PHASE-SEPARATION IN AMORPHOUS ERNI2HX PREPARED BY HYDROGENATION

Abstract

X-ray diffraction and observations of phase separation are reported in a hydrogen-induced amorphous ErNi2 alloy. Two separate diffuse maxima in x-ray diffraction and two principal halos in electron diffraction, which are evidence of phase separation into two amorphous states, are observed at hydrogenation conditions of 50 atm H-2 and elevated temperatures. The phenomenon of phase separation is manifest with increasing hydrogenation temperature. The compositions of both phases are estimated by comparing the nearest-neighbor distances calculated by using the Ehrenfest relation with all possible atomic distances in amorphous ErNi2 hydride. These distances are calculated by assuming that the configurational atomic arrangement in the amorphous phase is composed of tetrahedral units. The compositions of the two phases seem to correspond to an Er-rich phase (mixture of 4-Er and 3Er + 1Ni tetrahedra) and a Ni-rich phase (mixture of 4-Ni and 1Er + 3Ni tetrahedra). This phase separation may be caused by the motion of Ni atoms over short distances.