A tetragonal Cu20Bi9 intermetallic compound phase discovered in Cu75Sn13Bi12 alloy

This study reports the discovery and characterization of a previously unknown intermetallic compound phase, Cu20Bi9, within Cu75Sn13Bi12 alloy solidified both onboard China’s Space Station and under ground-based conditions. Combined with selected-area electron diffraction (SAED), atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive x-ray spectroscopy elemental mapping, its crystal structure was identified to be tetragonal (P4¯, No. 81) with lattice parameters a = b = 17.648 Å and c = 10.534 Å. A crystallographic model consisting of 80 Cu and 36 Bi atoms in a single unit cell was proposed, and its dynamic stability was validated by density functional theory-based molecular dynamics simulations. SAED patterns and HAADF images simulated based on this model perfectly reproduce the experimentally observed diffraction and imaging characteristics. Theoretical calculations of the formation and cohesive energies suggest that Cu20Bi9 is thermodynamically metastable, which is further supported by isothermal annealing experiments.