PHASE-TRANSITIONS IN C2O4DND4,1/2D2O - A COHERENT INELASTIC NEUTRON-SCATTERING STUDY

The low frequency dynamics related to the second order transition which takes place in ammonium deuterated oxalate has been investigated by coherent inelastic neutron scattering. This transition leads to a ferroelastic phase at zero pressure and to an incommensurate phase at 5kbar. At both pressures, our experiments show that the coupling between a reorientation motion of one family of ND4+ ions and the TA phonons propagating along c* and polarized along a drives the transition, the ND4+ being coupled through a pressure dependent ANNNI model. Weak changes in the parameters explain the changes with pressure of the static and dynamical properties of the transition.