Thermal Reactions of Hydrated Hexagonal RPO4·nH2O (R=Tb or Dy, n=0.5 to 1)

The thermal reactions of hydrated hexagonal RPO4·nH2O (R=Tb or Dy, n=0.5 to 1) were studied at 20° to 1800°C in air under atmospheric pressure. The hydrated hexagonal forms were dehydrated at 180° to 250°C. Thereafter, no significant changes in structure were seen up to 800°C (R=Tb) or 700°C (R=Dy). The water corresponding to nH2O was zeolitic water. Anhydrous hexagonal RPO4 gradually transformed into the monazite structure at 900°C (R=Tb) or 800°C (R=Dy), then into the xenotime structure at temperatures above 1100°C (R=Tb) or 900°C (R=Dy).