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

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 (O_I)=-0.095+or-0.012 AA and Delta (O_II)= Delta (O_III)=-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.