NMR studies of some lithium based fast ion conductors

The spin lattice and spin-spin relaxation times T1 and T2 when measured as a function of temperature and frequency provide information about the activation energies and the correlation times which help in understanding the conduction mechanism in fast ion conductors. Dilithium trititanate Li2Ti3O7 according to data reported by Huberman and Boyce shows an unusual frequency dependent T1 and an anomalously low value of attempt frequency of 4 x 10^7 Hz. was deduced. The activation energy from the motional narrowing region of T2 was 0.2 eV compared with 0.45 eV from the conductivity measurements. Various explanations were put forward to justify these anomalies including "the break-down of the absolute rate theory" and also Richards put forward a theory to account for these discrepancies assuming that the conduction in these channel type materials is one dimensional. T1 and T2 have been measured for 7Li in very pure, highly stoichiometric material and off-stoichiometric material at different temperatures and at several frequencies. The value of T1 and T2 obtained for the stoichiometric sample is more than an order of magnitude longer than those observed by Huberman and Boyce over most of the temperature range and the activation energy obtained from the motional narrowing region of T2 is very close to that obtained from the conductivity. Also the attempt frequency found through T1 minimum is more than two orders of magnitude higher than the reported value of Huberman and Boyce. Small departures from stoichiometry rapidly reduce the relaxation times. The results are discussed in terms of the presence of paramagnetic ions in this material. Some of the lisicon group such as Li12Mg2(SiO4)4 and Li12Zn2(SiO4)4 are also investigated through measurements of T1 and T2 in the temperature range between 150°C and 400°C. The activation energies obtained from T1 and the motional narrowing region of T2 are the same with values of 0.5 eV and 0.43 eV and attempt frequencies of 1 x 10^12 Hz. and 0.47 x 10^12 Hz. for Li12Mg2(SiO4)4 and Li12Zn2(SiO4)4 respectively. Below the minimum T1 varies with the frequency as ω^1.5. The results are also discussed in terms of the presence of paramagnetic ions in the materials