Nuclear Spin-Lattice Relaxation in Some Ferroelectric Ammonium Salts

The temperature dependence of H¹ and F¹⁹ spin-lattice relaxation times in polycrystalline (NH₄)₂SO₄, (NH₄)₂BeF₄, NH₄HSO₄, and (NH₄)₂{H}₃I{O}₆$ have been measured in both the ferroelectric and nonferroelectric phases, using a pulse technique at 30 Mc/sec. The spin-lattice relaxation processes were found to be determined by molecular reorientation. Definite changes in the proton relaxation behavior in the neighborhood of the Curie points indicate a change in the state of internal motion at the transition. The same was found to be true for two solid solutions of (NH₄)₂BeF₄ in (NH₄)₂SO₄. The results suggest that the ferroelectric phase changes in these solids are in general accompanied by a change in lattice structure which alters the potential in which the nuclei move, but do not involve an important change in the degree to which nuclei must cooperate in order to surmount the potential barriers.