Layered hydroxyl sulfate: Controlled crystallization, structure analysis, and green derivation of multi-color luminescent (La,RE)2O2SO4 and (La,RE)2O2S phosphors (RE = Pr, Sm, Eu, Tb, and Dy)

Abstract

The two important groups of Ln2O2SO4 and Ln2O2S compounds are traditionally synthesized with the involvements of environmentally harmful sulfur-containing reagents. We developed in this work a unique green approach for their synthesis, using Ln2(OH)4SO4·2H2O layered hydroxyl sulfate as the precursor (Ln-241 phase). Phase selective crystallization of La-241 under both atmospheric pressure and hydrothermal conditions was firstly optimized, followed by transformation into La2O2S and La2O2SO4 by controlled calcination. Rietveld structure refinement was performed for La-241, La(OH)SO4, La2O2SO4, and La2O2S, and the crystal structure and cell parameters of La-241 were originally reported. The photoluminescence performances of several important activators (Pr3+, Sm3+, Eu3+, Tb3+, and Dy3+) in the two hosts, in terms of excitation, emission, quantum yield, and color coordinates of emission, were thoroughly investigated, and multi-color luminescence including bright red, green, orange red, and yellow was obtained under ultraviolet excitation. Detailed investigations of Tb3+ photoluminescence revealed that the lack of 5D3 emission in La2O2S and the gradual quenching of 5D3 blue emission at a higher Tb3+ content (hence decreasing I488/I545 ratio and changing color coordinates) in La2O2SO4 were suggested to be due to thermal activation of the 5D3 electrons into the conduction band and cross relaxation between adjacent Tb3+, respectively. The synthesis approach developed in this work for La2O2SO4 and La2O2S, with water vapor as the only exhaust gas, is environmentally benign and holds great potential in the facile synthesis of analogous compounds of other lanthanides.