Hydrogen-induced disproportionation of Zr2M (M=Fe, Co, Ni) and reproportionation

Hydrogen induced disproportionation and reproportionation for Zr2Fe, Zr2Co and Zr2Ni were studied by means of volumetric measurements of hydrogen absorption and desorption, and X-ray diffraction analyses of the product phases. These alloys formed respective hydrides of Zr2MH5 (M=Fe, Co and Ni) type at room temperature only under conditions of very slow hydrogen absorption, otherwise the alloys disproportionated. Heating Zr2MH5 to 1073 K gave Zr2Ni and Zr2Co through decomposition, disproportionation and reproportionation. On the other hand, heating Zr2FeH5 to 1073 K yielded ZrFe2 and Zr3Fe. At the elevated temperature of 773 K, each alloy disproportionated very fast, within several tens of seconds, to ZrH2 and Zr-deficient alloys such as ZrCo, ZrNi and ZrFe2 for Zr2Co, Zr2Ni and Zr2Fe, respectively. ZrCo and ZrNi further disproportionated to ZrCo2 and Zr7Ni10 at this temperature, but the rates were very slow. These observations showed that the stability to hydrogen induced disproportionation is in the order of Zr2Ni>Zr2Co>Zr2Fe. This order is also valid for the ease of reproportionation, although the behavior of Zr2Fe was completely different from the others. (C) 2002 Elsevier Science B.V. All rights reserved.