Experimental study of the phase transition in KD2PO4

he ferroelectric phase transition in KD2PO4has been studied using coherent neutron inelastic scattering. The low frequency phonon dispersion relations were determined in the two principal symmetry directions. The results were fitted to a central force rigid ion model which gave good agreement with the acoustic branches and fee lowest frequency optic branches. Group theory was used to simplify all calculations. None of the phonon modes was temperature dependent and therefore the ferroelectric transition was not caused by a soft mode. Quasi-elastic critical scattering was observed, the intensity of which increased as fee transition temperature was approached. The scattering extended throughout each lirillouin zone and was peaked at reciprocal lattice points. The variation of fee intensity in the scattering plane showed that fee displacements of the atoms in the ferroelectric fluctuations were similar to those relating fee paraelectric and ferroelectric phases. The distribution of the intensity around each reciprocal lattice point showed the effect of the macroscopic field associated wife the ferroelectric fluctuations. A simple Ising model wife next nearest neighbour and Coulomb interactions described fee results well. A microscopic model for KDP type crystals is proposed. The model neglects sail dynamic effects, describes the experimental results and shows that the interaction of the phonons with the ferroelectric fluctuations is of more importance than previously thought. The model also predicts the shape of ferroelectric critical scattering to other ferroelectric and antiferroelectric KDP type crystals