Soft modes and the structure, spontaneous polarization and Curie constants of perovskite ferroelectrics: tetragonal potassium niobate

Publisher Website
Google Scholar
Abstract
The structure of potassium niobate KNbO3 in the ferroelectric tetragonal phase has been determined at 270 degrees C by neutron diffraction. The atomic displacements along (001) from the cubic perovskite positions are Delta (K)=-0.030+or-0.035 AA, Delta (Nb)=0.064 AA, Delta (OI)=-0.095+or-0.012 AA and Delta (OII)= Delta (OIII)=-0.102+or-0.012 AA. The phase transition is seen as the condensation of a soft lattice vibrational mode in which comparatively rigid oxygen octahedra vibrate against the K and Nb atoms. The spontaneous polarization, Curie constant and entropy change for this and the other perovskite ferroelectrics BaTiO3, PbTiO3 and K(Ta, Nb)O3 have been calculated according to Cochran's soft mode theory. The agreement with experiment is satisfactory.