Determination of C14/C15 phase abundance in Laves phase alloys

For the purpose of optimizing the gaseous phase and electrochemical advantages of both the C14 and C15 phases in AB2 metal hydride (MH) alloys, a systematic study was established to predict and understand the factors that affect the C14/C15 phase abundance. The study was constructed based on 6 series of transition metal-based AB2 alloys that were designed with the choices of A-site elements being Ti, Zr, and Hf and the choices of B-site elements being Cr, Co, and Cu. The average electron density (e/a) was proven to be one of the most influential parameters in determining the C14/C15 phase abundance: as e/a increased, C14/C15 became less/more dominant, respectively. However, with different A-site compositions but same overall e/a, a shift in e/a was observed in the C14/C15 phase abundance vs. e/a relationship. The average atomic radius of A atoms was shown to correlate very well with the C14/C15 threshold when the choice of A-site elements is limited to any of Ti + Zr combinations (pure or mixed). The average chemical potential for electronic charge of A atoms had a nearly perfect linear correlation to the C14/C15 threshold even with the addition of Hf in the selection of A-site elements. The combination of e/a and the average chemical potential for electronic charge can be used to predict the C14/C15 phase abundance and assist future AB2 MH alloy development.