Raman investigation of pretransition behaviour in potassium dihydrogen phosphate crystals and study of coupling between the PO4 distortion and hydrogen atom disorder in these crystals

Abstract

The mixed nature of the ferroelectric phase transition in potassium dihydrogen phosphate (KDP), involving both a displacive component associated with the distortion of PO₄ ions and an order–disorder component arising from H-atom disorder in O–H–O′ hydrogen bonds, was investigated using Raman scattering spectroscopy. By analyzing the subtle changes in the Raman frequencies of the crystal in the temperature range Tc < T < 173 K, it is argued that, prior to undergoing the ferroelectric phase transition at Tc, the crystal passes through a pretransitional phase within the temperature range Tc < T < 173 K. In this pretransitional phase, a large number of micropolar regions begin to form within the crystal owing to the dipolar distortion of PO₄ ions induced by the oscillating electric field generated by disordered H atoms. As the temperature approaches Tc, the size of these micropolar regions increases, and at Tc the crystal finally becomes polar.