Crystal structure and magnetic properties of the new ternary actinide compounds in An-Pd-Al system (An=U, Np)

We report the crystal structure and magnetic properties of new ternary actinide compounds. A series of compounds having AnPd₅Al₂ composition crystallize in the body-center tetragonal ZrNi₂Al ₅-type tetragonal structure (/4/mmm). Although the magnetic susceptibility of both compounds follows the Curie-Weiss behavior at high temperature, no magnetic phase transition was observed. UPd₅Al ₂ has a nonmagnetic ground state where the magnetic susceptibility saturates at low temperature, while NpPd5Al2 superconducts below 4.9 K as reported recently. Another new ternary uranium intermetallic compound U _2/3Pd₂Al₅ crystallizes in the hexagonal structure (P6₃/mmc) with the lattice parameters 4.3278 and c = 16.2870 Å. The composition analysis and the single crystal X-ray diffraction revealed the peculiar disordered structure where 1/3 of uranium atoms are replaced by three aluminum atoms. The similar structure is reported previously for the compound in U-Fe-Si system. Magnetization as well as specific heat measurements revealed a low temperature anomaly corresponding to the magnetic phase transition.