One-dimensional ordering in ferroelectric CsD2PO4and CsH2PO4as studied with neutron scattering

Abstract

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 Cs${\mathrm{D}}_{2}$P${\mathrm{O}}_{4}$ and Cs${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$. The diffuse scattering differs greatly from that observed previously in tetragonal K${\mathrm{D}}_{2}$P${\mathrm{O}}_{4}$, 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 $\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}$ 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 ${T}_{c}$. The width of the scattering along the chain direction has been measured accurately for Cs${\mathrm{D}}_{2}$P${\mathrm{O}}_{4}$ and yields an intrachain correlation of about 600 \AA{} near ${T}_{c}$ (264 K), decreasing to about 140 \AA{} at 315 K. Near ${T}_{c}$, the cross section develops short-range three-dimensional interchain correlations with no appreciable change in linewidth. Very near ${T}_{c}$, the range of the latter is only about 30 \AA{}.