Abstract: Monodispersed fine spherical particles(～100 nm)of complex rare earth hydroxy carbonates are prepared by hydrolizing mixed rare earth nitrates solution at elevated tempera ture in the presence of urea. By calcining the hydroxycarbonates at moderate temperature,complex rare earth sesquioxides(～70 nm)of C-type can be obtained.The mechnism of the decomposition of hydroxycarbonate at different temperatures have been verified to proceed through monoxydicarbonate, dioxymonocarbonate to sesquioxide of body-Centered structure by DTA,TGA,XRD and HREM. The experimental EDP diagrams of complex rare earth sesquioxides have been identified with those of calculated C-type structure. Complex rare earth oxysulfides of hexagonal structure can be prepared by firing a blend of complex rare earth hydroxycarbonate,S, Na₂CO₃and KH₂PO₄at 1000℃.

Key words: Superfine particles, Rare earth oxides, Rare earth oxysulfides, Hydrolvsis