Controlled Hydrothermal Crystallization of Anhydrous Ln(2)(OH)(4)SO4 (Ln = Eu-Lu, Y) as a New Family of Layered Rare Earth Metal Hydroxides

Anhydrous hydroxide sulfates Ln₂(OH)₄SO₄ (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 SO₄²⁻ 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)₄SO₄⋅2 H₂O, 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 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 Ln₂O₂SO₄ existence is narrowed. The newly discovered Ln₂(OH)₄SO₄, together with their hydrated counterparts, allow for the first time green synthesis of Ln₂O₂SO₄ with water as the only exhaust for the full spectrum of lanthanides. Calcining Ln₂(OH)₄SO₄ in H₂ yielded phase-pure Ln₂O₂S for Eu and Gd and a mixture of Ln₂O₂S and Ln₂O₃ 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.