Electrochemistry (Dec 2023)
Development and Research of Inorganic Energy Conversion Materials and Devices —The Effect of Carbonate Ions on the Ionic Conduction of B-type Carbonated Apatite—
Abstract
Na+-containing B-type carbonate apatite ([Ca10−xNa2x/3][(PO4)6−x(CO3)x][(OH)2−x/3(H2O)x], BCA) is a hydroxyapatite (Ca10(PO4)6(OH)2, HA)-derived compounds in which PO43− sites of HA are partially replaced with CO32−. When CO32−-composition x exceeds 1, the ionic conductivity of BCA increases four orders of magnitude greater than that of HA, achieving values between 10−4–10−3 S cm−1 at 600–800 °C. Owing to this unique compositional dependence of ionic conductivity, BCA is not only an important biomaterial, but also a promising material for solid electrolytes. Hence in this study, motivated by the elucidation of the ion conduction mechanism of BCA, two types of BCAs with different x values were prepared, and their physical and ionic conductive properties were investigated. Thermogravimetric analysis result of BCAs sintered under CO2 (S-BCAs) showed that dissociation of CO32− ions began at approximately 600 °C during heating in air. However, during three cycles of impedance measurements repeated from room temperature to 850 °C, the decrease in the x (up to −60 %) did not significantly affect the conductivity. The impedance measurements also revealed that the slope of the Arrhenius plot for conductivity exhibited a bend between 500 and 800 °C, which appeared repeatedly across measurement cycles. Based on first-principles calculations regarding the stable structure of the BCA, some OH− (or O2− in OH− sites) might get trapped at the oxygen vacancy close to CO32−, and a defect association could form between Na+ and CO32−. Thus, the unique conductivity behavior observed in S-BCAs can be attributed to the changes in the carrier density and conduction pathways, which are influenced by the thermally induced change in the O2−-trapping state at oxygen vacancies close to CO32−.
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