Structure and Dynamics of the - Conduction Plane: a Lead NMR Study

The beta^{' '}-alumina superionic conductor exhibits an unusual ability to exchange its monovalent charge carriers (usually sodium ions) for divalent or trivalent cations. However, the ionic conductivities of the higher valence isomorphs are reduced by more than three orders of magnitude. The single known exception is Pb^{2+} beta^{'' }-alumina, which is only a factor of three lower in conductivity than Na beta^ {''}-alumina. This thesis describes the use of ^{207} Pb NMR spectroscopic techniques to get equilibrium information about Pb ion positions and to get dynamical information about Pb ion diffusion. A single crystal of Na beta^{'' }-alumina was ion exchanged in molten PbCl _2 and enriched in ^ {207}Pb by vapor exchange. The product Pb beta^{'' }-alumina crystal was cooled to 140K to render the motion of Pb ions negligible, in order to examine its rigid lattice structure. A series of rotation patterns were taken at 10^circ intervals, with the axis of crystal rotation perpendicular to c. Dynamical information was extracted from relaxation measurements up to 400K and over a range of crystal orientations. Results of the low-temperature experiments show that the spectra are inhomogeneously broadened (~50kHz), due to displacive disordering of the Pb ions about their equilibrium sites. Deconvolution of the spectra shows that they can be described by two chemical shift tensors with similar isotropic parts and opposite anisotropies. Results of the high-temperature experiments indicate that ^{207}Pb relaxation is caused by a paramagnetic interaction. It is concluded that the equilibrium sites are highly disordered Beevers-Ross type, with approximately half of the Pb ions displaced toward mid-oxygen sites and half displaced toward mid-oxygen atoms.