Stabilization of divalent Eu²⁺ in fluorosilicate glass-ceramics via lattice site substitution

Fluorosilicate glasses and glass-ceramics with MF₂ (M = Ca, Sr, Ba), ZnF₂ or LaF₃ components were investigated to host divalent Eu²⁺ for photoluminescence (PL) application. X-ray diffraction phase identification and a series of spectroscopic analyses were performed to reveal the relationship between microstructure and the reduction of Eu³⁺ → Eu²⁺. The precursor glasses were believed being constituted by silicate-rich phases and fluoride-rich phases, due to the immiscibility of fluoride-and-silicate mixed glass system. After heat treatment, the fluoride-rich glass phases could transform into fluoride crystalline phase in the glass-ceramics. Europium tended to enrich in the fluoride-rich phases in the glasses or in the precipitated fluoride crystalline phases in the glass-ceramics. Small amounts of Eu³⁺ were reduced to Eu²⁺ in the glasses where the electronegativity had a crucial impact. In contrast, large amounts of Eu³⁺ were reduced to Eu²⁺ in the glass-ceramics containing MF₂ nanocrystals, where the reduction was determined by lattice site substitution. Using ZnAl₂O₄ containing glass-ceramics as reference, it was evidenced that the similar and a little larger radii between sites and substitution ions are the prerequisite for Eu³⁺/M²⁺ substitution. And using LaF₃ containing glass-ceramics as reference, it was certified that unbalanced charge at substitution sites induce the Eu³⁺ → Eu²⁺ reduction.