Proton N.M.R. study of the dynamics of the ammonium ion in ferroelectric langbeinite, (NH₄)₂Cd₂(SO₄)₃

Abstract

The proton N.M.R. lineshape of polycrystalline Langbeinite, (NH₄)₂Cd₂(SO₄)₃, has been studied in the temperature range 300 K to 1·8 K. The resonance line is motionally narrowed over the entire temperature range, and the low temperature proton line shows clear evidence for tunnelling motion of the ammonium ion between spin-symmetry states. From a computer simulation of the lineshape, we obtain an estimate for the tunnelling splitting parameter, J, of the torsional ground state of the ammonium ion, as 375±125 gauss. For an undistorted tetrahedral crystal field this corresponds to a tunnelling splitting Δ=4J=6·3±2·1 MHz. Pulsed proton N.M.R. studies have also been carried out on the above compound at 30·8 MHz and 48·2 MHz and the spin-lattice relaxation time (T₁) has been measured by the π-t-π/2 pulse sequence as a function of temperature down to 77 K. At 30·8 MHz, a T₁ minimum of 13 ms occurs at 105·8 K, and is ascribed to random reorientations of the NH₄⁺ ion. An activational energy barrier of 0·74±0·1 kcal/mole and an associated pre-exponential factor of 8·0×10^-13s are calculated for the above motional process, and the value of the activation energy is correlated with the tunnelling splitting of the torsional ground state. An anomaly in T₁ has been observed at the ferroelectric Curie point (95 K), indicating the order-disorder nature of the transition. This is the first experimental evidence relating to the nature of the transition in Langbeinite.