed in the CeCu₂(Si,Ge)₂ system at low Ge-concentration. Since in this alloy the first order transition also appears in the vicinity of a QCP, the question arises concerning how general this property is at AF critical points in Ce-based alloy systems.

PACS numbers: 71.10.Hf, 71.20.Lp, 71.27.+a, 75.20.Hr