Specific heat in NpNiSn and NpIrSn

We present the specific-heat study of the electronic properties of NpIrSn and NpNiSn, crystallizing in the ZrNiAl-type hexagonal and TiNiSi-type orthorhombic structure, respectively. NpIrSn orders antiferromagnetically below TN=38 K and undergoes a further magnetic phase transition at T1=23.5 K. The γ-coefficient of the electronic specific heat is only moderately enhanced to 75 mJ mol−1 K−2. ThIrSn, crystallizing in the same structure, is used to estimate the lattice specific heat to analyze the magnetic part. NpNiSn orders antiferromagnetically below TN=35.5 K. In both compounds the antiferromagnetic order is not affected by magnetic fields up to 14 T.