Note on the Lattice Dynamical Aspect of Ferroelectric Modes of KDP

Lattice dynamical consideration of the ferroelectric mode of potassium dihydrogen phosphate (KDP) is presented. All interaction parameters are calculated from the first principle and the dynamical matrix is reduced by the group theoretical technique. Six optically active modes (plus one rigid translational mode parallel to the c -axis), which give rise to the electric polarization parallel to the c -axis, are obtained and their eigen-frequencies and eigen-vectors are compared with experiments. One of these optically active modes is identified with the ferroelectric mode (soft mode). Furthermore the distortional motion of the oxygen tetrahedron is shown to play a very important role in the soft mode.