One example of glasses containing a RE is the lanthanum borate system, which exhibits a high refractive index due to the presence of heavy RE ions. The large number of electrons in RE elements play a key role in increasing the refractive index of optical glasses; furthermore, RE-doped optical-amplification fibers and low-loss optical fibers are currently being researched for new applications in advanced optical communication. Trivalent RE elements can also be generated as fission products of nuclear fuel, and they are found in spent nuclear fuel. Because nuclear waste is mixed and solidified into borosilicate glass for its final disposal, it is essential to understand the nature of borate glass containing trivalent REs to gain a fundamental understanding of nuclear-waste immobilization. Binary xLa 2 O 3 -(1 -x)B 2 O 3 glass systems have been studied in a wide composition range. Glasses with x = 0.25 to 0.28 can form homogeneously without liquid-liquid phase separation, and they show high refractive indices (n = 1.70-1.77). Glasses with RE-rich compositions represent a new group of materials, and they have been extensively studied in recent years; 10, these new materials are known to exhibit characteristic optical properties related to their peculiar structural features. It has been suggested that the introduction of excess trivalent REs in borate glasses can produce abundant non-bridging oxygen for charge compensation, resulting in isolated BO 3 3units, as revealed by solid-state 11 B and 17 O nuclear magnetic resonance (NMR) measurements. Such an isolated BO 3 3structure formed solely from non-bridging oxygen is also found in the LaBO 3 crystal, as shown in Fig. , in which the charge of La 3+ is balanced by three non-bridging oxygens. Interestingly, the borate structure in YBO 3 with the same trivalent RE has a ring with three BO 4 -units in contrast to the isolated BO 3 units in LaBO 3 , as shown in Fig. . In a previous report, we explained that this structural change in borate units for YBO 3 and LaBO 3 was the result of a difference in the ionicity of Y 3+ and La 3+ .