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

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 (Gd0.99Tb0.01)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.