The precipitation behavior of Sr-doped LaP3O9 from phosphoric acid solutions was investigated at 190-270 degrees C in the ambient atmosphere or at 300-360 degrees C in water vapor (1 atm) for the purpose of obtaining large single crystals and dense plate-like polycrystals. Slow heating of the solutions in the "humidified and high-temperature" conditions is effective for inhibiting too much nucleation and accelerating crystal growth. Under such conditions, columnar single crystals with length up to 2 mm can be obtained. The c-axis is the preferred growth direction; it is directed parallel to the longer direction of the columnar crystals. The Sr-doping level in the crystals reaches up to 5.1%, which surpasses the highest value reported for LaP3O9. When the amount of precipitates is large relative to the surface area of crucible, the plate-like polycrystals can be obtained. The morphology of the plate-like polycrystals is heating-rate dependent: dense plates with elongated grains of 100-200 mu m perpendicularly-directed to the surfaces are obtained by slow heating, whereas loose plates with small grains are obtained by immediate heating. The longer direction of the elongated grains is identical to the c-axis of LaP3O9. (C) 2013 Elsevier B.V. All rights reserved.