Single-crystal microtubes of a novel apatite-type compound, (Na_2.5Bi_2.5)(PO₄)₃(F,OH), with well-faceted hexagonal cross sections

A novel apatite-type compound, (Na_2.5Bi_2.5)(PO₄)₃(F,OH), NBPF, in the form of single-crystal microtubes with well-faceted, hexagonal morphologies in both exterior and interior surfaces, has been synthesized under hydrothermal conditions without addition of any template, surfactant or metal catalyst. Single crystal X-ray diffraction analysis confirms that Na⁺ and Bi³⁺ ions completely replace the Ca²⁺ ions in the apatite-like structure and jointly occupy three crystallographic sites with significantly different site preferences [Bi_0.61Na_0.39 at the 6c site, and Bi_0.45Na_0.55 and Bi_0.23Na_0.77 at the 2b sites, space groupP6₃ (No. 173)]. Time-dependent experiments show that these NBPF microtubes form via a fluoride-induced, in situ chemical reaction nucleation–dissolution–recrystallization growth mechanism.