High-temperature oxygen monolayer structures on W(110) revisited

Systematic studies of the two high-temperature monolayer oxygen structures that exist on the (110) tungsten surface were performed using low-energy electron microscopy and diffraction measurements. Our work questions the commonly accepted interpretation from the literature that striped oxygen superstructures arise from alternating site-exchanged (S-E) domains. We postulate that the superstructures originate from a misfit between tungsten and oxygen lattices while the striped appearance corresponds to a moir\'e pattern. Moreover, we show that the two structures, indicated as 113- and 337-phases due to the characteristic directions of the respective moir\'e patterns, differ considerably in their symmetry properties. This suggests that oxygen atoms in the two overlayers occupy different adsorption sites on average. In particular, the 113-phase features rotational domains that retain mirror symmetries with respect to the [001] and [1-10] directions, whereas the 337-phase is characterized by the appearance of additional domains due to the breaking of these symmetries. We propose structural models for both phases that consistently explain their unusual properties and suggest a universal mechanism for the thermal evolution of oxygen monolayer adsorbed on W(110).