Synthesis of monazite (RPO4, R=La, Ce, Nd, or Sm) by solid state reaction.

Syntheses of monazite (RPO4, R=La, Ce, Nd, or Sm) were carried out by heating the mixture of [RCl3, R2O3, Ce(NO3)3, or CeO2] and [(NH4)2HPO4, K2HPO4, Na2HPO4, or H3PO4)] with the atomic ratio (P/R) of 1 at 200–1000°C for 1 day. Products observed were monazite-(La, Ce, Nd, or Sm), hexagonal RPO4·nH2O, Ce(PO3)3, Ce(PO3)4, and CeP2O7. Single monazite-(La, Ce, Nd, or Sm) phase was synthesized by heating (1) the mixture of [RCl3 or R2O3] and H3PO4 at 400–1000°C or (2) the mixture of [CeCl3 or Ce(NO3)3] and [(NH4)2HPO4, K2HPO4, or Na2PO4] at 200°C, or (3) the poorly crystallized monazite-(Ce) (synthesized from the mixture of CeCl3 and (NH4)2HPO4 at 200°C at 400–1000°C. The method (1) is most convenient.