Magnetic characterization of some hyperstoichiometric ZrCr2-based alloys and their hydrides

Abstract

It is well known that many zirconium-based Laves phase intermetallics react reversibly and absorb large quantities of hydrogen. They have therefore attracted a great deal of attention as possible hydrogen storage materials [1-5]. Although ZrCr2 reacts reversibly and absorbs more than three hydrogen atoms per formula unit this compound is not technologically interesting because of its low hydrogen equilibrium dissociation pressure [6]. However, it was found that partial substitution of chromium by other transition metals and/or addition of these metals to ZrCr2 may considerably affect the hydrogen dissociation pressure of ZrCr2 [1, 5, 7-11]. Apart from their possible practical applications, these materials are also of great interest because profound changes in the electronic properties can occur. This is a concomitant of the substitution of one metal in binary compounds by a third metal, addition of a third metal to binary compounds (hyperstoichiometry) and/or hydrogen absorption. For example, it was found that the Pauli paramagnetism of ZrCr 2 is changed into ferromagnetism in the Zr(Crl_xFexh system, strongly enhanced in the Zr(Crl_xCOx)2 system, only moderately enhanced in the Zr(Ch_xNij2 system, and in the Zr(Ch_xCu,)2 system a ferromagnetic contribution to total magnetization is developed. Hydrogen uptake acts in favour of these effects [12-15]. The results of magnetization measurements for ZrCr2 when subjected to addition of selected quantities of iron, cobalt or nickel are reported here and represent the continuation of our studies of the hydrogen sorption and magnetic properties of ZrCr2-based alloys. The intermetallic compounds corresponding to the ZrCr2T0.8 (T = Fe, Co, Ni) stoichiometry were prepared by arc melting of the high-purity constituent metals (Ventron, at least 99.9 wt%), and then charged with hydrogen. The details on alloy preparation, the hydrogen absorption procedure, the hydrogen pressure-composition desorption measurements, as well as the apparatus used, have been described previously [7]. X-ray powder diffraction patterns of alloys and their hydrides were obtained using a Philips PW 1050 diffractometer equipped with a pulse height analyser; nickel filtered CuKo~ radiation was employed. Magnetization measurements were performed on a vibrating sample magnetometer (VSM) in the Physics Department, University of Durham, in the