STUDY OF ELECTRICAL PROPERTIES AND STRUCTURE OF NASICON-TYPE SOLID ELECTROLYTES (FAST IONIC CONDUCTORS, MICROWAVE MEASUREMENTS, IMPEDANCE SPECTROSCOPY)

Dense, polycrystalline samples of Na(,2)Zr(,2)SiP(,2)O(,12) and Na(,3)Zr(,2)Si(,2)PO(,12) were prepared by solid state reactions between mechanically mixed reagents. The electrical properties were investigated in the temperature range 20 to 450(DEGREES)C by the a.c. impedance spectroscopy at frequencies 1 to 7 x 10('5)Hz and using waveguide techniques at microwave frequencies.;New techniques for measuring the complex permittivity at microwave frequencies were developed involving: (1) use of nonlinear least-squares estimation for variable termination measurements in a waveguide; (2) a rigorous solution of the boundary value problem for determination of the permittivity of a centered E-plan slab in a rectangular waveguide. The microwave conductivities of both compounds are higher than their low-frequency bulk conductivities and increase with frequency according to a power-law dependence in agreement with the predictions of hopping models. At room temperature, the local motion of ions dominates the microwave conductivity.;Crystal structures of the two compounds were determined at 20 and 300(DEGREES)C using neutron powder diffraction and profile refinement. The high ionic conductivity of Na(,3)Zr(,2)Si(,2)PO(,12) at temperatures around 300(DEGREES)C was correlated with the wide openings between the Na(1) and Na(2) sites and the high relative occupancy factor of the Na(2) positions.;A computer program was developed for the nonlinear least-squares analysis of the complex impedance spectra which made use of statistical weights reflecting resolution of the measuring equipment. The a.c. impedance spectra of both compounds (samples with ion-blocking electrodes) were modelled using equivalent circuits which contained constant phase elements. For Na(,2)Zr(,2)SiP(,2)O(,12), the grain-boundary polarization was identified over the temperature range 50 to 410(DEGREES)C. For Na(,3)Zr(,2)Si(,2)PO(,12), two distinct polarization processes were observed in high and low-temperature regions. The ion-blocking electrodes were represented by a constant phase element. Onset of dispersion observed in the high-frequency part of the admittance spectra at room temperature was associated with frequency-dependent hopping conductivity.