Effect of polyhedral co-substitution on the crystal structure and luminescence color of Sr₂(Al_1–xMg_x)(Al_1–xSi_1+x)O₇: Eu²⁺

A series of Sr 1.98 (Al 1– x Mg x )(Al 1– x Si 1+ x )O 7 : 2%Eu 2+ phosphors is prepared by the high-temperature solid-state reaction method, the crystal structures and luminescent properties of the prepared phosphors are investigated by measuring the X-ray diffraction, luminescent spectra and optical microscope. The isomorphic compounds of Sr 2 Al 2 SiO 7 and Sr 2 MgSi 2 O 7 contain tetrahedra including [MgO 4 ], [SiO 4 ] and [AlO 4 ]. Although the valences of the [MgO 4 ] 6– , [SiO 4 ] 4– and [AlO 4 ] 5– groups are different, the charge imbalance occurs when the [MgO 4 ] 6– and [SiO 4 ] 4– substitutes of [AlO 4 ] 5– and [AlO 4 ] 5– , respectively. While the groups are co-substituted, the charge imbalance disappears. And the larger volume of [MgO 4 ] and the smaller volume of [SiO 4 ] together replaces the similar volume of [AlO 4 ], resulting in the decrease of [(Si/Al)O 4 ] and increase of [(Mg/Al)O 4 ]. Moreover, the decrease of unit cell parameters c and the increase of a and V due to the increased replacement of Mg 2+ (0.57 Å for CN = 4) by Al 3+ (0.39 Å for CN = 4) and Si 4+ (0.26 Å for CN = 4) by Al 3+ (0.39 Å for CN = 4) cause the ambient temperature to change, the crystal field splitting of the Eu 2+ cation to be weakened, and the emission spectra to be blue-shifted from 503 nm to 467 nm, which are closely related to the local coordination environment of the Eu 2+ , in addition, this reveals that the emission color of this series of phosphors can be tuned from green with color coordinate (0.2384, 0.3919) to blue (0.1342, 0.1673) by adjusting the chemical compositions via the [MgO 4 ] 6– and [SiO 4 ] 4– groups’ co-substitution for [AlO 4 ] 5– . The full width at half maximumof emission band is 120 nm when x = 0, the photoluminescence emission width decreases monotonically from 89 to 50 nm as x is increased from 0.25 to 1. In other words, the full width at half maximum of emission band exhibits a decreasing trend. The internal quantum efficiency is enhanced with increasing x in Sr 1.98 (Al 1– x Mg x )(Al 1– x Si 1+ x )O 7 : 2%Eu 2+ phosphors. These results verify that the groups’ substitutions are enhanced with polyhedron changing in the solid solutions and contribute largely to the luminescence properties of the phosphor.