ORIGIN OF THE 1ST-ORDER PHASE-CHANGE AT THE CURIE-TEMPERATURE IN KNBO3

As judged by the difference between Curie temperature T(c) and Curie-Weiss temperature (H) (T(c) - (H) approximately 53-degrees-C) KNbO3 goes through a very strongly first-order phase change at T(c) into a tetragonal ferroelectric phase. For BaTiO3, it is known that the weaker first-order change at T(c) (T(c) - (H) approximately 10-degrees-C) derives from the electrostrictive coupling and that the strain-free crystal would go directly to rhombohedral symmetry by a second-order transition. It is the purpose of this note to point up the fact that in spite of the much stronger first-order change in KNbO3, the origin is again in the electrostrictive coupling, and that the cubically clamped crystal would go through a second-order phase change.