Refractive index behavior in phosphate glass from the view point of B3+ and Al3+ coordination

Higher refractive index and higher Abbe value were obtained simultaneously at B2O3/Al2O3 = 1 in the system of 65P2O5–xAl2O3–(20 − x)B2O3–10CaO–5Li2O, although the refractive index generally shows a positive correlation with the dispersion. We investigated the molar volume and the molar refraction by Lorentz–Lorenz’s formula and also studied the coordination number of 11B and 27Al by MAS NMR. By the replacement of Al2O3 with B2O3, the molar refraction constantly decreased, but the molar volume was minimized at B2O3/Al2O3 = 1. The refractive index behavior of the glasses was mainly determined by the molar volume in the system. The coordination number of B3+ was only IV and it did not change if the composition was changed. On the other hand the coordinations IV, V and VI were observed for Al3+. The ratio of Al3+(VI) was maximized at B2O3/Al2O3 = 1. It is considered that the higher coordination of Al3+ brings the improvement of the packing and it leads to high refractive index.