THE PREPARATION, STRUCTURE, AND CONDUCTIVITY OF SCANDIUM-SUBSTITUTED NASICONS
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A series of NASICON type compounds was prepared in which Sc(III) partially replaced Zr(IV). By simultaneously correlating the extent of this substitution with the amount of Si(IV) replacement of P(V) it was possible to obtain a series with a constant amount (3 mole) of sodium ion. All of the rhombohedral members of this series exhibited similar conductivities and activation energies over the temperature range 30-350C. Transformation to the monoclinic phase occurred at a composition of Na3Sc0.5Zr1.5Si1.5P1.5O12 and was accompanied by a threefold increase in conductivity. Activation energies of the monoclinic (room temperature) phases in the temperature range 200-300C were 0.2-0.23 eV. Crystal structures of the rhombohedral phase of composition Na3ScZrSiP2O12 and the monoclinic phase, Na3Sc0.5Zr1.5Si1.5P1.5O12, were carried out by Rietveld refinement of their X-ray powder patterns. The structures confirmed those presented by Hong, and also confirmed the stoichiometry of these phases as evidenced from site occupancy refinement. These results were compared with the structure of a nonstoichiometric phase, Na3.20Zr1.68Si1.84P1.16O11.54, prepared by a hydrothermal route. In the latter case it was shown that the low Zr content is due to Na+ substitution for Zr4+. No such substitution occurred in the scandium containing phases, and it is suggested that when Sc3+ substitution for zirconium takes place it prevents the sodium ion from occupying these sites. 1985.
