Increase in the refractive index of alkali titanium-hafnium silicate glass upon Na+ (glass)-Li+ (melt) ion exchange

An increase in the refractive index upon ion exchange in the Li₂SO₄ + Li₂MoO₄ salt melt has been studied for glasses of composition (mol %)xLi₂O-(25 -x)Na₂O-15TiO₂-6HfO₂-54SiO₂, wherex = 0, 5, 10, 15, 20, and 25. The ordinary transparent single-layer and two-layer diffusion zones are obtained. In the latter zone, the optically opaque near-surface layer gives way to a deeper optically transparent layer with the refractive index gradient. The opaque layer produced by the low-temperature exchange exhibits a unique structure, which is permeable to melt. An increase in the refractive index as large as 0.055 is achieved for the first time without fracture and crystallization of glass.