HE-ATOM-SCATTERING STUDIES OF THE STRUCTURE AND DYNAMICS OF THE KMNF3(001) SURFACE AND THE 186-K PHASE-TRANSITION

A phase transition at the surface of KMnF3(001) is observed by helium-atom-scattering diffraction and surface-phonon-dispersion-curve measurements at T(c)s = 191 K. This transition temperature is only slightly above the corresponding bulk phase transition at T(c)b = 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 MBAR point. Critical scattering is observed up to 320 K. At temperatures higher than almost-equal-to 420 K an irreversible change in the diffraction intensity is observed, indicating a transition to a partially disordered surface.