Nuclear magnetic resonance investigations of certain protonic conductors.

Abstract

Ionic motion in the hydrate of ammonium ferrocyanide has been investigated by high?pressure wide?line NMR, high?resolution NMR and conductivity techniques. In the high?pressure NMR study, the room?temperature proton NMR spectra recorded at different pressures show evidence for the occurrence of a pressure?induced phase transition at 0.45 GPa, resulting in the complete freezing of the translational motion of one of the mobile species in the compound. This has been confirmed by a compressibility study of the sample in which a sudden 3.7% change in volume is observed at the same pressure. The pressure?dependent second?moment study at room temperature has yielded an activation volume of 13.63 cm³/mole. The temperature?variation study up to 0.425 GPa shows an increase in the temperature of line narrowing with increasing pressure, with corresponding increase in activation energy and attempt frequency. The high?resolution NMR study has shown the presence of two chemically shifted signals attributed to NH?? and H?O protons and leads to the conclusion that in addition to NH?? ions, water molecules also participate in diffusion in this compound. The electrical conductivity study by complex admittance method has yielded values of ? and D of the same order as that estimated from the NMR data of Whittingham et al., thereby showing that essentially all the ammonium ions in AFC take part in charge transport. The information obtained from these different studies, though valuable, is by no means complete. There is still scope for further investigations. Relaxation?time studies might throw more light on the nature of the mobile species in the compound, PFG NMR study would give a direct and more accurate estimate of diffusion coefficient, and an X?ray structural study at high pressures would give direct evidence for the structural phase transition observed at 0.45 GPa. Such experiments are being carried out in our laboratory. Proton motion in HNbWO?·1.5H?O has been studied by both cw and pulsed NMR experiments. The low?temperature cw spectrum shows evidence for the existence of rigid water molecules and protons as separate entities. The narrow spectra above 190 K suggest the dominance of translational motion of protons throughout the lattice. The low activation energy (8 kJ/mole) and the “abnormally” low prefactor (0.9 × 10¹? sec?¹) are consistent with the values found for several other superionic conductors with layered and channelled structures. The activation energy obtained from the T? study is much different from that obtained by the line?narrowing experiment. This anomaly suggests non?BPP relaxation behaviour. The wide?line NMR study at high pressures has indicated the absence of pressure effects on ionic motion up to 1.4 GPa. The wide?line NMR study of N?HgSO? from room temperature up to 478 K shows evidence for translational diffusion of ions around 475 K. This inference is consistent with an earlier deuteron resonance and T? study of Harrel and Peterson. The NMR observations are confirmed by the ESR study of Mn²? doped single crystal of N?HgSO? in which an abrupt change in the behaviour of ?H is observed around 463 K. Hexagonal ammonium tungsten bronze was prepared by the reduction of ammonium paratungstate in hydrogen and characterised by powder X?ray diffraction study. Proton NMR study of the sample was made at high pressure. The study indicates the absence of pressure effects on the ionic motion in the compound up to 1.4 GPa. It is speculated that the cause for this behaviour might be attributed to the channel structure of the material.