Site preference-based luminescence studies in Eu doped calcium magnesium silicate phosphor: A combined experimental and DFT approach

Abstract

The luminescence of Eu3+ doped calcium magnesium silicate (CMS: Eu3+) phosphor is studied experimentally by optimizing the single diopside structure in the solid state reaction method and theoretically verified using the first principle density functional theory (DFT) calculation. Field Emission Scanning Electron Microscopy (FESEM) analysis shows a transition from agglomerated morphology to particle formation at 900 °C. A combination of diopside and akermanite structures are formed at 800 °C, whereas a single diopside phase is optimized from 900 °C onwards. DFT calculation has been carried out to identify the lattice parameters of the diopside structure of calcium magnesium silicate (CMS) and Eu3+ doped CMS by replacing Eu3+ in the lattice site of Ca and Mg to identify the stable structure. The lattice expansion is minimal when Eu3+ replaces the Ca2+ position. Lattice parameters of the diopside phase calculated using Rietveld refinement of the XRD pattern are in agreement with DFT findings. FTIR analysis confirms a redshift corresponding to Ca–O and a blueshift corresponding to Mg–O stretching wavenumbers due to the structural modification of CMS due to Eu addition in the lattice. The stability and site preference of Eu3+ ion in CMS is determined from the mixing energy (ΔHm) of Eu doped in the Ca site and Mg site of CMS separately via DFT calculation and confirm that the mixing energy is minimum when Eu3+ replaces Ca lattice position. UV–visible, DRS spectroscopy confirms a bandgap of ∼6 eV for CMS and CMS: Eu3+. Luminescence studies confirm constant peak splitting in the 5D0→7FJ (J = 1,2,4) transitions due to the crystal field effect caused due to the incorporation of Eu3+ in the diopside structure with dodecahedral coordination. Eight coordinated site preference of Eu3+ in diopside is confirmed via DFT calculations. Modification in asymmetry ratio (R-ratio) with temperature also corroborates with modification in the intensity of photoluminescence spectra dictated by the reduction in inversion symmetry. CIE color chromaticity analysis shows a coordinate of (0.61, 0.39) in the red region of visible spectra with a color purity of 83.49% irrespective of annealing temperature.