High Oxide Ion Conductivity in Al-Doped Germanium Oxyapatite

The apatite La<INF>10</INF><INF>-</INF><INF>x</INF><INF></INF>□<INF>x</INF><INF></INF>(Ge<INF>5.5</INF>Al<INF>0.5</INF>O<INF>24</INF>)O<INF>2.75</INF><INF>-</INF><INF>1.5</INF><INF>x</INF><INF></INF> (10 − x = 9.80, 9.75, 9.67, 9.60, 9.50, and 9.40) series has been prepared and the single phase existence range has been established, 9.75 ≥ 10 − x ≥ 9.45. The hexagonal crystal structures of La<INF>9.5</INF>□<INF>0.5</INF>(Ge<INF>5.5</INF>Al<INF>0.5</INF>O<INF>24</INF>)O<INF>2</INF> have been determined at room temperature, 500 °C, and 900 °C from neutron powder diffraction data using the Rietveld method. The room-temperature unit cell parameters were a = 9.9206(4) Å, c = 7.2893(3) Å, V = 621.29(6) ų, and Z = 1, and this refinement converged to R<INF>WP</INF> = 3.03 and R<INF>F</INF> = 1.30%. The most important structural result is the presence of interstitial oxygen ion associated with vacancies at the apatite oxide anions channels. Oxide ion conductivities have been measured by impedance spectroscopy. La<INF>9.5</INF>□<INF>0.5</INF>(Ge<INF>5.5</INF>Al<INF>0.5</INF>O<INF>24</INF>)O<INF>2</INF> shows very high oxide conductivity, 0.16(1) S·cm^-1 at 800 °C, with negligible electronic contribution. The ionic transport number, obtained by combination of impedance and ion-blocking data, is higher than 0.99 in the studied oxygen partial pressure range, 0.21 to 10^-20 atm.