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© 2021 The American Ceramic SocietyOxyfluoride glasses of xLaF3–(60 − x)LaO3/2–40NbO5/2 (x = 0, 5, 10, 35) and xLaF3–(60 − x)LaO3/2–30NbO5/2–10AlO3/2 (x = 0, 10, 20, 30) were prepared using a levitation technique. Both the glass–transition temperature, Tg, and onset crystallization temperature, Tc, were lowered by substituting a part of the oxygen with fluorine in the glasses. An appropriate amount of fluorine maximized the difference between the temperatures, ΔT (= Tc − Tg), indicating the improvement in the glass–forming ability. The atomic packing densities of the glasses were approximately 60%, which gradually increased with the fluorine content. The absorption edge of the glasses shifted toward the shorter wavelength region in the ultraviolet spectra and toward the longer region in the infrared spectra by fluorine substitution. In addition, in one of the oxyfluoride glasses, a wide transparency from 307 nm to 9.2 µm was realized. Furthermore, the glass exhibited superior optical properties, with a combination of a high refractive index, nd, of 2.020 and low wavelength dispersion, vd, of 30.1. The effect of fluorine substitution on the nd and its vd was analyzed using the Lorentz–Lorenz dispersion formula.
