Pretransitional Phenomena at the First-Order Phase Transition in LaNbO₄

Abstract

X-ray diffraction experiments have been performed near the ferroelastic first-order phase transition temperature in lanthanum niobate LaNbO₄, which transforms from a tetragonal phase to a monoclinic phase at T_c=768, K with a discontinuous change of the lattice parameter Δβ =0.4°. With regard to the temperature dependence of the Bragg reflection near T_c, such pretransitional phenomena as decreasing of the peak intensity and broadening of the profile were observed. Furthermore, in the high temperature phase, as the temperature was close to T_c, the anisotropic diffuse scattering around the Bragg reflections extended to the directions where the Bragg reflections in the low temperature phase appeared was observed. These behaviors were contrasted with the well known precursor phenomena of the martensitic phase transformation in metals and alloys.