Ab Initio Calculations of Fluoride–Phosphate Glasses: Impact of O/P Ratio on the Structures and Electronic States

Ab initio molecular dynamics (AIMD) simulations coupled with x‐ray diffraction experiments are performed to investigate the impact of the oxygen‐to‐phosphorus (O/P) ratio on the local structures and electronic states of fluoride–phosphate glasses. At low O/P ratios (3.0–3.4), increasing the O/P ratio initially replaces fluorine in the P─F terminal bonds with oxygen without significant change in the Q n distribution. At higher O/P ratios (3.6–4.0), excess oxygen disrupts P─O─P linkages and induces a drastic structural transformation from interconnected Q 2 units to isolated Q 0 units. These structural changes are attributed to the selective bonding behavior whereby P prefers to coordinate with O and other less electronegative cations (Al, Ca, and Ba) prefer to coordinate with F. Analysis of the electronic density of states reveals that the oxygen‐related valence band increases in intensity and shows upward shifts relative to the fluorine‐related band with increasing O/P ratio, in agreement with the experimentally observed bandgap narrowing. The O/P ratio dependence of the electronic states of oxygen is further investigated via Mulliken analysis, and the experimental results are consistent with the AIMD‐simulated structural and electronic trends, providing valuable insight into the factors affecting the optical properties of fluoride–phosphate glasses.