MIXED IONIC CONDUCTIVITIES IN SODIUM FLUOROAPATITES

The substitution of sodium for calcium in the fluoroapatite Ca10-xNax(PO4)(6-x)SO4)(x)F-2 has been investigated. For 0 less than or equal to x less than or equal to 6, a solid solution is obtained. From solid state Na-23 MAS-NMR spectroscopy, it is shown that for the high substitution ratios, the sodium ions go into me Ca-II sites only, whereas at small contents they are distributed among various sites. As a consequence a drastic increase of the ionic conductivity is observed for the small amounts of sodium with a maximum at x=0.4. From impedance measurements with sodium conducting electrodes, we can attribute this increase of conductivity to sodium mobility. Upon increasing the sodium content, conductivity decreases again and becomes purely fluorine-type again.