Crystallization of nanoheterogeneities in Ga-containing germanosilicate glass: Dielectric and refractive response changes

Modification of the optical and dielectric response upon nanocrystallization represents a tool for functionalizing glass-based materials and an alternative approach for data storage in optical media. Here we report results for Ga-containing alkali-germanosilicate glass undergoing thermally activated secondary phase separation and crystallization of native nanoheterogeneities with formation of dense dispersions (105 μm−3) of γ-Ga2O3 nanocrystals in transparent glass ceramics. The shift of the Urbach tail in the UV absorption and the refractive index increase upon nanocrystallization are analyzed following Duffy's approach and the Maxwell-Garnett description of refractive index in composite systems. The results clarify that native nanoheterogeneities consist of Ge-containing Ga-oxide and, consequently, composition changes occur between matrix and nanophase during nanocrystallization. Complex impedance data are analyzed to disentangle the electronic and ionic contributions to the dielectric response, pointing to a lowering of ionic polarizability parallel to the electronic polarizability increase. The analysis, aided by 71Ga NMR data, gives an insight into the relation between coordination changes and refractive and dielectric response, highlighting the role of non-bridging-oxygens and interphase.