Samples of Ta-free material were prepared by using solid-state synthesis and sol-gel techniques, and the structure was investigated by using the Rietveld method and X-ray diffractometry data. The powder diffraction data revealed evidence of a triclinic distortion of the basic monoclinic structure. The ionic conductivity was determined at temperatures ranging from 130 to 560C, using alternating-current impedance measurements. It was found that the Na+ ion mobility was limited, with an activation energy of 1.04eV and a conductivity at 300C of between 2 x 10-7 and 9 x 10-7S/cm. Partially Ta-substituted samples, Na2-x(Zr1-xTax)Si4O11 (where x was less than 0.2), exhibited the same evidence of triclinic distortion. These materials had lower activation energies (about 0.75eV) but only slightly higher conductivities at 300C.

G.Vitins, J.Grins, T.Hoerlin: Solid State Ionics, 1996, 86-88[1], 119-24