CHEMICAL RECONSTRUCTION OF THE TIAL(010) SURFACE

The atomic structure of a clean (010) surface of the ordered binary al loy TiAl (with tetragonal bulk structure of the CuAu I type) is studie d with quantitative low-energy electron diffraction (QLEED). Two diffe rent surface phases are found depending on the preparation procedure. After a cleaning step in vacuo by means of Ar-ion bombardments, anneal s at 750-850 degrees C produce a 2 x 1 surface and anneals at about 90 0 degrees C produce a 1 x 1 surface. A QLEED intensity analysis of the 1 x 1 structure reveals the occurrence of chemical reconstruction, wh ereby the Ti atoms in the first layer exchange places with the Al atom s in the second layer. Thus, while any bulk (010) plane contains 50% A l and 50% Ti, the top atomic layer of a (010) surface contains 100% Al and the second atomic layer contains 100% Ti. Both layers are slightl y buckled and the first interlayer distance is compressed about 7.1% w hile the second interlayer spacing is expanded about 7.4% with respect to the bulk value.