Green apatites: hydride ions, electrons and their interconversion in the crystallographic channel

Abstract

Hydride (H⁻) ions and electrons in channel sites of the lattice of calcium phosphate apatites are characterized. Solid-state chemical reduction using TiH₂ is effective for doping of H⁻ ions into apatites. Irradiation of the H⁻ ion-doped apatite with ultraviolet (UV) light induces green coloration. Electron paramagnetic resonance (EPR) reveals that this colour centre is attributed to electrons captured at a vacant anion site in the crystallographic channel, forming F⁺ centres. Transient H⁰ atoms are detected at low temperatures by EPR. The concentration of UV-induced electrons in the apatite at room temperature decays according to second-order kinetics because of the chemical reactions involving two electrons; overall, electron generation and thermal decay can be described as: H⁻ + O²⁻ ↔ 2e⁻ + OH⁻. ¹H magic angle spinning nuclear magnetic resonance spectroscopy is used to identify H⁻ ions in the apatite, which are characterized by a chemical shift of +3.4 ppm. Various types of O–H groups including OH⁻ ions in the channel and protons bound to phosphate groups are concurrently formed, and are identified by considering the relationship between the O–H stretching frequency and the ¹H chemical shift. The complementary results obtained by EPR and NMR reveal that the H⁻ ions and transient H⁰ atoms are located at the centre of Ca₃ triangles in the apatite, while the electrons are located in the centre of Ca₆ octahedra. These findings provide an effective approach for identifying new classes of mixed-oxide-hydride or -electride crystals.