Magic angle spinning NMR study of the ferroelectric transition of KH2PO4: definitive evidence of a displacive component

Abstract

Variable temperature magic angle spinning (MAS) NMR measurements are reported on 1H and 31P nuclei in KH2PO4 (KDP) in the vicinity of its paraelectric–ferroelectric phase transition temperature, Tc, of 123 K, to examine the transition mechanism, in particular if this is a model order–disorder type or whether it also involves a displacive component. It has been well established that the temperature variation of the isotropic chemical shift, δiso, in NMR measurements of the nuclei directly involved in the transition should remain constant or change smoothly through Tc for an order–disorder type transition but it should show an anomalous change for a displacive one. Here we demonstrate that the δiso for both 31P and 1H nuclei in KDP show clear anomalies as a function of temperature around KDP’s Tc, providing direct evidence of a displacive component for the phase transition of KDP in contrast to the generally accepted notion that it is a model order–disorder type.