Temperature dependent sign reversal of the surface contraction of Ag(111)

We have studied the structure of the Ag(111) surface as a function of temperature using medium energy ion scattering. We find that the first interlayer spacing is contracted at temperatures below 670 K, but increases at higher temperatures in a nonlinear fashion and is expanded same 10% 80 K below the melting point. As the surface vibration amplitudes increase significantly at the same time, we attribute this sign reversal to enhanced anharmonic effects at the surface.