Stabilization of divalent Eu(2+) in fluorosilicate glass-ceramics via lattice site substitution

Fluorosilicate glasses and glass-ceramics with MF(2) (M = Ca, Sr, Ba), ZnF(2) or LaF(3) components were investigated to host divalent Eu(2+) 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 Eu(3+) → Eu(2+). 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 Eu(3+) were reduced to Eu(2+) in the glasses where the electronegativity had a crucial impact. In contrast, large amounts of Eu(3+) were reduced to Eu(2+) in the glass-ceramics containing MF(2) nanocrystals, where the reduction was determined by lattice site substitution. Using ZnAl(2)O(4) 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 Eu(3+)/M(2+) substitution. And using LaF(3) containing glass-ceramics as reference, it was certified that unbalanced charge at substitution sites induce the Eu(3+) → Eu(2+) reduction.