Neutron-diffraction studies of the heavy-atom structure in h-ordering materials

Abstract

We have made a high-resolution neutron-diffraction study of KD₂PO₄ (DKDP) at T_c+5K. The results indicate that above T_c, in the paraelectric phase, the thermal motion of the P atom is highly anharmonic along the z axis, and can be better modelled using a disordered three-site distribution. Analysis of the P-atom thermal motion in a neutron-diffraction study of KH₂PO₄ (KDP) at 16.5kbars and 10K, close to the point where T_c ↪ ok, indicates that the separation of the disordered P-atom sites remains relatively unchanged on cooling or under pressure. This may suggest that T_c is not directly related to the P-atom separation.