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.