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

Abstract Higher refractive index and higher Abbe value were obtained simultaneously at B 2 O 3 /Al 2 O 3 = 1 in the system of 65P 2 O 5 – x Al 2 O 3 –(20 − x )B 2 O 3 –10CaO–5Li 2 O, 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 11 B and 27 Al by MAS NMR. By the replacement of Al 2 O 3 with B 2 O 3 , the molar refraction constantly decreased, but the molar volume was minimized at B 2 O 3 /Al 2 O 3 = 1. The refractive index behavior of the glasses was mainly determined by the molar volume in the system. The coordination number of B 3+ 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 Al 3+ . The ratio of Al 3+ (VI) was maximized at B 2 O 3 /Al 2 O 3 = 1. It is considered that the higher coordination of Al 3+ brings the improvement of the packing and it leads to high refractive index.