Abstract
A phase transition at the surface of KMnF3(001) is observed by helium-atom-scattering diffraction and surface-phonon-dispersion-curve measurements at Tcs=191 K. This transition temperature is only slightly above the corresponding bulk phase transition at Tcb=187.5 K. As in the bulk, the phase transition is found to be weakly first order and is driven by the softening of the Rayleigh mode at the M¯ point. Critical scattering is observed up to 320 K. At temperatures higher than ≊420 K an irreversible change in the diffraction intensity is observed, indicating a transition to a partially disordered surface.