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

Variable temperature magic angle spinning (MAS) NMR measurements are reported on H and P nuclei in KHPO (KDP) in the vicinity of its paraelectric-ferroelectric phase transition temperature, T , 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, δ , in NMR measurements of the nuclei directly involved in the transition should remain constant or change smoothly through T for an order-disorder type transition but it should show an anomalous change for a displacive one. Here we demonstrate that the δ for both P and H nuclei in KDP show clear anomalies as a function of temperature around KDP's T , 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.