REM IN A UHV TEM FOR THE OBSERVATION OF DYNAMIC PHASE-TRANSFORMATION PROCESSES ON THE CU3AU(111) SURFACE

Reflection electron microscopy (REM) in a transmission electron microscope (TEM) has been used to observe the order-disorder phase transformation on the Cu3Au(111) surface. The samples are spherical Cu3Au single crystals with (111) facets. The observation is in situ over a range of temperatures in UHV environment. Stable 1 X 1 and 2 X 2 structures, corresponding to the ideal truncation of the disordered and ordered bulk crystal, respectively, have been seen by RHEED. A long-period structure has also been found on the ordered 2 X 2 surface. The surfaces under different sample treatments have been imaged with REM. It was established that atomically flat surfaces can be obtained on the cleaned sample, but the surface morphology of the disordered crystal was quite different from that of ordered crystal. The former was very flat, similar to the clean Au(111) surface, whereas the latter was flat on a local scale but rough and wavy on a large scale. The dynamic change of the surface morphology in the phase transformation process has, for the first time, been observed directly by an electron microscopy method. The surface morphology change is believed to be caused by the formation of the tetragonal long-period Cu3Au(II) phase during the bulk disorder-order transition. The results indicate that REM and RHEED are very useful techniques for studying surface phase transformations and morphology changes.