Diffuse scattering due to anisotropic phonon softening in ferroelastic compounds NdNb04 and LaNO4

X-ray diffraction studies have been performed in a ferroelastic compound NdNbO4 by synchrotron radiation at BL02B1 in SPring-8. Anisotropic diffuse scattering around Bragg reflections is observed prior to the phase transition. The diffuse streaks extend to the directions deviated from the crystal axes by 20 deg on the tetragonal basal plane of the high temperature phase. Diffuse streaks in LaNbO4 have been also observed by usual X-ray source, and the deviation angle of 23 deg is obtained. The origin of the diffuse scattering is considered for the most part the anisotropic phonon softening of the transverse acoustic mode at the Brillouin zone center. The deviation from the axes is caused by the low symmetry of the tetragonal phase with 4/m point group. The propagating directions of the soft phonon coincide with the orientations of the domain boundaries in the low temperature, monoclinic phase. (author)