Precipitation behavior of highly Sr-doped LaPO4 in phosphoric acid solutions

Abstract

The precipitation condition of LaPO₄ in phosphoric acid solution was investigated under both atmospheric and humidified conditions. The crystal morphology, Sr doping level, H₂O incorporation, and high temperature stability of Sr-doped LaPO₄ precipitates were also analyzed. Increasing pH₂O from 0.01 to 1 atm extends the precipitation region of LaPO₄ in phosphoric acid solutions toward higher temperatures by nearly 100 °C. Under such conditions (e.g. T = 250 °C and pH₂O = 1 atm), it is possible to obtain aggregated crystals up to 200 μm. The Sr doping level in LaPO₄ shows a strong dependence on the precipitation ratio of La, X_La. The Sr doping level is higher in the low X_La region, reaching 20 mol% at maximum. This value is one order of magnitude higher than the previously reported solubilities. It was confirmed H₂O is certainly incorporated in the bulk by Sr doping. The solubility of Sr in LaPO₄ would be sufficiently high (at least 20.1%) even at 600 °C; however, it becomes considerably lower at 1200 °C.