Effect of CaF2 substitution by CaO on spectroscopic properties of oxyfluoride glasses

The main purpose of the present study was to investigate the optical properties and structural changes of oxyfluoride glasses containing various amounts of CaF2 (CaO). Four nominal chemical compositions, i. e., 45SiO(2)-20Al(2)O(3)-(30-x)CaF2-(5+x)CaO (x = 0, 5, 10, 15) (mole ratio), were chosen and glass samples were obtained by convenient melt- quenching process. On the basis of DSC curves, the peak temperature of CaF2 crystallization was increased from 673 degrees C to 805 degrees C by replacing CaF2 with CaO. This result was related to the reduction of the number of non-bridging fluorines (NBFs) and non-bridging oxygens (NBOs) in presence of higher CaO amounts. Increment of density (from 2.74 to 2.92 g/cm(3)) and decrement of molar volume (from 27.24 to 24.48 cm(3)) of samples approved this claim to some extent. In addition, according to XRD patterns, the size of CaF2 nanocrystals got smaller (from 29.21 to 10.13 nm) by increasing the quantity of CaO. This was ascribed to the difficulty of the penetration of ions through the diffusion barriers around fluoride nanocrystals in glasses with more continuous structure. Fluorine loss measurements and Structural studies showed that more CaO addition decreased NBFs and NBOs and more Al-F bonds were increased compare to Si-O-Al bonds. UV-vis spectra were used to determine the optical properties comprising Fermi energy level, optical band gaps and Urbach energy. Calculations of Fermi energy and optical band gaps demonstrated the reduction of weak and dangling bonds in samples with more CaO. Moreover, Urbach energy, the indicator of the disorderliness of the network, was decreased from 0.275 to 0.245 eV. The sample containing 25 and 10 mol ratio of CaF2 and CaO respectively, showed higher transmittance in IR region and its cut-off was red-shifted to higher wavelengths in comparison to other glasses.