INCOMMENSURATE STRUCTURAL PHASE-TRANSFORMATION IN NA2CO3

Na2CO3 has an incommensurate phase transition at 619.5 K. The temperature dependence of a satellite reflection and of the length of the modulation wave vector q→0 has been investigated below this temperature using neutron diffraction. The transition has been found to be of second order with |q→0| decreasing steeply when approaching Tc. Also, the critical diffuse scattering along q→0 has been measured in the incommensurate and in the normal high-temperature phase. Apart from a diffuse peak at q→0, a strong diffuse streak is observed extending from main reflections in the direction of q→0. The energy spectra of this diffuse scattering are overdamped with a q-dependent width. The diffuse scattering is in evidence of a soft transverse acoustic branch in the β phase. At high temperatures the branch is soft over a large distance along q→0. On lowering the temperature it hardens but softens again at q→0, thus giving rise to the incommensurate structural deformation. Below 300 K the temperature dependence of the components of q→0 has been measured. In this temperature range the sum of the components is constant: q1+q3=12. At 130 K the wave vector locks-in to the commensurate value q→c=16a→*+13c→*. It is shown on the basis of Landau theory that the commensurate phase is nonpolar. © 1979 The American Physical Society.