Zinc borosilicate glasses doped with Co2+ ions: Synthesis and optical properties

New promising optical glasses containing cobalt ions in tetrahedral coordination have been synthesized, their mechanical and optical properties, as well as their characteristic temperatures, have been studied. High-quality glasses of the composition (70-x)ZnO–10SiO2-(20 + x)B2O3 (x = 0; 5; 10 wt%) doped with 0.1, 0.3, and 0.6 wt% CoO have been synthesized for the first time. The selected glass compositions were close to the eutectic one in the ZnO–SiO–B2O3 system. This made it possible to reduce the glass synthesis temperature from the 1350–1500 °C, described in the literature for similar zinc borosilicate glasses, to the 1170 °C. The presence of tetrahedral cobalt ions (CoO4) was identified by the absorption bands at 17000 cm−1 (4A2(4F) → 4T1(4P)) and at 6954 cm−1 (4A2(4F) → 4T1(4F)). Then the intense absorption bands in the visible region have been attributed to the 4A2(4F) → 2A1(2G), 4A2(4F) → 4T1(4P), 4A2(4F) → 2T2(2G), 4A2(4F) → 2T1(2G) and 4A2(4F) → 2E(2G) transitions. The most intense absorption band of octahedral cobalt ions (CoO6) corresponds to the 4T1(F) → 4T1(P) transition and is located in the region of 18000–20000 cm−1. According to analysis of Co2+ absorption spectra, the tetrahedral Co2+ ions concentration increased with increasing the CoO concentration in glasses. To study the nature of the bond between cobalt ion and ligands, the values of the splitting force of the ligand field (10Dq) and Racah parameter B were determined.