The structural evolution and spectral blue shift of solid solution phosphors Sr3−mCamB2O6:Eu2+

Abstract

An unexpected spectral blue shift of the emission from yellow to green is observed for the solid solution phosphors Sr_3−mCa_mB₂O₆:Eu²⁺ (m = 0–3), when larger Sr²⁺ ions are substituted by smaller Ca²⁺ ions in Sr₃B₂O₆ lattices. The powder X-ray patterns of all the phases indicate a structural similarity to the end compounds and show a smooth variation of the structural parameters with composition in the full range. The linear structural evolution of the iso-structural Sr_3−mCa_mB₂O₆ solid solutions obeying Vegard's rule has also been examined and verified by Rietveld refinement, high resolution transmission electron microscopy and Fourier transform infrared spectroscopy. The average bond lengths of the central cations to the coordinating anions and the distortion indexes of the coordination polyhedra are investigated. As a result, the symmetry of the coordination crystal field is found to be higher with increasing Ca content in the solid solutions phosphors, which should be primarily responsible for the continuous blue shift of the emission spectra of Sr_2.96−mCa_mB₂O₆:0.04Eu²⁺ with the growth of the m value.