LAUSR

Abstract Y–Co intermetallic compounds and their hydrides were investigated as magnetic materials, but the hydrogenation of these alloys was not observed. We established the existence of Y5Co19 in the phase diagrams of the Y–Co system. Y5Co19, an intermetallic compound, was synthesized and its crystal structure was determined by the Rietveld refinement of X-ray diffraction (XRD) data. The structural model of Y5Co19 is a Ce5Co19-type model with lattice parameters a = 0.4994 (1) nm and c = 4.800 (1) nm. The crystal structure of the original alloy and its hydride phase is closely related to the hydrogen absorption–desorption property. The hydrogen capacity of Y5Co19 was 0.63 H/M, while two plateaus were observed in the P–C isotherm. We discovered two hydride phases: Y5Co19H3.8 (phase I) and Y5Co19H14.9 (phase II), and the structural model of these phases (I and II) was Ce5Co19-type, which was the same as the original alloy. The unit cell of phase I showed an expansion of 3.9% only along the c-axis from the original alloy, whereas that of phase II indicated an expansion along the a- and c-axes. Additionally, a close isotropic expansion was observed in phase II.