Structure and properties of gallate and germanate langasite infrared optical glass materials

Glasses in the system BaO - La2O3 - Ga2O3 - GeO2 and Langasite crystalline phases with the same stoichiometry have been studied considering the possible use of these glasses for laser-induced space-selective growth of crystalline architectures in the glass compositions. For better understanding of mechanisms involved in such controlled crystallization process, crystalline and glass compounds with same stoichiometry and different La/Ba ratios have been synthesized to compare their local structure. Solid-state Nuclear Magnetic Resonance evidence 4- and 5-coordinated gallium units in the glass matrix, denying unambiguously the formation of 6-coordinated gallium sites observed in crystalline phases. Moreover vibrational spectroscopies suggest a link between a 3D network formed by 4- and 5-coordinated gallium, connected to each other and to 4- coordinated Q4 and Q3 germanium units in the glass and their congruent crystallization. The transition from 4- and 5-coordinated gallium to 6-coordinated gallium sites observed in Langasite crystalline phases remains not still elucidated.