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

Phase boundaries in the LaNi5-D system determined by in situ neutron diffraction experiments have shown good agreement with those predicted from the LaNi5-D pressure-composition phase diagram by intersecting extrapolated pressure plateaux and pure phase isotherms. Lattice parameter measurements show strong dilational strain in the alpha phase and severe 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 partially hydrided powder is microscopically inhomogeneous, i.e. individual micron-sized particles contain coexisting nanoscale alpha and beta phase domains. The alternative model, consisting of a mixture of particles which are wholly alpha and particles which are wholly beta phase, is not supported by these data. We observed hysteresis in the lattice parameters of the beta phase and the linewidths of the alpha phase in the two-phase legion, implying a fundamental asymmetry in the D absorption and desorption mechanisms.