A new phase in stoichiometric Cu ₆Sn ₅

The intermetallic compound Cu ₆Sn ₅ is a significant microstructural feature of many electronic devices where it is present at the solder-substrate interfaces. The time- and temperature-dependent thermomechanical properties of Cu ₆Sn ₅ are dependent on the nature and stability of its crystal structure, which has been shown to exist in at least four variants (η, η', η ⁶ and η ⁸). This research details an additional newly identified monoclinic-based structure in directly alloyed stoichiometric Cu ₆Sn ₅ using variable-temperature synchrotron X-ray diffraction (XRD) and transmission electron microscopy. The phase is associated with a departure from the equilibrium temperature of the polymorphic monoclinic-hexagonal transformation temperature. The new monoclinic phase can be treated as a modulation of four η ⁸-Cu ₅Sn ₄ unit cells plus one η'-Cu ₆Sn ₅ unit cell. It has been labeled as η ⁴⁺¹ and has cell parameters of a = 92.241 Å, b = 7.311 Å, c = 9.880 Å and β = 118.95° determined from electron diffraction patterns. The XRD results could be fitted well to a Rietveld refinement using the new crystal parameters.