MAGNETIC-ORDERING IN SILICON DEFECT ND-SI COMPOUNDS STUDIED BY NEUTRON-DIFFRACTION AND MAGNETIC MEASUREMENTS

Earlier investigations had indicated the presence of a compound (of unknown structure) with the composition Nd3Si4. Our neutron diffraction data show that the compound crystallizes in a novel type of structure related to the CrB and alpha-GdSi2 structure types. The structure is orthorhombic and comprises two neodymium sites and three silicon sites. It is a defect structure in which about 50% of one of the silicon sites is unoccupied, leading to the composition Nd2Si3-delta with delta = 0.5. The compound orders ferromagnetically below T(C) = 80 K and the neodymium moments are oriented along the c axis. The experimental results obtained for Nd2Si3-delta are discussed together with the defect structures of the alpha-GdSi2 type observed for NdSix in the range 1.75 less-than-or-equal-to x less-than-or-equal-to 1.85. The distribution of the vacancies changes substantially within this range and the magnetic ordering changes with increasing x from ferromagnetic to antiferromagnetic.