Evidence for the dynamic disordering of the NH4 dipoles in the paraelectric phase of (NH4)2SO4 by proton spin thermometry

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Abstract
The proton Zeeman (T1) and dipolar (T1D) relaxation rates were measured in (NH4)2SO4 from 344 to 137 K. Below the phase transition (223 K) from 220 to 190 K, the ratio T1/T1D is about 2 and above Tc, from 224 to 280 K, T1/T1D is about 3. From an analysis of the rf field (H1) dependence of T1ρ at 236 and 210 K, this difference in T1/T1D is attributed to a contribution from cross relaxation to 14N which is comparable to the proton–proton relaxation in the paraelectric phase and negligible in the ferroelectric phase. The cross relaxation appears to be based on the dynamic disorder in the paraelectric phase. Above 300 K, the onset of ammonium diffusion, with activation energy 18±2 kcal/mole, is evident in the temperature dependence of T1D.