Thermal and optical properties of La_2O_3-Nb_2O_5 high refractive index glasses

(100−x)La2O3-xNb2O5 glasses with high refractive indices (>2.1) and low wavelength dispersion were prepared by containerless processing. All the glasses were colorless and transparent in the visible region. The glass forming region was divided into two regions: a high-La2O3-content (39 ≤ x ≤ 42) region and a high-Nb2O5-content (60 ≤ x ≤ 75) region. The dependence of the physical properties on the composition for these two types of glasses was different, implying that the high-La2O3-content glasses (La glasses) and high-Nb2O5-content glasses (Nb glasses) were intrinsically different. A large difference in the molar volumes of the two types of glasses indicated that the Nb glasses were more densely packed than the La glasses. Furthermore, the oxygen polarizabilities estimated from the molar volumes and refractive indices were greater than 2.43 A3 for the La glasses, while those for the Nb glasses decreased from 2.43 A3 with decreasing x. The large oxygen polarizabilities, as compared to those of conventional optical glasses, indicate a particularly high degree of ionic character for the component elements in the glasses. These results suggest that both the La and Nb glasses are desirable materials for high-valued optics, such as lenses with high power and high resolution as well as wide viewing angles used for a digital camera in a smartphone.