A Pressure-Induced Incommensurate Phase in Ammonium Hydrogen Oxalate Hemihydrate

Abstract

We report evidence for the existence of a new incommensurate phase in a crystal of ammonium hydrogen oxalate hemihydrate. This phase is remarkable in two aspects: it exists only above a critical pressure P,, and the incommensurate wave vector, which is parallel to the vector e* of the reciprocal lattice, has the largest variation ever reported, varying continuously from 0.147c* at 4.3 kbar to -0.25c* at the maximum pressure (8 kbar) used to date. This letter reports results of a collaborative research, performed by complementary techniques, which has led to the discovery of an incommensurate phase in an insulating material, C204HNH4, BHzO, with two very unusual features:

Phase transitions in C2O4H NH4 1/2 H2O : a light scattering study of the high pressure phase

“…A new phase, called phase III, has been discovered, phase which exists only above the pressure (Py ) and below the temperature ( TY ) of a triple point, the coordinates of which are different for AHO and AHOD. Phase III has been later identified as an incommensurate phase [6] and the analysis of our data suggests a possible mechanism for its existence. Consequently, the present paper is divided as follows.…”

“…It was proposed that phase III would be an incommensurate phase, characterized by a wave vector qo which remains parallel to a given direction qo, but with a modulus varying strongly with pressure and not with temperature. This interpretation was confirmed by a neutron elastic experiment [6] which showed that qo was parallel to the cristallographic axis The two low frequencies modes of figure 4 were then found to be two acoustic phonons with wave vector qo, which are detected in the incommensurate phase through their coupling with the frozen-in modulation. Using the elastic data of I, these acoustic branches were identified as the LA mode (upper branch) and the TA mode, b polarized (lower branch of Fig.…”

“…Finally our results are discussed in section 4. We first recall (paragraph 4.1) how the behaviour of new, low frequency modes, detected in phase III has been explained [6] by the fact that phase III was incommensurate, with a large variation with pressure of the incommensurate wave vector. We then propose (paragraph 4-2) a model for the phase transition which takes into account the ordering of the NH' ions and their possible coupling with an acoustic phonon.…”

“…This means that, along this transition line, the application of pressure does not substantially modidy the interactions governing the phase I-phase II transition. Finally, from the extrapolation of the higher pressure results, the two transition lines [1][2][3][4][5][6][7][8][9][10][11] and II-III are found to have the same tangent at the Y point, as is predicted in a Landau theory with two competing order parameters [8]. On the other hand, extrapolating from the low pressure side yields no change of slope between the 1-11 and the 1-111 transition lines, a result which is not predicted by the theory but too uncertain to be further discussed.…”

Phase transition in C2O4DND4, [Formula: see text]D2O : a coherent inelastic neutron scattering study

“…This suggestion was confirmed by X ray [8] and elastic neutron experiments [9] : qo was found to remain parallel to c* and to vary, for AHOD, from at most 0.06 c* at P = 2.8 kbar to at least 0.24 c* at 8 kbar.…”