ONE-DIMENSIONAL ORDERING IN FERROELECTRIC CSD2PO4 AND CSH2PO4 AS STUDIED WITH NEUTRON-SCATTERING

The triple-axis neutron technique has been used in a detailed study of the intensity distribution and temperature dependence of the diffuse quasielastic scattering associated with the ferroelectric transitions in monoclinic CsD2PO4 and CsH2PO4. The diffuse scattering differs greatly from that observed previously in tetragonal KD2PO4, in which case the intensity distribution clearly exhibits characteristics of three-dimensional dipolar interactions. In the present study, the nature and extent of the diffuse scattering can be explained quantitatively on the basis of chainlike ordering with temperature-dependent interchain correlations. The variation in intensity measured perpendicular to the chains (which are parallel to the ferroelectric b axis) is mostly due to variations in the dynamical structure factor of a hydrogen-bonded chain of phosphate groups, and approaches a purely-one- dimensional form at temperatures well above Tc. The width of the scattering along the chain direction has been measured accurately for CsD2PO4 and yields an intrachain correlation of about 600 near Tc (264 K), decreasing to about 140 at 315 K. Near Tc, the cross section develops short-range three-dimensional interchain correlations with no appreciable change in linewidth. Very near Tc, the range of the latter is only about 30. © 1979 The American Physical Society.