Controlled Hydrothermal Crystallization of Anhydrous Ln 2 (OH) 4 SO 4 (Ln=Eu-Lu, Y) as a New Family of Layered Rare Earth Metal Hydroxides.

Anhydrous hydroxide sulfates Ln2 (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 C2/m) with structures built up by alternate stacking of interlayer SO4 2- and the two-dimensional host layer composed of tricapped [LnO9 ] trigonal prisms along the a axis. In distinct contrast to the recently discovered hydrated LRHs Ln2 (OH)4 SO4 ⋅2 H2 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 SO4 2- tetrahedron. Rietveld refinement showed that the cell dimension tends to decrease for smaller Ln3+ , while the axis angle (β=98.78-100.31°) 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 Ln2 O2 SO4 existence is narrowed. The newly discovered Ln2 (OH)4 SO4 , together with their hydrated counterparts, allow for the first time green synthesis of Ln2 O2 SO4 with water as the only exhaust for the full spectrum of lanthanides. Calcining Ln2 (OH)4 SO4 in H2 yielded phase-pure Ln2 O2 S for Eu and Gd and a mixture of Ln2 O2 S and Ln2 O3 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.