A high-pressure praseodymium fluoride borate linking multiple structural features of apatite-type compounds.

Pr5(BO4)3-x(BO3)x(F,OH)2.67O0.28 (x ≈ 1.6), a boron-containing fluoride-oxoapatite-like compound, was obtained by the application of high-pressure/high-temperature synthesis. It exhibits a superstructure of the fluorite type with a tripled c lattice parameter (space group P63/m) and shows complex anion disorder along the 63 screw axis and occupation of distorted octahedral as well as almost trigonal planar sites by oxygen and fluorine atoms. Furthermore, a distinct BO4 / (BO3 + F) group disorder is found, 46% of the sites being occupied by BO4 groups and 54% by BO3 groups with a fluoride ion located near the missing oxygen atom. The rare earth cations in the 4f sites exhibit a specific distorted tricapped trigonal prismatic coordination with a mean metaprism angle of 21.3°. The crystal structure of Pr5(BO4)3-x(BO3)x(F,OH)2.67O0.28 (x ≈ 1.6) shows much "flexibility" resulting in split and off-site positions of all other rare earth cations. The title compound therefore combines many structural features of apatite-like compounds, for example biologically highly important carbonated apatites, shading more light onto the complex structural chemistry of apatites.