Abstract
It is shown that the strong short-range interactions which exist in hydrogen bond ferroelectrics, produce strong anisotropy in the wavevector (q) dependence in the polarisation fluctuations in these systems. It is suggested that this anisotropy is the origin of the observed strong suppression neutron intensity with wavevectors pointing along the ferroelectric axis of KD2PO4. Calculations of the q-dependent susceptibility has been made using an Ising model which includes four spins per unit cell representing the motion of the four deuterons surrounding each PO4 group. The short-range spin-spin interaction reproduces the known energies of the different hydrogen bonds configurations in KD2PO4. The neutron scattering is then calculated using the mean-field approximation, and the anisotropy of the scattering is found to be in good agreement with experiments. Finally in the ice-rule limit of the model, the scattering has a dipolar singularity as q to 0.