Order-Disorder and Displacive Behavior of the Cation (NH₄⁺) Sites in the Hydrogen-Bonded Antiferroelectric NH₄H₂AsO₄: ¹⁵N NMR Evidence

We report high resolution ¹⁵N NMR measurements of the paraelectric to antiferroelectric phase transition (T_N = 216 K) of the model hydrogen-bonded antiferroelectric NH₄H₂AsO₄ (ADA). We specifically examined whether the NH₄ ⁺ ions undergo a displacive or an order-disorder behavior at the phase transition. High resolution NMR measurements were made on ¹⁵N-enriched single crystals, using the CPMAS technique at a frequency of 50.6 MHz (11.4T). The temperature dependence of the isotropic chemical shift, δ_iso, and of the spin-lattice relaxation time, T₁, shows clear anomalies at the phase transition. These results are interpreted as evidence for the coexistence of an order-disorder and displacive behavior at the NH₄ ⁺ site.

Keywords:

• ¹⁵N NMR
• hydrogen bonding
• antiferroelectric transition
• order-disorder phase transition
• displacive phase transition

Acknowledgments

Paper originally presented at IMF-11, Iguassu Falls, Brazil, September 5–9, 2005