Ionic Motion in Solid Electrolytes: A Solid State NMR Study of Sodium and Lithium in β-Alumina
<p>Solid state NMR techniques have been used as a microscopic probe of the structural and dynamical properties of the mobile cations in the solid electrolyte β-alumina. The first order quadrupole shifts in the (±3/2 ↔ ±1/2) transitions in the spin 3/2 nuclei sodium and lithium have allowed the...
| Summary: | <p>Solid state NMR techniques have been used as a microscopic probe of the structural and dynamical properties of the mobile cations in the solid electrolyte β-alumina. The first order quadrupole shifts in the (±3/2 ↔ ±1/2) transitions in the spin 3/2 nuclei sodium and lithium have allowed the electric field gradients (EFG) for the nominal Beevers and Ross (BR) and mid-oxygen (MO) sites to be characterized. The details of the EFG's for and the distribution of cations among BR and MO sites reflect the differences in the potential wells for lithium and sodium in β-alumina. In particular, unlike sodium, the MO site for lithium is lower in energy than the BR site which results in a different structural arrangement of lithium ions. In sodium β-alumina, the equilibrium position for sodium in the BR site is displaced from the three-fold symmetry axis. It is believed that this is due to the presence of nearby MO-MO pairs. Furthermore, an activation energy of 0.04 eV is observed for the motion of sodium ions among the displaced BR sites which is associated with the correlated motion of MO-MO pairs. The effects of a second motional process with an activation energy of 0.08 eV are observed in both lithium and sodium spectra which are associated with the motion of cations among BR and MO sites. The interaction of cations in MO-MO pairs was further investigated by observing the effect of varying the ratio of lithium and sodium in mixed lithium-sodium β-alumina on the distribution of cations among the available sites. The results were interpreted using a theory previously developed to explain the mixed-alkali effect in glasses and indicate that there is a strong interaction between unlike MO-MO pairs which governs the cation site distribution and raises the activation energy for conduction.</p> |
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