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 micros cope (TEM) has been used to observe the order-disorder phase transform ation on the Cu3Au(111) surface. The samples are spherical Cu3Au singl e 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 b ulk crystal, respectively, have been seen by RHEED. A long-period stru cture has also been found on the ordered 2 X 2 surface. The surfaces u nder different sample treatments have been imaged with REM. It was est ablished 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, sim ilar to the clean Au(111) surface, whereas the latter was flat on a lo cal scale but rough and wavy on a large scale. The dynamic change of t he 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 formatio n of the tetragonal long-period Cu3Au(II) phase during the bulk disord er-order transition. The results indicate that REM and RHEED are very useful techniques for studying surface phase transformations and morph ology changes.