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 (TN = 216 K) of the model hydrogen-bonded antiferroelectric NH4H2AsO4 (ADA). We specifically examined whether the NH + 4 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, δiso, 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.