Spin–Lattice Relaxation Times in Deuterated Ferroelectric Ammonium Sulfate and Fluoroberyllate

The temperature dependences of the deuteron spin–lattice relaxation times ${\mathrm{(T}}_1)$ in deuterated ammonium sulfate, (ND₄)₂SO₄, and ammonium fluoroberyllate, (ND₄)₂BeF₄, have been studied by transient methods over the range 85°–475°K, which includes the ferroelectric transition temperatures for both materials. The temperature dependence of $T_1$ in both salts is interpreted in terms of reorientation of the two nonequivalent ND₄⁺ ions. Within certain temperature regions in both materials, the deuterons in each type of ND₄⁺ ion have a $T_1$ smaller than the spin‐diffusion time with the result that they relax independently, making it possible to measure $T_1$ for each group. The activation energy changes little if at all at the phase transition, although there is an abrupt change in $T_1$ for the deuterons in both ND₄⁺ ions in ammonium sulfate and in one ND₄⁺ ion in ammonium fluoroberyllate. This latter fact has provoked the suggestion that both ammonium ions are involved in the ferroelectric behavior of ammonium sulfate, whereas only one is involved in the fluoroberyllate.