An insight into local environment of lanthanide ions in Sr2SiO4:Ln (Ln = Sm, Eu and Dy)

Abstract

Sr₂SiO₄ is an important inorganic host for lanthanide-doped white-light-emitting diodes. In order to probe the local structure and symmetry around lanthanide ions in Sr₂SiO₄, detailed experimental and theoretical investigations have been carried out. Samples prepared via sol–gel methods were thoroughly characterized using X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) spectroscopy. The local symmetries of SrO₉ and SrO₁₀ polyhedra were determined using emission spectroscopy with europium as the probe ion. Despite having the same oxidation state and comparable ionic radii, Sm³⁺, Dy³⁺ and Eu³⁺ behave differently in terms of their site occupation at the Sr sites of SrO₉ and SrO₁₀ polyhedra. While Eu replaces Sr in both polyhedra, Sm and Dy reside specifically at the 9-coordinated Sr sites only. Based on density functional theory (DFT) calculations, it has been established that for Dy and Sm, the strong metal–oxygen bonding leads to significant distortion and hence the destabilization of MO₁₀ polyhedra in the host.