Hydrogen Ion Nuclear Magnetic Resonance Study of the Metal-Ammonia Compounds

Proton spin-lattice relaxation times and line widths have been measured for A(NH(,3))(,6), where A stands for Ca, Sr, Yb, and Ba. The results of this study indicate that the molecular motion in these materials exhibit three regions of different dynamic behavior. First, in the low -temperature region, the molecules are slowly tunneling between symmetric minima of the potential. Second, in the intermediate-temperature range, there is substantial evidence for two populations of dynamically inequivalent NH(,3) molecules. One of these populations is involved in structural phase transitions which are inferred for Sr(NH(,3))(,6) and Ba(NH(,3))(,6), while the second is unaffected by them. The existence of the first population in Ca(NH(,3))(,6) and Yb(NH(,3))(,6) is taken as evidence that these two compounds also approach, but do not undergo, phase transitions of a similar kind. Finally, in the high -temperature region, the molecules exhibit rotational diffusion through the lattice, giving near-liquid line widths. Details of the complex T(,1) and line width behavior are reviewed, and a unified model for the molecular motion of ammonia in these compounds is discussed.