Structural insight of fluorophosphate glasses through F/O ratio: Case study of Raman and NMR spectra

Fluorophosphate glass has excellent characteristics such as low nonlinear refractive index, anomalous partial dispersion, low phonon energy, high UV transparency and high rare earth doping concentration, etc. The two anion ions of fluorine (F) and oxygen (O) and their F/O ratio in fluorophosphate glasses provide important structural characteristics of the specific glass. To explore this, a series of fluorophosphate glass samples with theoretical F/O ratios ranging from 1 to 7 were prepared, Raman spectroscopy and 31P nuclear magnetic resonance (NMR) spectroscopy were mainly employed to elucidate the structural polyhedrons made of the network chain. The underlying correlation between the Qn units and F/O ratio was explored, i.e., the Q2 unit dominate when the F/O ratio is less than 0.3, and predominantly Q2 or Q1 unit is predominant as F/O ratio ranges from 0.3 to 1, it comes to be Q1 or Q0 unit as F/O ratio further increase to 4, while it becomes Q0 when the F/O ratio is greater than 4. The applicability of this interval estimation was verified by comprehensive analyzing a variety of structural results of fluorophosphate glasses with various glass compositions. The explored interrelation between the F/O ratio and structural property is of great significance for developing low nonlinear and UV transparent fluorophosphate glasses.