Structural Transitions

As a first example we present the study of Thermal motions in AuCu3 near the order-disorder phase transition (Tc=668K) performed on D9. The outstanding feature of the results is that whereas at low temperatures an anharmonic treatment of thermal motion is necessary to give an adequate account of the data, at Tc-4 thermal motions are harmonic with however strong anisotropy of the copper atoms; the amplitude is increased perpendicular to the plane of their four gold nearest neighbours. Such behaviour suggests that phonon anomalies may be responsible for the first order transition.

As a second example we chose a high-resolution study of KDP and its deuterated equivalent. Experiments on these materials have been going on for several years, but experiments close to the ferro-electric phase transition have enabled a better understanding of the phase transition in these "classical" materials. D9 seems to be the best instrument for these kinds of studies. High-resolution diffraction data from monodomain crystals, obtained by applying an electric field, were collected at several temperatures in the ordering range below the tetragonal to orthorhombic phase transition. The results support a picture of the ferro-electric soft mode above Tc that includes local centre-of-mass fluctuations (freezing out below Tc) and show the spontaneous polarization, the proton ordering (Figure below) and the heavy atom displacements below Tc, all have the same temperature dependence.

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Proton distribution in KDP at Tc+2 and Tc-1.3 in the a-b plane.

Last updated by Andrew Crowe on 12/02/1996