SEGREGATION-INDUCED SURFACE-STRUCTURES OF CARBON-DOPED MO0.75RE0.25(100)

The carbon segregation at the Mo0.75Re0.25(100) surface was investigat ed by means of AES and quantitative LEED structure determination. The equilibrium surface carbon concentration is found to be a function of the bulk concentration and the annealing temperature. With increasing surface carbon concentration four different phases have been observed: c(2 x 2), (3 x 3)diag, p(1 x 1), (1 x 1)(S). The two (1 x 1) phases d iffer substantially both by Auger amplitudes and LEED I(V) spectra whi ch makes evident that they indeed correspond to different structural p hases. Quantitative LEED structure determination of the c(2 x 2) and ( 1 x 1) phases reveals the occupation of top layer hollow sites by carb on. In the case of the (1 x 1)(S) phase carbon atoms reside additional ly in subsurface octahedral interstitial sites, There is good theory-e xperiment agreement as mirrored by a best fit Pendry R-factor of less than or equal to 0.23 for the three structures.