Sulfate ion motion in ammonium sulfate crystal as studied by low field proton magnetic relaxation of 17O enriched species

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Abstract
Proton T1D and T1 ρ are measured on ammonium sulfate crystal with 0.8% and 7.3% 17O content, at 30 and 10 MHz. A marked reduction is observed in T1D and T1 ρ compared with normal specimens at temperatures above about 200 K. At 30 MHz, T1D showed a minimum at about 240 K while two minima were observed for T1 ρ in the similar temperature range. Both T1D and T1 ρ at 10 MHz showed different behaviors from those measured at 30 MHz. These experimental results are interpreted in terms of H–O dipole modulation by the SO2−4 reorientation and by the fluctuation of the z component of 17O spin due to strong quadrupole relaxation. From the temperature dependence of the relaxation times, an activation energy of 21 kJ mol−1 and a preexponential factor of 3×10−11 s are obtained for the correlation time of the SO2−4 ion reorientation in the paraelectric phase. The correlation time exhibited a discontinuous change at Tc. Anomalous temperature dependence of the correlation time is observed in the ferroelectric phase. This behavior closely resembles that found for ammonium ions.