Luminescence and Cationic-Size-Driven Site Selection of Eu ³⁺ and Ce ³⁺ Ions in Ca ₈ Mg(SiO ₄ ) ₄ Cl ₂

In this work, the morphology, composition, crystal, and electronic structure of Ca ₈ Mg(SiO ₄ ) ₄ Cl ₂ (CMSOC) prepared by a high-temperature solid-state reaction technique are characterized first. To investigate the site occupancies of Eu ³⁺ and Ce ³⁺ in CMSOC, the emission spectra under well-chosen wavelength excitations and the corresponding excitation spectra by monitoring of the specific wavelength emissions are measured in detail for singly doped samples with different concentrations. Two kinds of Eu ³⁺ or Ce ³⁺ luminescence spectra are found. On the basis of the chemical environments of two Ca ²⁺ sites and dielectric chemical bond theory, the sites of these two kinds of Eu ³⁺ and Ce ³⁺ luminescence spectra are respectively assigned. Because energy transfer between the two types of luminescent centers, concentration-dependent emission-wavelength shifting, and luminescence concentration quenching are negligible, the emission spectra of Eu ³⁺ and Ce ³⁺ give us a hint of their occupation preferences on two Ca ²⁺ sites. The results indicate that, with an increase of the doping concentration, the Eu ³⁺ ions with smaller cationic size show an occupation preference on the smaller Ca ²⁺ (1) sites, but the Ce ³⁺ ions with larger cationic size are inclined to enter the larger Ca ²⁺ (2) sites. These opposite occupation preferences of Eu ³⁺ and Ce ³⁺ in CMSOC are thought to be the cationic-size-driven site selection.