Synthesis and characterisation of chloro-vanadato-apatites M5(VO4)3ClM5(VO4)3Cl (M = Ca, Sr, Ba)

The synthesis of chloro-vanadato-apatites M5(VO4)3Cl (M   = Ca, Sr, Ba) in a two step solid state procedure is reported. IR spectra show a considerable shift of the frequencies attributed to the tetrahedral group to lower energies compared with the phosphato-apatites. The structure of these compounds is determined by powder diffraction data and refined using Rietveld methods. An elaborate analysis of the data shows that the chloride anions are to be found at different positions along a channel along [001][001] with varying probability. They are fairly well localised in the case of the Sr and the Ba compound. In the Ca compound they are spread over a larger region along c. A refinement according to a new concept of “metric distortion” gives a good fit of the data and indicates that the delocalisation of electron density seen in a Difference-Fourier plot is best described by a static picture of disorder.

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