Extended solubility of Sr in LaPO4 monazite

•Methods for producing the highest reported dopant concentration of Sr to date are discussed•The highest Sr dopant concentration in LaPO4 was achieved•The solid solution stability and the ionic conductivity as a function of temperature and atmosphere are established

Monazite-type LnPO4 is a stable phase for many of the larger rare earths. The unusually asymmetric 9-fold coordinated La3 + sites can be substituted by other large ions including aliovalents such as Sr2 +. In the case of divalent ions, “charge balance” can be maintained by substituted monovalent anionic units such as (OH)−. The solid solution series has the chemical formula La1 − xSrxPO4 − x(OH)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 LaPO4 up to that level, including proton transport, have been measured. At high temperatures, (> 400 °C) proton ionic conductivity in the bulk is expected to be high, but the structure becomes unstable. As (OH)− 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)− in the structure.