Drastic Connectivity Change in High Refractive Index Lanthanum Niobate Glasses

The highly ionic, high refractive index La2O3–Nb2O5 system has a La-rich glass forming region and another Nb-rich glass forming region. The La-rich and Nb-rich regions have markedly different structural and physical properties. Structural analyses using diffraction and spectroscopic measurements combined with structural modeling show that the Nb-rich glass, which has unusually high oxygen packing density, is a network of distorted NbOn polyhedra with mainly corner-sharing, and LaOx polyhedra with both corner-sharing and edge-sharing. Contrastingly, in the La-rich glass, small-sized symmetrical NbOn polyhedra with a large amount of edge-sharing are inhomogenously distributed in the network of LaOx polyhedra. The drastic connectivity change of cation–oxygen polyhedra and the dense oxygen packing due to edge-sharing polyhedra contravene long-established rules of oxide glass formation. These results raise the possibility that novel higher refractive index with lower wavelength dispersion glasses, which contain highly ionic heavy elements at the lower left in the periodic table, may be synthesized.