High Resolution NMR Evidence for Displacive Behavior in Hydrogen-Bonded Ferroelectrics and Antiferroelectrics

The high NMR spectral resolution attained via magic angle spinning (MAS-NMR) on single crystals have enabled us to detect new aspects of mechanism of phase transitions in hydrogen-bonded ferroelectrics and antiferroelectrics. Results are summarized for anomalous changes in the temperature dependence of the isotropic part of the NMR chemical shifts for ¹³C and ¹⁷O in squaric acid (SQA), and for ³¹P from NH₄H₂PO₄, KD₂PO₄ and RbH₂PO₄, and more recently on ¹⁵N in NH₄H₂AsO₄ and NH₄H₂PO₄.

These data are interpreted as providing an unambiguous evidence of a displacive component, together with an order-disorder behaviour at the same time scale for both SQA and KDP-type lattices.

Keywords:

• KDP-type ferroelectrics
• order-disorder phase transition
• displacive phase transition
• high resolution NMR

Acknowledgments

This research was supported in part by a grant from the National Science Foundation and Florida State University.

Paper originally presented at IMF-11, Iguassu Falls, Brazil, September 5–9, 2005; received for publication January 26, 2006.