POSSIBLE DISORDER OF THE P-ATOM IN KD2PO4

Very high-resolution neutron diffraction data have been collected from KD2P04 (DKPD) at Tc+5K. Analysis of these data shows that the P-atom distribution is highly anharmonic, so much so that it can be modelled as disorder over three sites-a central site at the mean position, and two others displaced symmetrically along k x, where x is the direction that becomes the polar axis of the low-temperature ferroelectric (FE) phase below T,. The displaced sites correspond to the two FE configurations of the PO4 groups. It is concluded that the population of these sites is ≥ 40% in each, leaving only ≤ 20% at the central site. This indicates a much greater preponderance of the FE configurations just above Tc than predicted by the standard model of phase transitions of this type. © 1990 IOP Publishing Ltd.