Oxygen adsorption on the LaB₆(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ∼3.8 eV by an oxygen exposure of ∼2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ∼2 L where the work function has a maximum value of ∼4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ∼2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ∼4.4 to ∼3.8 eV until an oxygen exposure of ∼100L. The initial sticking coefficient on LaB₆(110) has the highest value of ∼1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.