Nature of Ferroelectricity in KNO3

The lattice parameters of KN${\mathrm{O}}_3$ were measured in the temperature region of the ferroelectric transition. The changes in the lattice parameters as the crystal undergoes the paraelectric-to-ferroelectric transition have been interpreted as being due to the electrostriction effect. The dielectric constants and losses along the ferroelectric axis have been measured in the audio, megacycle, and microwave frequency regions. The occurrence of a relaxation type of dielectric dispersion in the microwave region, together with the contraction of the ferroelectric axis at the transition, suggests that ferroelectricity in KN${\mathrm{O}}_3$ is an order-disorder phennomenon. The observed first-order ferroelectric transition is caused by the large lattice-dipole coupling. A statistical theory based on a double well potential for the nitrate ion and on the assumption of a lattice-dependent internal field explains the origin of the electrostriction effect and the observed anomalies of the ferroelectric transition. The statistical theory is related to Devonshire's thermodynamic theory of the ferro-electric transition, and good agreement is obtained between the theoretically predicted and the experimentally determined coefficients of Devonshire's free energy.