Structural transformations and optical properties of glass-ceramics based on ZnO, β- and α-Zn2SiO4 nanocrystals and doped with Er2O3 and Yb2O3: Part I. The role of heat-treatment

Differential scanning calorimetry, X-ray diffraction analysis, transmission electron microscopy, high-and low-frequency Raman and optical spectroscopy were applied to the study of structural transformations in glass of the K2O - ZnO - Al2O3 - SiO2 system codoped with Er2O3 and Yb2O3 and subjected to heat-treatments at 680-1300 degrees C. Transparent glass-ceramics based on ZnO nanocrystals are prepared by heat-treatments at 680-800 degrees C, while at 860-900 degrees C, beta-Zn2SiO4 (beta-willemite) nanocrystals precipitate in addition to ZnO; alpha-Zn2SiO4 nanocrystals appear additionally at 950 degrees C, and the material loses transparency. Opaque glass-ceramic prepared by heat-treatments at 1000 degrees C contains only crystals of ZnO and alpha-Zn2SiO4, while crystallization of potassium aluminosilicates, KAlSi2O6 (from the surface) and KAlSiO4 (from the bulk) in addition to ZnO and alpha-Zn2SiO4, is observed at above 1000 degrees C. Optical absorption and luminescence of the initial glass and GCs are reported. A shift of the UV absorption edge and the near-IR losses due to the free charge carrier scattering are indications of the ZnO crystallization. Multiple visible emissions from the glass and GCs are determined by the Er3+ ions, nanocrystals of alpha-Zn2SiO4 and defects of the ZnO nanocrystals. The color of the latter emission can be tuned by varying the heat-treatment temperature. The rare-earth ions (RE3+) do not enter into the Zn-containing nanocrystals and remain in the residual glassy phase until similar to 1200 degrees C, when the RE silicates (RE2Si2O7) are formed resulting in structured intense Er3+ emission.