Crystal Growth Mechanism of Highly -Axis-Oriented Apatite-Type Lanthanum Borosilicate Using BO 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 -axis-oriented oxide-ion conductivity. To obtain solid electrolytes with high oxide-ion conductivity, a technique for growing crystals oriented along the -axis is required. For mass production and upscaling, we have thus far focused on the vapor-phase synthesis of -axis-oriented apatite-type LSO and successfully grew polycrystals of highly -axis-oriented boron-substituted apatite-type lanthanum silicate (-LSBO) using BO vapor. Here, we investigated the mechanism of -LSBO crystal growth to determine why the utilization of BO vapor resulted in such a strong -axis crystal orientation. The synthesis of -LSBO by the BO vapor-phase method results in crystal growth accompanied by the diffusion of BO supplied from another new compound that formed on the surface of the LaSiO disk, LaBO. In addition, -LSBO crystals are formed not only by vapor-solid reactions but also by solid-solid and liquid-solid reactions. The increase in the -axis orientation degree might be due to the increase in the amount of the liquid-phase interface.