The role of networking in the optical anisotropy of hot-extruded calcium phosphate glass

We have investigated the structural modifications that occur concomitantly with the generation of optical anisotropy in the binary glass with nominal starting composition 0.51(CaO)0.49(P2O5) as a result of hot-extrusion processes. Micro-Raman and X-ray-fluorescence micro-analysis have been used to map the structure and composition along the directions parallel and orthogonal to the extrusion axis, with respect to which we have detected birefringence and refractive index modifications up to 10−3. The results show an annular region, midway between the axis and the outer side, in which Ca is slightly depleted and the phosphate chains are shortened and packed through crosslinking sites in anisotropic arrangement.

► The structural origin of the optical anisotropy in a hot-extruded phosphate glass was identified.
► Alkali depletion causes shortening and anisotropic packing of phosphate chains in the network.
► Anomalous relations between composition and connectivity hold during hot-working processes.