Crystal structure, hydrogen absorption-desorption behavior and magnetic properties of the Nd^sub 3-x^Mg^sub x^Co9 alloys

New Nd3-xMgxCo9 alloys have been synthesized by powder sintering method and their crystal structure, hydrogen storage and magnetic properties have been systematically studied. X-ray diffraction analysis showed that Nd3-xMgxCo9 (x ≤ 1.5) alloys belong to the PuNi3-type structure and NdMg2Co9 is an individual ternary compound with YIn2Ni9-type structure. Hydrogen storage capacity of the PuNi3-type Nd3-xMgxCo9 alloys is in the range 1.1-1.6 wt % H at room temperature, while for NdMg2Co9 it is below 0.5 wt % H. PCT measurements demonstrated that thermodynamic stability of hydrides decrease with increasing Mg content. Hydrides of the alloys with lower Mg content (x ≤ 1.0) are stable in air at room temperature. XRD study of these hydrides showed preserved PuNi3-type structure with hydrogen-induced volume expansion 16.7-24%. The positions of hydrogen atoms were determined by neutron powder diffraction study of Nd2MgCo9D10. In the Nd2MgCo9D10 structure D atoms partially occupy four types of interstices, including octahedral Nd2Co4 sites, tetrahedral Nd2Co2 sites and two types of trigonal bipyramids (Nd,Mg)3Co2. Electrochemical investigations of the Nd2MgCo9 electrode demonstrated rather low discharge capacity, 100 mAh/g, which is about 30% of its theoretical capacity. Magnetic measurements have shown that Nd2MgCo9 is ferromagnetic with high Curie temperature (TC = 633 K) and spin reorientation at 225 K, its hydrogenation causes a significant decrease of the magnetization.