Crystal Growth Mechanism of Highly c-Axis-Oriented Apatite-Type Lanthanum Borosilicate Using B2O3 Vapor

Apatite-type lanthanum silicate (LSO) exhibits high oxide-ion conductivity and has recently garnered attention as a potential solid electrolyte for high-temperature solid oxide fuel cells and oxygen sensors that operate in the low- and intermediate-temperature ranges (300–500 °C). LSO exhibits anisotropic oxide-ion conduction along with high c-axis-oriented oxide-ion conductivity. To obtain solid electrolytes with high oxide-ion conductivity, a technique for growing crystals oriented along the c-axis is required. For mass production and upscaling, we have thus far focused on the vapor-phase synthesis of c-axis-oriented apatite-type LSO and successfully grew polycrystals of highly c-axis-oriented boron-substituted apatite-type lanthanum silicate (c-LSBO) using B2O3 vapor. Here, we investigated the mechanism of c-LSBO crystal growth to determine why the utilization of B2O3 vapor resulted in such a strong c-axis crystal orientation. The synthesis of c-LSBO by the B2O3 vapor-phase method results in crystal growth accompanied by the diffusion of B2O3 supplied from another new compound that formed on the surface of the La2SiO5 disk, LaBO3. In addition, c-LSBO crystals are formed not only by vapor–solid reactions but also by solid–solid and liquid–solid reactions. The increase in the c-axis orientation degree might be due to the increase in the amount of the liquid-phase interface.