Complex impedance measurements were made of Zr2-xMgxNa1+2x(PO4)3 compositions. Either 1 or 2 semicircles, which depended strongly upon x, were observed in the resultant plots. A transition from 1 semicircle to 2 semicircles occurred at x = 0.4. These results could be explained by assuming that the semicircle in the region of x = 0.1 to 0.3 was caused by bulk conduction. Therefore, the conductivity increased with increasing x-value, and rigid material formed below x = 0.3. The 2 semicircles in the region of x = 0.4 were attributed to 2 conduction mechanisms. One was bulk conduction at high frequencies, and the other was grain boundary diffusion at low frequencies. Two semicircles in Na2Zr1.5Mg0.5(PO4)3 were attributed to 2 mechanisms which involved both bulk conduction and grain boundary diffusion of the Na ion. In the present study, the longitudinal modulus depended strongly upon x, and soft material formed at x = 0.4 to 1.0. This suggested that grain boundary layers formed when x was between 0.4 and 1.0; thus leading to the observed transition from 1 semicircle to 2 semicircles at x = 0.4.

N.Inoue, M.Tagami, Y.Nakamura, Y.Matsumoto, Y.Ogawa, T.Hasegawa: Solid State Ionics, 1994, 72, 285-8