The Lower Curie Point of Ferro-electric Salts

Abstract

COMPARATIVELY little attention has been given to the behaviour of the group of ferro-electric salts analogous to potassium dihydrogen phosphate (KH₂PO₄) at the so-called ‘lower Curie point’. In this region dielectric measurements in an alternating field show a rapid decrease in the magnitude of the reversible polarization with decreasing temperature¹. There has been indirect evidence that this can be attributed simply to the sharp rise in the coercive field, itself unexplained, which has been observed to accompany it, and that the domains of the crystal retain their high spontaneous polarization at lower temperatures. This concept of the ‘freezing-in’ of the electric moment has been supported by the lack of a specific heat anomaly as observed in this salt², and in the case of potassium di-deuterium phosphate by the constancy of the saturation polarization as inferred from electro-optical measurements³, and by the absence of any change detectable by X-ray analysis; an angular deformation has been clearly shown by this method to accompany the onset of spontaneous polarization at the upper Curie point⁴. While in the Rochelle salt group the lower Curie point corresponds to a real disappearance of the spontaneous polarization⁵, no direct experimental evidence exists in any published work for the potassium dihydrogen phosphate group of ferro-electric salts.