Abstract
We investigate how 5 at. % Ni influences the relative stability of eta and eta(') Cu6Sn5. Synchrotron x-ray diffraction shows that, while Cu6Sn5 exists as eta(') at 25 and 150 degrees C and transforms to eta on heating to 200 degrees C, Cu5.5Ni0.5Sn5 is best fit to eta throughout 25-200 degrees C. Our first principles calculations predict that eta(') is stable at T=0 K in both Cu6Sn5 and Cu5.5Ni0.5Sn5, but that the energy difference is substantially reduced from 1.21 to 0.90 eV per 22 atom cell by the Ni addition. This effect is attributed to Ni developing distinct bonding to both Cu and Sn in the eta phase.