Abstract
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 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)2.67O0.28 (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, shedding more light onto the complex structural chemistry of apatites.
Highlights
Due to their enormous biological significance as main mineral components of human and animal hard tissues such as bones and teeth, apatite Ca5(PO4)3(F,Cl,OH) and compounds with similar structures have been objects of extensive studies
The rare earth cations surrounding the 63 screw axis form trigonal planar, tetrahedral, and octahedral sites for the anions in the channels along [0 0 1], as well as the metaprisms formed by the rare earth cations Pr3 and Pr5
Pr5(BO4)3Àx(BO3)x(F,OH)2.67O0.28 (x % 1.6) thereby combines structural features that have only been reported in different apatitic compounds, for example, the AO4/AO3 group disorder in carbonate hydroxyapatite, the main component of human and www.chemeurj.org
Summary
Conductors like La8Y2Ge6O27 have come into focus as potential electrolytes for solid oxide fuel cells (SOFC).[5]. Pr5(BO4)3Àx(BO3)x(F,OH)2.67O0.28 (x % 1.6) represents a high-pressure rare earth fluoride-oxoapatite-like compound exhibiting a superstructure featuring a unit cell tripled along [0 0 1], in combination with anion disorder along the 63 screw axis and BO4/ (BO3 + F) group disorder. This compound combines the two topics described above in a way that has not been reported so far, and could be a good basis for further understanding of the crystal chemistry in natural hydroxyapatites. These data are provided free of charge by The Cambridge Crystallographic Data Centre / FIZ Karlsruhe deposition service
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