Extended solubility of Sr in LaPO sub(4) monazite

• Published in: Solid state ionics 2016-10, Vol.293, p.44-50
• Main Authors: Ohtaki, Kenta K, Heravi, Neshat J, Leadbetter, Joanne W, Morgan, Peter ED, Mecartney, Martha L
• Format: Article
• Language: English
• Subjects: Asymmetry; Conduction; Ionic conductivity; Physical properties; Rare earth metals; Solid solutions; Solubility; Strontium
• ISSN: 0167-2738
• DOI: 10.1016/j.ssi.2016.05.022

Monazite-type LnPO sub(4) is a stable phase for many of the larger rare earths. The unusually asymmetric 9-fold coordinated La super(3 +) sites can be substituted by other large ions including aliovalents such as Sr super(2 +). In the case of divalent ions, "charge balance" can be maintained by substituted monovalent anionic units such as (OH) super(-). The solid solution series has the chemical formula La sub(1 - x)Sr sub(x)PO sub(4 - x)(OH) sub(x), which may exist without defects such as vacancies as long as sufficient water is present. X up to as high as 0.3 is found, much higher than previously reported, when using a direct precipitation process in hot, strong phosphoric acid. Physical properties of Sr-doped LaPO sub(4) up to that level, including proton transport, have been measured. At high temperatures, (> 400 degree C) proton ionic conductivity in the bulk is expected to be high, but the structure becomes unstable. As (OH) super(-) is given off, Sr also leaves the structure and forms an intergranular phase with phosphorus, a process that detrimentally affects the ionic conductivity and cannot be suppressed even when conducting measurements in water vapor that should encourage retaining (OH) super(-) in the structure.