Oxysulfate, oxysulfide, and oxide red-phosphors from one single hydroxyl sulfate precursor (Gd,Eu)2(OH)4SO4⋅nH2O: Phase evolution, and photoluminescence

Abstract

Three kinds of important red phosphors, the monoclinic Ln2O2SO4 (Ln = Gd, Eu), cubic Ln2O3 and hexagonal Ln2O2S were obtained through controlled calcination of single hydroxyl sulfate Ln2(OH)4SO4⋅nH2O (LLnHs) precursor. The conditions of the hydrothermal synthesis of LGdHs were investigated, and it was found that LGdHs can be obtained in pH range of 7–9. The micro-morphology changes from aggregated spheres to micro plates with increasing pH. Gd2(OH)4SO4⋅nH2O was transformed into Gd2O2SO4via removal of hydration water (up to 340 °C) and dehydroxylation (340–800 °C), and finally change into Gd2O3via desulfuration (800–1230 °C) in the air. Gd2O2S was obtained from Gd2(OH)4SO4⋅nH2O by calcination in reducing atmosphere (800–1200 °C). Photoluminescence excitation (PLE) studies showed broad and strong O-Eu CT bands at 260, 275, and 267 nm in Ln2O3, Ln2O2SO4 and Ln2O2S, respectively, and extra S-Eu CT band locates at 334 nm was observed in Ln2O2S. The strongest red emission corresponding to the electric dipole 5D0 → 7F2 transitions of the Eu3+ ions were observed at 613, 618, and 626 nm in Ln2O3, Ln2O2SO4 and Ln2O2S system, respectively. The quenching concentration of Ln2O2S (3 %) is smaller than those of Ln2O2SO4 (6 %) and Ln2O3 (6 %), which is due to the fact that the average Eu3+-Eu3+ distance is smaller in Ln2O2S. The quenching mechanism was investigated, and it was found that the observed luminescence quenching was dominated by exchange interactions among Eu3+ ions.