SOLID-STATE PHASE-EQUILIBRIA IN THE ER-GA-AS SYSTEM

The Er-Ga-As solid state equilibrium phase diagram was determined at 8 00-degrees-C with the use of powder X-ray diffraction, electron probe microanalysis and scanning electron microscopy. No ternary phases were found and very limited solid solubilities were measured in the consti tuent binary Er-Ga and Er-As compounds, with the exception of Er5Ga3 w hich showed a broad homogeneity range with an As-rich limit correspond ing to the formula Er5Ga2As. GaAs, ErAs and gallium metal form a three -phase region that dominates the GaAs side of the phase diagram. The e xistence of this three-phase region confirms previous observations tha t the metallic compound ErAs is thermodynamically stable on GaAs and t herefore, since lattice matched, a potential candidate for epitactic c ontacts. The results of the Er-Ga-As phase diagram, confirmed by the e xperimental determination of the Ho-Ga-As diagram, can be extended to the other heavy rare earths (Gd --> Lu) and to chemically related elem ents such as scandium and yttrium. In the case of the light rare earth s (La --> Sm) the occurrence of an As-rich binary modifies the bottom part of the ternary diagram below 600-degrees-C as confirmed by the pa rtial determination of the Pr-Ga-As diagram.