Unraveling the role of crystal structural variations in modifying the luminescence properties of CMS: Eu3+ phosphor

Abstract

Pristine and europium doped calcium magnesium silicate (CMS and CMS: Eu³⁺) phosphor having akermanite (Ca₂MgSi₂O₇), monticellite (CaMgSiO₄) and merwinite (Ca₃MgSi₂O₈) phases are synthesized via solid state reaction method. The modification of photoluminescence properties such as emission intensity, decay time and quantum yield (QY) due to the variation of the crystal structure, local site symmetry and coordination geometry of akermanite, monticellite and merwinite phases are studied. The merwinite phase is optimized at an annealing temperature of 900 °C, whereas monticellite phase is at 1100 °C. The agglomerated morphology changes to particle formation as the annealing temperature changes from 900 °C to 1100 °C. The lattice parameters and site preference of Eu³⁺ ions are determined using Density Functional Theory (DFT) calculations. The analysis of lattice expansion, formation enthalpy (${\Delta H}_f$) and mixing energy (${\Delta H}_m$) reveal the preference of Eu³⁺ occupation in the Ca²⁺ cationic site over Mg²⁺ for all three phases. The blueshift and redshift of Mg-O and Ca-O stretching in the Fourier Transform Infra-Red (FTIR) analysis agree with the DFT calculation. UV–visible spectra analyses reveal a modification in optical bandgap with Eu³⁺ addition. The highest intensity ⁵D₀→⁷F₂ induced electric dipole (ED), hypersensitive transition indicates the preference of Eu³⁺ ions in a non-inversion center for all the phases. This agrees with the higher ${\Omega }_2$ values from Judd-Ofelt (J-O) parameter calculations and local site symmetry analysis of DFT-optimized structures. The monticellite phase exhibits maximum crystal field splitting due to its octahedral geometry, whereas the akermanite phase, with its distinct dodecahedral geometry, displays maximum emission intensity, an extended decay time, and the highest quantum yield (QY). The modification of photoluminescence properties of the three phases is analyzed in detail based on the coordination geometry and the distortion in local sites due to Eu³⁺ doping in the Ca and Mg sites. CIE color chromaticity analysis confirms the orange-red emission with 91.94 %, 90.88 % and 90.24 % color purity for akermanite, monticellite and merwinite phases respectively. Hence, the present study throws light on the potency of the akermanite phase of CMS: Eu³⁺ phosphor with 70 % QY as the optimal matrix for Eu³⁺ ions over monticellite and merwinite host matrices.