Crystal formation in Eu³⁺ - Doped oxyfluorophosphate glass-ceramics for luminescence thermometry

Eu³⁺ doped-Oxyfluorophosphate glasses with a general composition 75 NaPO₃ - (25-x) CaO – x CaF₂ (with x ranging from 0 to 25 mol%) were prepared and their spectroscopic changes induced by the addition of fluorine and when applying a thermal treatment in order to grow crystals in the glasses were investigated. The addition of CaF₂ at the expense of CaO reduces Ω₂ and Ω₄ parameters indicating that the Eu³⁺ ions are located in less covalent and well separated sites in oxyfluorophosphate glasses. An increase in the CaF₂ content at the expense of CaO also leads to an increase in the absolute thermometric sensitivity from (4.2 × 10⁻⁴) K⁻¹ to (17.6 × 10⁻⁴) K⁻¹ at 523 K while decreasing the relative thermometric sensitivity from 3.0% to 1.4%. The heat treatment leads to the precipitation of crystals, the composition of crystal phase depends on the glass composition. For the glasses with x = 0 and 10, the thermal treatment increases Ω₆ while reducing Ω₄, as a result of an enhanced stiffness and reduced covalency of dopant's environment due to the precipitation of various crystals. As Ω₂ remains unchanged, the Eu³⁺ are suspected to remain mainly in the amorphous part of the glass-ceramics in agreement with the lack of sharp peaks in the emission spectrum. For the glasses with x = 20 and 25, Ω₂ increases indicating a change in the site symmetry of Eu³⁺ resulting from the dopant entering the CaF₂ crystalline phase. Due to the changes in the Eu³⁺ sites in the heat treated glasses, the heat treatment of the glass increases the absolute thermal sensitivity, but reduces the relative thermal sensitivity. The oxyfluorophosphate glass-ceramic with x = 25 exhibits high absolute thermometric sensitivity compared to oxyfluorophosphate glass, but also comparatively low relative sensitivity of 0.5% K⁻¹.