Coexistence of nano-scale phase separation and micro-scale surface crystallization in Gd2O3–WO3–B2O3 glasses

Glasses with the compositions of 25Gd2O3–xWO3–(75 − x)B2O3 with x = 25–65 were prepared by using a conventional melt quenching method, and their thermal stability and crystallization behavior were examined. The glass transition and crystallization temperatures decreased with increasing WO3 content. It was found from observations with the scanning transmission electron microscopy in high angle annular dark-field imaging mode that a glass of 25Gd2O3–40WO3–35B2O3 with x = 40 showed the nano-scale (2–3 nm) phase separation of tungsten-rich and boron-rich glassy phases prior to the crystallization. The glass with x = 40 has a prominent surface crystallization, providing the formation of α-Gd2(WO4)3. It was also found that even in sufficiently crystallized glasses their micro-structure consists of both nano-scale phase separation and micro-scale α-Gd2(WO4)3 crystals. The appearance of the phase separation was discussed from the glass forming tendency in the Gd2O3–WO3–B2O3 system.