Hydrothermal crystallization of a Ln2(OH)4SO4·nH2O layered compound for a wide range of Ln (Ln = La–Dy), thermolysis, and facile transformation into oxysulfate and oxysulfide phosphors

Abstract

The synthesis of a layered Ln₂(OH)₄SO₄·nH₂O material (Ln-241) with a smaller lanthanide ion (Dy³⁺) was successfully achieved through the optimization of the hydrothermal conditions, and the effect of lanthanide contraction on the chemical composition, phase structure, and crystallite/particle morphology of the products was investigated and discussed. Structure refinement showed that the lattice parameters (a, b, and c), cell volume, and axis angle across the series (Ln = La–Dy) monotonously decrease as the size of Ln³⁺ decreases. Comparative TG/DTA analysis in air indicated that the dehydroxylation temperature of Ln-241 tends to increase, whereas the dehydration and desulfurization temperatures decrease as the size of Ln³⁺ decreases, thus narrowing the stable temperature range for Ln₂O₂SO₄. Taking advantage of the fact that Ln-241 has exactly the same Ln/S molar ratio as Ln₂O₂SO₄ and Ln₂O₂S, the latter two groups of important compounds (excluding Ce) were facilely transformed from the former via the removal of water by calcination. The photoluminescence properties of Eu³⁺ and Tb³⁺, in terms of excitation, emission, fluorescence decay, quantum yield, and emission color, were investigated and compared for the two hosts Gd₂O₂S and Gd₂O₂SO₄, and the (Gd_0.99Tb_0.01)₂O₂S phosphor was shown to be stable under electron beam irradiation in the studied range and exhibited an increasingly higher emission brightness as the acceleration voltage (up to 7 kV) or beam current (up to 50 μA) increased.