# Phonon anomalies in displacive phase transitions by surface x-ray scattering

Author(s): G. Landmesser, T.R. Finlayson, R.L. Johnson, M. Aspelmeyer, A. Plech, J. Peisl, U. Klemradt

Journal: J. Phys. IV France

Volume: 112

Page(s): 123-127

Year: 2003

DOI Number: 10.1051/jp4:2003848

Phonon anomalies in displacive phase transitions have long been suspected to aid the nucleation of the new phase. In materials with strong but incomplete phonon softening, the crystal surface might additionally play an important role, since the enhanced softening of associated surface phonons effectively lowers the nucleation barrier. X-ray scattering under grazing angles is discussed with regard to experimental studies of such phase transitions between 2 and 3 dimensions, as in this geometry all x-ray techniques provide information from a controlled depth below the surface adjustable from nanometers to microns. However, the sensitivity to premonitory phonon softening varies greatly with the scattering technique employed. It is argued that soft modes confined to a narrow interval in q need not show up in the Debye-Waller factor while giving a strong signal in thermal diffuse scattering. Synchrotron radiation data from Ni 2MnGa provide evidence for this behavior and confirm also the feasibility of depth-resolved studies of the [ $\zeta$ $\zeta$0] TA 2 phonon softening.