  Strain relaxations of a p(1x2) ordered oxygen layer on W(110) are measured as
a function of temperature across the disordering transition using low-energy
electron diffraction. The measured strains approach values of 0.027 in the
[1-10] and -0.053 in the [001] direction. On the basis of the measured strain
relaxations, we give quantitative information on temperature-dependent surface
stress using the results of ab initio calculations. From the surface formation
energy for different strains, determined by first-principles calculations, we
estimate that surface stress changes from -1.1 for the ordered phase to -0.2N/m
for the disordered one along [1-10], and from 5.1 to 3.4 N/m along [001].
Moreover, our observation that the strains scale inversely with domain size
confirms that the strain relaxation takes place at the domain boundaries.
