A High-Pressure Praseodymium Fluoride Borate Linking Multiple Structural Features of Apatite-Type Compounds

Pr₅ (BO₄ )_3-x (BO₃ )_x (F,OH)_2.67 O_0.28 (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 apatite 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 octahedra, 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 twist angle of 21.3°. The crystal structure of Pr₅ (BO₄ )_3-x (BO₃ )_x (F,OH)_2.67 O_0.28 (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, shedding more light onto the complex structural chemistry of apatites.