Structural changes of the W(211) surface induced by ultrathin films of Ph,Pt, and Pd

A W(211) surface covered with a thin film (between 0.5 and 1 physical monol ayer) of Ph, Pt, or Pd is found to exhibit an n x 1 superstructure when ann ealed above a threshold temperature of similar to 900 K (500 K for Pd). The superstructure is observed using low energy electron diffraction; phase di agrams are presented to indicate the coverage range and the temperature thr eshold where the new structure appears. Scanning tunneling microscopy resul ts indicate that in the case of Pd/W(211) the superstructure phase is due t o missing overlayer rows. In the case of Rh/W(211), there are two possible interpretations to the observed periodic structure: either the adlayer form s an ordered superstructure or the W surface itself forms a microfaceted st ructure with {110} faces. On the basis of our STM observations of gas adsor ption on the surface, we found the explanation involving microfacets more p lausible. A careful calibration of overlayer coverage is performed using a quartz crystal monitor combined with temperature programmed desorption meas urements performed at high temperatures (up to 2400 K). Auger electron spec troscopy used in combination with thermal annealing series demonstrates tha t overlayer coverages above one physical monolayer are thermally unstable, which is explained by the formation of an ultrathin alloy film.