Research and development of the coprecipitation process for lanthanum germanate oxyapatite

Abstract

Although lanthanum germanate oxyapatite (La-Ge-O) has shown good potential for use as a solid electrolyte in energy storage applications, its synthesis has been challenging by either solid- or solution-state methods. In this study, a new synthesis of La-Ge-O was developed through a coprecipitation technique, in which a highly concentrated homogeneous aqueous solution of La and Ge was prepared from aqueous ammonium germanate and lanthanum nitrate solutions with the addition of dilute nitric acid. Several precipitates were formed by pH manipulation, including an amorphous material obtained at pH > 3. Compared to the individual precipitation behaviors of the parent compounds, the amorphous precipitate was formed only from the aqueous two-component mixture, and appeared to contain both metals. This material was transformed into crystalline mixtures upon heating at 1273 K. The crystalline phases were La2Ge3O9 and hexagonal-type GeO2 when the precipitate was formed below pH 8, and the La-Ge-O and La2Ge2O7 phases when the precipitate was formed around pH 8. Product formation from the coprecipitate was discussed based on X-ray diffraction and thermal analyses. The improved availability of La-Ge-O will allow more extensive investigations of its useful properties.