HIGH-PRESSURE ELASTIC NEUTRON-SCATTERING STUDY OF THE INCOMMENSURABILITY IN ND4DC2O4-1 2 D2O (ANOD)/

Several experimental studies have identified, in ND4DC2O4-1/2 D2O (AHO D), the existence of an incommensurate phase above a critical pressure , P-c approximate to 2.6 kbar. This phase, and the transition leading to it, has been well described by a compressible ANNNI model up to app roximately 8 kbar. In order to further test the model, we have perform ed elastic neutron scattering measurements from 7 kbar up to 13 kbar a nd have followed a Bragg peak satellite as a function of temperature. The results show that, for intermediate pressures, the system remains incommensurate over a broad range of temperatures below the transition and that the c/4 and c*/5 lock-in phases appear only at low temperat ures. At higher pressures, the c/3 phase appears to be stable even at 0 K; the reasons for these differences with the rigid ANNNI model are discussed.