High Pressure Elastic Neutron Scattering Study of the Incommensurability in ND ₄DC ₂O ₄-1/2 D ₂O (AHOD)

(Received 20 April 1994, received in final form 20 October 1994, accepted 27 October 1994)

Abstract
Several exprimental studies have identified, in ND ₄DC ₂O ₄-1/2 D ₂O (AHOD), the existence of an incommensurate phase above a critical pressure,  kbar. This phase, and the transition leading to it, has been well described by a compressible ANNNI model up to approximately 8 kbar. In order to further test the model, we have performed elastic neutron scattering measurements from 7 kbar up to 13 kbar and have followed a Bragg peak satellite as a function of temperature. The results show that, for intermediate pressures, the system remains incommensurate over a braod range of tempertures below the transition and that the and c lock-in phases appear only at low tempertures. At higher pressures, the phase appears to be stable even at 0 K; the reasons for these differences with the rigid ANNNI model are discussed.