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

Abstract

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.