Abstract
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.
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