Effect of Boron Substitution on Oxide-Ion Conduction in c-Axis-Oriented Apatite-Type Lanthanum Silicate

Apatite-type lanthanum silicate (LSO) is a material with high oxide-ion conductivity in the low- and intermediateerature range (573-873 K) and is, therefore, a promising solid electrolyte for lowerature applications such as solid oxide fuel cells and oxygen sensors. Herein, the effect of B substitution at the Si site in a c-axis-oriented apatite-type lanthanum silicate (La_9.7Si_5.3B_0.7O_26.2, c-LSBO) polycrystal on oxide-ion conduction is investigated. A highly c-axis-oriented LSBO polycrystal is fabricated by a vapor-solid reaction in which a dense La₂SiO₅ disk is heated in B₂O₃ vapor at ≥1673 K. The oxide-ion conductivity of c-LSBO reaches 16 mS cm^-1 at 678 K with an activation energy of 0.4 eV. The obtained oxide-ion conductivity of c-LSBO is approximately 190 times higher than that of yttria-stabilized zirconia and 5.8 times higher than that of the polycrystalline c-axis-oriented nondoped lanthanum silicate. Based on ¹¹B nuclear magnetic resonance measurements, B is located at the SiO₄ site as BO₄, suggesting the formation of an oxygen vacancy at the O4 site located along the c-axis due to charge compensation. In addition, molecular dynamics simulations indicate that the oxide-ion diffusion coefficient of the B-doped LSO is higher than that of the nondoped LSO. The high oxide-ion conductivity of c-LSBO is likely attributable to the formation of an oxygen vacancy at the O4 site by B doping, which has a lower valency than Si. Therefore, c-LSBO is a promising candidate as a solid electrolyte in electrochemical devices operating at low and moderately high temperatures.