Crystal structure, hydrogen absorption-desorption behavior and magnetic properties of the Nd3−xMgxCo9 alloys

Abstract New Nd 3− x Mg x Co 9 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 Nd 3− x Mg x Co 9 ( x  ≤ 1.5) alloys belong to the PuNi 3 -type structure and NdMg 2 Co 9 is an individual ternary compound with YIn 2 Ni 9 -type structure. Hydrogen storage capacity of the PuNi 3 -type Nd 3− x Mg x Co 9 alloys is in the range 1.1–1.6 wt % H at room temperature, while for NdMg 2 Co 9 it is below 0.5 wt % H. PСT 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 PuNi 3 -type structure with hydrogen-induced volume expansion 16.7–24%. The positions of hydrogen atoms were determined by neutron powder diffraction study of Nd 2 MgCo 9 D 10 . In the Nd 2 MgCo 9 D 10 structure D atoms partially occupy four types of interstices, including octahedral Nd 2 Co 4 sites, tetrahedral Nd 2 Co 2 sites and two types of trigonal bipyramids (Nd,Mg) 3 Co 2 . Electrochemical investigations of the Nd 2 MgCo 9 electrode demonstrated rather low discharge capacity, 100 mAh/g, which is about 30% of its theoretical capacity. Magnetic measurements have shown that Nd 2 MgCo 9 is ferromagnetic with high Curie temperature (T C  = 633 K) and spin reorientation at 225 K, its hydrogenation causes a significant decrease of the magnetization.