Site preference of fluoride anion and potential ionic conductivity in fluorapatites A₅(VO₄)₃F (A = Pb, Sr, and Ba)

Abstract

The crystallographic parameters of fluoro-apatite A₅(VO₄)₃F (A = Pb, Sr, and Ba) were refined by the Rietveld method. The refined parameters indicate that the F⁻ anions in Pb₅(VO₄)₃F occupy sites different from those in Sr₅(VO₄)₃F and Ba₅(VO₄)₃F. The F⁻ anions in Pb₅(VO₄)₃F are located around center of the Pb₆ octahedra, while those for Sr₅(VO₄)₃F and Ba₅(VO₄)₃F are located around the center of Sr₃ or Ba₃ triangle. Our structural analysis suggested that this difference is attributed to the over-bonding state of F⁻ anion in Pb₅(VO₄)₃F. The electron density distribution for F⁻ anions suggests the potential ionic conductivity along the c axis in Pb₅(VO₄)₃F.