Origin of the first-order phase change at the Curie temperature in KNbO3

As judged by the difference between Curie temperature Tc and Curie-Weiss temperature Θ (Tc−Θ∼53 °C) KNbO3 goes through a very strongly first-order phase change at Tc into a tetragonal ferroelectric phase. For BaTiO3, it is known that the weaker first-order change at Tc (Tc−Θ∼10 °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.