Transition to body-centered cubic structure in Au thin films under electron-beam irradiation

Gold (Au) is well known to transition to a body-centered cubic (bcc) structure at high temperature and pressure. However, this study shows that such conditions may not be necessary. In this study, Au thin film specimens for transmission electron microscopy (TEM) undergo a transition from face-centered cubic (fcc) to bcc structure under electron-beam irradiation in TEM. Some regions in the Au thin film specimens are selectively thinned during electron-beam irradiation. The thin regions are considered to be subjected to the localization of energy deposition by electronic excitations by incident electrons due to their finite size, which causes the regions to be heated. Of the excitations, Auger decays are a possible source of Au heating. The temperature rise due to Auger decays is successfully simulated to be accompanied by stress development in the thin regions. Sometimes the thin regions transition from face-centered cubic (fcc)-to-bcc phases. The fcc matrix and the bcc phase have orientation relationships similar to those found in the fcc-to-bcc phase transformation in steels, which indicate that the transition to the bcc structure occurs through a displacive mechanism. It is suggested that the phase transition is facilitated for thin film morphology and that crystal defects caused by electron-beam irradiation and stress development stabilize the more open bcc structure.