Structural analysis of oxyfluoride borate glass and BaF2 crystallization from phase separation

Oxyfluoride glass and glass-ceramics are being studied due to the possibility of their utilization in many applications, such as thermoluminescent dosimeters, optical fibers, fluorescent concentrators, and temperature sensors. In order to improve the properties of glass, they are often crystallized. For this crystallization to be effective, knowledge and control of the structure of the starting material is required. Much work was done on silicate glass; however, a great interest exists in obtaining these kinds of materials in other matrices such as germanates, phosphates, and borates. Here, we present a structural study of some oxyfluoride borate glass with different concentrations of PbF₂ by means of Nuclear Magnetic Resonance (NMR), Raman, and Fourier Transform Infrared (FTIR) spectroscopies. We also analyzed glass-crystallization using Differential Scanning Calorimetry (DSC). The crystallization study was complemented with the use of Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED), for the purpose of identifying possible mechanisms of crystallization. Our results indicate, from the structural point of view, that the glass present groups with one or two BO₄ species, such as diborate or pentaborate. The small variation in the BO₄ fraction with the increase in PbF₂ concentration would be an indication of the formation of F⁻BO_3/2 species. Oxyfluoride borate glass containing BaO as a cation modifier and 30% PbF₂ represents a good starting point for obtaining glass-ceramics with a narrow size distribution of BaF₂ nanocrystals. We also propose that the possible mechanism for oxyfluoride borate glass crystallization started with a separation of phases.