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.