Formation of ferroelectric phases in KNbO3 and other niobates with perovskite structure

The Curie-Weiss temperatures T-CW of NaNbO3 and AgNbO3 are determined for the first time by extrapolating the temperature dependence of the reciprocal permittivity from the cubic phase. It is established that the values of T-CW of ANbO(3) perovskites (A = Na, Ag, K) are practically equal, i.e., that the cubic phases of all the compounds are potentially unstable to an almost equal extent with respect to polarization. From an analysis of the EXAFS spectra of the NaNbO3 and KNbO3 cubic phases, it follows that the potential-energy surfaces of the Nb cation in both compounds are similar and exhibit eight minima shifted by 0.16 and 0.19 angstrom, respectively, from the oxygen-octahedron center along the [111]-type directions. Based on these facts, it is inferred that the ANbO(3) instability with respect to the occurrence of polarization is due to the tendency of the Nb cations to be ordered over the eight minima and that the phase transitions induced by this instability are of the order-disorder type. A statistical model capable of describing the sequence of ferroelectric phase transitions occurring in KNbO3 and in (Ba, Sr)TiO3 is proposed and studied.