STRUCTURAL-PROPERTIES AND HYDRIDING-PROPERTIES OF THE ZR(V0.25NI0.75)(ALPHA) SS-THAN-OR-EQUAL-TO-ALPHA-LESS-THAN-OR-EQUAL-TO-4) ALLOY SYSTEM

Previous investigations on the Zr(VxNi1-x)(2) alloy system showed that Zr(V0.25Ni0.75)(2) has a very high reversible capacity (approximate t o 365 mA h g(-1), discharge current i(dis)=100 mA g(-1)). This alloy d oes not crystallize in a perfect single phase, a second phase (Zr7Ni10 ) was observed. In order to understand the influence of the second pha se and to optimize the alloy composition for high capacity, the influe nce of the stoichiometry was investigated. A series of alloys Zr(V0.25 Ni0.75)(alpha) (1 less than or equal to alpha less than or equal to 4) were prepared by r.f. levitation melting. The main phase in all the t ested alloys was the cubic C15-Laves phase. A second phase was observe d in the samples with alpha less than or equal to 2.5, whereas the all oys with alpha>2.5 are single phase. The lattice parameter shows a sig nificant step in the range of 2 less than or equal to alpha less than or equal to 2.5. The electrochemically measured capacity is highest fo r alpha=2 (C=1.0 H/M, 394 mA h g(-1), i(dis)=5 mA g(-1)). The alloy sy stem Zr(V0.25Ni0.75)(alpha) (1 less than or equal to alpha less than o r equal to 4) is very adaptable to changes in the stoichiometry. Howev er, the distribution of A and B elements on the A and B sites is cruci al for high capacity. A part of the vanadium atoms move from B sites t o A sites in the overstoichiometric systems Zr(V0.25Ni0.75)(alpha)(2<a lpha). The overstoichiometric (AB(3)) alloy ZrV1.5Ni1.5 crystallizes i n the cubic C15-Laves phase and shows a very high capacity of 800 mA h g(-1) (i(dis)=2 mA g(-1)). However, only 400 mA h g(-1) can be discha rged at a 20 mA g(-1) discharge current.