Order-disorder and displacive behavior of the cation (NH+4) sites in the hydrogen-bonded antiferroelectric NH4H2AsO4:: 15N NMR evidence

We report high resolution N-15 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 N-15-enriched single crystals, using the CPMAS technique at a frequency of 50.6 MHz (11.4T). The temperature dependence of the isotropic chemical shift, delta(iso), and of the spin- lattice relaxation time, T-1, 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.