NMR-STUDIES OF HYDROGEN DIFFUSION IN HYDROGEN URANYL PHOSPHATE TETRAHYDRATE (HUP)

**1H NMR spin-lattice relaxation times, T//1 (Zeeman) and T//1// rho (rotating frame) and spin-spin relaxation times, T//2, and **3**1P NMR solid-echoes are reported for phase I and II of hydrogen uranyl phosphate tetrahydrate (HUP) at temperatures in the range 200-323 K. The spectral density functions extracted from the measured relaxation times for phases I and II are consistent with a 2D diffusion mechanism for hydrogen motion. **3**1P second moments determined from the solid-echoes show that all the hydrogens diffuse rapidly in phase I, and that the hydrogen-bond site nearest to the phosphate oxygen is not occupied in phase II. The mechanism for diffusion in phase II is discussed.