A NEUTRON-DIFFRACTION INVESTIGATION OF THE LANI5-D PHASE-DIAGRAM

Phase boundaries in the LaNi5-D system determined by in situ neutron d iffraction experiments have shown good agreement with those predicted from the LaNi5-D pressure-composition phase diagram by intersecting ex trapolated pressure plateaux and pure phase isotherms. Lattice paramet er measurements show strong dilational strain in the alpha phase and s evere compressive strain in the beta phase in the two-phase region. In addition, there are systematic changes in linewidths in the two-phase region. Taken together, these provide strong evidence that the partia lly hydrided powder is microscopically inhomogeneous, i.e. individual micron-sized particles contain coexisting nanoscale alpha and beta pha se domains. The alternative model, consisting of a mixture of particle s which are wholly alpha and particles which are wholly beta phase, is not supported by these data. We observed hysteresis in the lattice pa rameters of the beta phase and the linewidths of the alpha phase in th e two-phase legion, implying a fundamental asymmetry in the D absorpti on and desorption mechanisms.