Structural evidence for centre-of-mass motion in the ferroelectric fluctuation of KDP and DKDP

Abstract
Using high-resolution neutron diffraction data, the thermal vibration amplitudes of the K, P, and O atoms, and the positions of the H(D) atoms have been determined as a function of temperature near Tc for KH2PO4 (KDP) and 95%-deuterated K(DxH1-x)2PO4 (DKDP). The data were collected from KDP at 2K above Tc (123K) and at 1, 10 and 20K below Tc, and from DKDP at 2K above Tc (224K) and at 2, 6 and 10K below Tc. The results show changes in some thermal amplitudes and in the H(D) positions, at and below Tc, that strongly support a model of the ferroelectric fluctuation revised to include a local fluctuation of the centre of mass along the ferroelectric axis.

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