Abstract
Equilibria of the aqueous Ln + O + SO 4 ( Ln = La, Pr, Nd) system have been investigated at water pressures of 120 ± 20 MPa and temperatures of 450 ± 50°C. At low sulfate contents, the UCl 3-type trihydroxides coexist with the orthorhombic dioxide monosulfates. A second two-phase region, Ln 2O 2SO 4 + Ln(OH)SO 4, appearing at higher sulfate concentrations is followed by a third two-phase region, Ln(OH)SO 4 + Ln 2(SO 4) 3 · nH 2O. Whereas the monohydroxide monosulfates all have the same monoclinic structure, several new hydrate compositions and structures form in hydrothermal media. X-ray diffraction data indicate that the n = 7 hydrates of La and Pr and the monohydrate of La have closely related orthorhombic structures similar to that of the anhydrous La 2(SO 4) 3 phase obtained by decomposition of the hydrates at low temperature. The n = 5 sulfate hydrates of Pr and Nd have also been characterized, and a metastable La 2(OH) 4SO 4 phase has been identified. Lattice parameters and diffraction data obtained by powder and single crystal methods are reported, and the thermal decomposition reactions of the products are described.
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