Rietveld refinement and ionic conductivity of Ca8.4Bi1.6(PO4)6O1.8

The structure of Ca8.4Bi1.6(PO4)6O1.8, isostructural with Fluoroapatite, was determined by X-ray powder diffraction methods. The results of Rietveld refinement revealed that the formula of this compound is [Ca4]4f[Ca4.4Bi1.6]6h(PO4)6[O1.8]2a, space group P63/m (a=9.468 (3) Å, c=6.957 (3) Å). A total substitution of Bi3+ ions in the (6h) sites was related particularly to the high polarizability of the Bi3+ ion compared to Ca2+. The observed frequencies in the Raman and infrared spectra were explained and discussed on the basis of unit-cell group analyses and in comparison with Fluoroapatite and other oxyapatites. The ionic conductivity over a wide range of temperature was investigated according to the complex impedance method. The highest overall conductivity values were found at σ700 °C =5.03×10−7 S cm−1 and Ea=0.50 eV.