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

Abstract

Apatite-type lanthanum silicate (LSO) is a material with high oxide-ion conductivity in the low- and intermediate-temperature range (573–873 K) and is, therefore, a promising solid electrolyte for low-temperature 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 (La9.7Si5.3B0.7O26.2, 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.