Structural study of potassium niobate between 200 and 823 K

The lattice parameters and structures of the four phases of pure potassium niobate (KNO) have been determined at 10 K intervals between 200 and 823 K. The experiments were performed on the high-resolution powder diffractometer at the neutron spallation source, ISIS facility, Rutherford Appleton Laboratory. A similar experiment performed on barium titanate (BTO) was reported recently. We have analysed the results obtained for KNO in a similar way and make comparisons between the two materials; both show the same sequence of ferroelectric transitions; rhombohedral - orthorhombic - tetragonal- cubic with increasing temperature. As in BTO, the only deformation of the octahedron in KNO is due to electrostrictive coupling between strain and the spontaneous polarisation; other distortions allowed by symmetry are vanishingly small. We have calculated the lattice parameters from an expansion of the Gibbs free energy in even powers of the spontaneous polarisation with excellent agreement with experimental values; the coefficients in the expansion were found by a least-squares technique. Using these values with all sixth-order terms in the expansion the transition temperatures are reproduced well, which was not the case for BM. Using apparent charges and "absolute" displacements the spontaneous polarisation can be calculated giving excellent agreement with experiment. Analysis of the temperature dependence of the Debye-Waller factors in the cubic phase indicates a softening of the one-particle potential for niobium. The bond valencies of both potassium and niobium show a small, abrupt change at the cubic-tetragonal transition.