Abstract
X-ray scattering techniques have been used to study the critical fluctuations and domain walls of KDP and of DKDP. Above Tc the shape of the critical scattering about each reciprocal lattice vector is shown to be very similar in DKP and DKDP showing that the static properties of the fluctuations are very similar even though their dynamics are quite different. The shapes observed in both the (h0l) and (hk0) planes are consistent with the known properties of fluctuations in DKDP and the elastic constants. The intensity of the critical scattering near to different reciprocal lattice points was determined and used to find the atomic displacements in the ferroelectric fluctuations. The results are consistent with the results of NMR and of accurate structure determinations. The X-ray scattering from the domain walls was observed below Tc and enabled measurements to be made of the width of the domain walls and of the atomic displacements in the walls. The width of the domain walls increases as the temperature is raised towards Tc.