Influence of fluorine substitution on the proton conductivity of hydroxyapatite

Abstract

Polycrystalline hydroxyfluoro-apatite solid solutions exhibit pronounced proton conductivity, σ(H⁺), in the range 250–500 °C. For samples with between 0 and 50% F the activation energy for σ(H⁺) remains constant, 0.6 (±0.1) eV, but rises sharply for samples with 65 (1.1 eV), 75 (1.3 eV), and for ca. 100% F (1.4 eV). The frequency factor, σ₀, increases with increasing F content up to 50% F, but drops sharply for higher percentages of F substitution. The dominant protonic charge carriers are defect protons as deduced from thermopotential measurements.