NH+3 as a paramagnetic probe of molecular motion: Application to phase transitions in (NH4)2SO4-type solids

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Abstract
Using line broadening effects in the EPR of NH+3 radicals, correlation times (τ) were measured over a wide temperature range around the ferroelectric transition (Tc) of (NH4)2SO4. An anomalous decrease in τ is observed well above Tc, as is a critical slowing down and coexistence of para- and ferroelectric phases about Tc, suggesting a Landau-type mean-field approach as an alternative model for this improper ferroelectric. The good agreement between the τ for the NH+3 fluctuations measured with EPR and that for the NH+4 measured earlier via NMR, infrared, and neutron scattering over a wide temperature range, suggests that the NH+3 radicals can serve as sensitive probes of the low frequency (∼1010 Hz) reorientations of the NH+4 ions in the NH4+-containing lattices.