Phase transitions in C2O4HNH4, 1/2 H2O. A light scattering study at normal pressure

Abstract

C2O4HNH4, 1/2 H2O undergoes, at Tc = 145 K, a second order ferroelastic transition. The present Raman study allows us to identify the soft degree of freedom responsible for the transition as the ordering of one family of NH+4 ions. This ordering process appears as a central peak which displays a critical narrowing in the vicinity of Tc. Nevertheless, its linewidth is minimum not at Tc but below Tc . The combined analysis of its intensity and linewidth shows that this unusual behaviour is due to the evolution of the individual ionic relaxation time. The latter is strongly modified, below Tc, by the inequivalence between the two potential wells, as well as by the change of the potential barrier. This model gives quantitative agreement with our results

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

“…Let us first note that Albers and Kippers measurements agree with the model developed here: as was already shown in [5], the four pseudospins contained in the high temperature unit cell generate, at q = 0, four variables belonging to four different irreducible representations.…”

“…One is in the B2g repreientation, which dynamics has been studied in [5] and in this paper; a second is in the B3u representation and is responsible for the results reported in [15]; a third is in the Big representation, active in Raman spectroscopy and its dynamics was also detected in [5], while the last, Au representation is silent in all these techniques. Each of these variables has its own freezing temperature, in agreement with [16], though their dynamics are diffusive, and not propagative as suggested by Poon. Nevertheless, as can be inferred from section 3, One may also analyse, within the interaction model developed in section 3, what would be the effect of taking into account Poon's type of coupling.…”

“…These two orientations can be represented by the two values of a one half pseudopsin, and one of the linear combinations of the four pseudospins existing in the unit cell generates a spin variable with the B2g symmetry. This These Raman experiments enabled one of us [5] to analyse the NH( individual reorientational dynamics. It was found that this dynamics could be described by a Markovian process, characterized by a mean life time, T, in a given orientation, which, in phase I, was simply governed by an Arrhenius process T = To exP <Pi)…”

“…Raman scattering experiments [5,6] revealed, in the vicinity of this transition, the existence of a central peak visible in the B2g <ac> geometry, the intensity of which increased while its linewidth decreased close to the transition. These Brillouin and Raman experiments were interpreted as the result of a strong coupling between the NH( ordering process, and the e5 shear deformation.…”

“…Another set of Raman scattering experiments Were also performed under hydrostatic pressure [5,7]. It showed that, below a critical temperature Tc = 146 K, and above a critical pressure Pc = 2.6 kbar, the second order phase transition leads to a new phase, called phase III; by decreasing further the temperature, at constant pressure, phase II was recovered through a first order phase III phase II transition.…”