Diffuse scattering due to anisotropic phonon softening in ferroelastic compounds NdNbO4 and LaNbO4

X-ray diffraction studies have been performed in a ferroelastic compound Nd NbO4 by synchratron radiation at BL02B1 in SPring-8. Anisotropic diffuse sc attering around Bragg reflections is observed prior to the phase transition . The diffuse steaks extend to the directions deviated from the crystal axe s by 20 degrees 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 degrees is obtained. The origin of the diffus e scattering is considered for the mast part the anisotropic phonon softeni ng of the transverse acoustic mode at the Brillouin zone center. The deviat ion from the axes is caused by the low symmetry of the tetragonal phase wit h 4/m point group. The propagating directions of the soft phonon coincide w ith the orientations of the domain boundaries in the low temperature, monoc linic phase.