Laves phases in RE(Mg,Cd)_2 (RE = rare earth) pseudo-binary alloys

摘要

Trends in the stability of Laves phases in the pseudo-binary alloy systems RE(Mg,Cd)_2 (RE: rare earth element) are studied experimentally. Alloys are synthesized with various compositional ratios between Mg and Cd and with different REs. For most REs except Yb, a Laves phase forms only in the Mg-rich part of the phase diagram including the binary limit REMg2, while for Yb a Laves phase is maintained throughout the whole compositional range between YbMg2 and YbCd_2. A thorough analysis on the compositions of the Laves phases in the Mg-rich part is performed, which reveals a systematic tendency towards off-stoichiometry with Mg-enrichment of about 5-10 atomic %. The phenomenon can be understood as a consequence of an unbalanced mixing of atomic species between two kinds of atomic sites. Rietveld refinements of the crystal structures reveal that a better fit is achieved by allowing the RE-positions to be replaced partially by Mg, which provides a supporting evidence for the atomistic model of off-stoichiometry.