Nuclear Magnetic Resonance Relaxation Study of the Phase Transformations of LiNH4SO4 and LiND4SO4 Single Crystals: The Roles of Li, NH4 and ND4 Ions

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The NMR spectrum and spin–lattice relaxation times, $T_{1}$, of the 1H, 2H, and 7Li nuclei in LiNH4SO4 and LiND4SO4 crystals were obtained, and from these measurements, the rotational correlation times were calculated. In addition, the activation energies for the reorientation of 1H, 2H, and 7Li were obtained for each phase. For both types of crystal, each curve of $T_{1}$ versus inverse temperature exhibited a minimum value, which is attributed to the effects of reorientational motion. From the 1H, 2H, and 7Li $T_{1}$ curves, we conclude that the relaxation process is affected by molecular motion, as described by the Bloembergen–Purcell–Pound theory. The differences in activation energy among the various phases indicate that the 1H, 2H, and 7Li ions are significantly affected by the transitions between phases. The present results indicate that the contributions from the Li–O–S of the skeleton and the dipole moments of the ammonium groups change discontinuously at each phase transition temperature. This study furthers our understanding of the roles of Li+, NH4+, and ND4+ ions in the phase transformation process in LiNH4SO4 and LiND4SO4 crystals.
2009-10-15