Electroconductivity and transport numbers of solid electrolytes La_10−x Ca_xA₆O_27−δ and La_9.33+δA_6−x Al_xO₂₆ (A = Si, Ge) with apatite-like structure

Abstract

The ion-oxygen conductivity of apatite-like compounds based on lanthanum silicates and germanates La₁₀A₆O₂₇ (A = Si, Ge), La_10−x Ca_xSi₆O_27−δ (x = 0.25, 0.5, 1.0), La_9.75Ca_0.25Ge₆O_27−δ and La_9.33+δSi_6−x Al_xO₂₆(x=0.4, 0.8, 1.5) is studied in the interval of partial oxygen pressures pO₂ extending from 10⁻¹⁶ to 10⁵ Pa, at temperatures of 500–1000°C. The electroconductivity of undoped compounds La₁₀A₆O₂₇ (A = Si, Ge) exceeds that of yttria-stabilized zirconia. The electroconductivity of lanthanum germanate (1.7 × 10⁻² and 8.5 × 10⁻²S cm⁻¹ at 700 and 900°C, respectively) is substantially higher than that of lanthanum silicate (9.8 × 10⁻³ and 3.5 × 10⁻² S cm⁻¹ at 700 and 900°C). Doping lanthanum germanate with calcium raises its electroconductivity (2.7 × 10⁻² and 1.3 × 10⁻¹ S cm⁻¹ for La_9.75Ca_0.25Ge₆O_27−δ at 700 and 900°C). Conversely, doping lanthanum silicate with ions of calcium or aluminum reduces the conductivity. In the pO₂ interval studied, the above compounds are ionic conductors and represent a class of solid electrolytes of promise for various electrochemical devices.