Pulsed neclear magnetic resonance investigations of certain proton conductors

Abstract

In conclusion, we have assembled a pulsed field gradient spin-echo NMR spectrometer by interfacing a home-built microprocessor-based pulsed programmer and data acquisition system with imported RF units and an electromagnet. The spectrometer was calibrated using known samples. Measurements of relaxation times as a function of temperature and frequency have been carried out using n-decanol and water. At present, the spectrometer is being used for measuring relaxation times in unknown SIC samples such as AFC, layered HNbWO?·1.5H?O, pyrochlore HNbWO?·H?O, etc. The results of the relaxation time data for these samples are presented in Chapters 4, 5, and 6. Measurement of the diffusion coefficient (D) as a function of temperature and frequency on FICs is under progress. Proton NMR relaxation data suggest that both HNbWO?·1.5H?O and HTaWO?·1.5H?O (layered) are good protonic conductors. The ionic conduction occurs via protons of water molecules and protons of HMWO?. The low-temperature spectra show that rotational motion does not exist in both samples. Our T? and T?? results suggest that at temperatures above the transition temperature or the temperature corresponding to T?? minimum (?200 K), the translational motion of protons dominates, and the system enters a liquid-like state. The temperature variation of relaxation studies shows non-BPP behavior due to anomalies such as low prefactor values. Therefore, we analyzed the relaxation data using a two-dimensional model as proposed by Richards [30], which confirms the 2-D character of ionic diffusion, consistent with the layered structure. Since HMWO?·1.5H?O (M = Nb or Ta) has a layered structure, we expect the protons to be mobile in the interlayer region. These anomalies are common in superionic conductors with layered structures [4-15]. Ionic diffusion occurs by the vacancy-jump mechanism, where a proton jumps into a vacancy site, creating a vacancy at its former site. Protonic motion in pyrochlore HMWO?·H?O has been studied by pulsed NMR experiments. The low-temperature signal shows evidence for the existence of rigid water molecules and protons as separate entities in cubic HMWO?·H?O (3-D). The three important results are: (1) Rigid lattice state to liquid-like state transition behavior and its temperature range. (2) Temperature corresponding to T?? minimum. (3) Diffusion coefficient (D). As a primer for future work, we suggest that NMR studies of one-dimensional HMWO?·xH?O would provide further understanding of dimensionality effects on NMR results.