Controlled Hydrothermal Crystallization of Anhydrous Ln2(OH)4SO4 (Ln = Eu-Lu, Y) as a New Family of Layered Rare Earth Metal Hydroxides

Anhydrous hydroxide sulfates Ln(2)(OH)(4)SO4 (Ln = Eu-Lu, Y) were hydrothermally synthesized as a new family of layered rare earth metal hydroxides (LRHs). They crystallize in the monoclinic system (space group C-2/m) with structures built up by alternate stacking of interlayer SO42- and the two-dimensional host layer composed of tricapped [LnO(9)] trigonal prisms along the a axis. In distinct contrast to the recently discovered hydrated LRHs Ln(2)(OH)(4)SO4 center dot 2H(2)O, which only exist for Ln = La-Dy, the host layers of the anhydrous phase are linked together by sharing edges instead of an O node of the SO42- tetrahedron. Rietveld refinement showed that the cell dimension tends to decrease for smaller Ln(3+), while the axis angle (beta = 98.78-100.318) behaves oppositely. Comparative thermogravimetric/differential thermal analysis in air revealed that the dehydroxylation and desulfurization temperatures become gradually higher and lower, respectively, for smaller Ln3+, and thus the temperature range of Ln(2)O(2)SO(4) existence is narrowed. The newly discovered Ln(2)(OH)(4)SO4, together with their hydrated counterparts, allow for the first time green synthesis of Ln(2)O(2)SO(4) with water as the only exhaust for the full spectrum of lanthanides. Calcining Ln(2)(OH)(4)SO4 in H-2 yielded phase-pure Ln(2)O(2)S for Eu and Gd and a mixture of Ln(2)O(2)S and Ln(2)O(3) for the other Ln. The effects of the lanthanide contraction were clearly revealed, and photoluminescence was found for the anhydrous LRHs of Eu and Tb.