Constitutional and Thermal Structure Defects in NiGa

Abstract

The structure defects due to deviation from the equiatomic composition within the structure stability range of NiGa consists of Ni antistructure atoms or Ga‐site vacancies, depending on the composition. Large concentrations of thermal vacancies form on heating and can be readily retained by quenching. The effective formation and mobility activation energies have been determined throughout a range of compositions. The ordered B2 structure was found in stoichiometric NiGa to temperatures of about 1000°C. It is shown that in ordered compound lattices thermal defects other than single vacancies must also form. The interaction of the constitutional and thermal defects is qualitatively discussed. Because of possibly complex defect relaxation, the conventional mobility measurements cannot be interpreted as corresponding to simple vacancy relaxation. It is concluded that observations on any single composition cannot be considered representative of the compound throughout its stable structure composition range. Similar considerations apply to other B2 compounds, and the relevance to reported observations in AuCd is discussed.