Reversible hydrogen absorption and desorption achieved by irreversible phase transition

Phase relations in the Zr7N10-H system were examined with a conventional Sieverts'-type apparatus. The hydrogen pressure-composition-temperature (PCT) relations obtained under the conditions such as a temperature of 410 K and a pressure range of 0.01-3 MPa demonstrated the existence of two hydride phases (β, γ) in the system and indicated the following unprecedented hydrogenation behavior of the Zr7Ni10 compound: (1) the β-hydride phase, which has lower hydrogen content than the other (γ), appears only in hydrogen desorption but not in hydrogen absorption; (2) the hydrogen capacity of the continuous solid solution phase (α) is 0.56 H/M (H/M; the atomic ratio of hydrogen to metal) in hydrogen absorption and only 0.40 H/M in hydrogen desorption; (3) The Zr7Ni10 compound absorbs hydrogen accompanied by the phase transition from α to γ(α&rarrγ transition) and desorbs hydrogen accompanied by the γ&rarrβ and β&rarrα transitions, however, the reverse of these three transitions, i.e. γ&rarrα, β&rarrγ and α&rarrβ transitions are not observed in the PCT relations under the conditions mentioned above. From the fact about the irreversibility of the phase transitions, the Zr7Ni10 compound can reversibly absorb and desorb hydrogen by the irreversible phase transition.