In order to investigate the contribution which cations made to the transport properties of these materials, measurements were made of their self-diffusion coefficients in ceramic samples of solid solutions by using suitable radio-tracers (table 132). It was found that, as the Ti content of the solid solution increased, the self-diffusion coefficients of 44Ti and 90Sr increased, and that the Ti diffusivity was greater than the Sr diffusivity for all of the compositions within the homogeneity range. This was attributed to the appreciable difference between the ionic radii of Ti (0.0605nm for a coordination number of 6) and Sr (0.118nm for a coordination number of 6 and 0.144nm for a coordination number of 12). This difference was suggested to be responsible for a distortion of the crystal structure of Sr6Nb2O11; thus producing new paths for both the short-circuit and intercrystalline diffusion of Sr ions. This seemed to be the cause of the increase in the 90Sr effective diffusion coefficient with increasing Ti content. Overall, the present Sr diffusion coefficients in these niobate phases were comparable to those reported for other niobates. A comparison of Ti and Nb mobilities suggested that Nb was fixed more securely within the NbO6 octahedron, than was Ti, because of a strong polarization of the neighboring O atoms. The Nb therefore had a noticeably smaller self-diffusion coefficient. The data on self-diffusion provided further insight into the nature of ionic conduction in these solid solutions. Although the mobilities of Ti and Sr ions increased with the TiO2 content of the solid solution, their contribution to the total conductivity was insignificant. The total conductivity was determined mainly by the mobility of the O ions, and decreased as the concentration of structural O vacancies decreased. It was proposed that these solid solutions should be regarded as being solid electrolytes with a disordered O sub-lattice.

A.L.Podkorytov, M.I.Pantyukhina, V.M.Zhukovskii, N.A.Simaranova, N.V.Sherstneva, G.I.Dontsov: Neorganicheskie Materialy, 1995, 31[8], 1097-9 (Inorganic Materials, 1995, 31[8], 1007-9)

Table 132

Self-Diffusion of 44Ti and 90Sr in Strontium Niobium Titanates