Optical properties of the europium (II) and (III) ions doped metal oxides obtained from sintering layered double hydroxides, and their fine structures

Abstract

We examined the optical properties of sintered layered double hydroxides (LDH) as phosphors. Eu3+-doped MgAl LDH and CaAl LDH were synthesized via co-precipitation methods. We sintered the obtained LDH at temperatures ranging from 300 to 1000 °C. The sintered MgAl LDH showed the same X-ray diffraction (XRD) pattern as rock salt in the temperature range between 300 and 800 °C and partly transitioned to spinel at 1000 °C. The sintered LDH showed red luminescence with a main peak at 613 nm. This dominant red luminescence is assigned to the 5D0-7F2 electric dipole transition of Eu3+, which is allowed for structures without inversion symmetry. The dominant intensity of the electric dipole transition is inconsistent with the highly symmetric rock salt structure. We concluded that this phenomenon is triggered by the reduction of symmetry around the Eu3+ ion due to the introduction of the Al3+ ion and the cation vacancy in the MgO rock salt host and/or the strain induced by the introduction of these dopants. Our conclusion is based on the X-ray absorption near edge structure (XANES) data and the extended X-ray absorption fine structure (EXAFS) analysis of the sintered NiTi LDH with the same rock salt structure as the sintered MgAl LDH.Eu2+-doped MgAl and CaAl binary metal oxides, synthesized by sintering Eu3+-doped MgAl and CaAl LDH in quartz tubes in the presence of Ti powder, showed broad peaks at the green (540 nm) and blue (440 nm) light regions, respectively. These results have demonstrated that red-green-blue (RGB) luminescence can be obtained using a uni-type host structure.