Network coordination in low germanium alkaline-earth gallate systems: influence on glass formation

Abstract

Glass formation in a newly formulated low germanium alkaline earth gallate (LGMGa, M = Ca/Mg/Zn/Sr/Ba) system by a high temperature melt quenching route is reported here. The proportionate ratios of tetrahedral to octahedral coordination of both Ga³⁺ and Ge⁴⁺ network cations are found to be crucial for vitrification. The enhancement in octahedral coordination led to surface crystallization and phase separation in cast melts. This behavior has been attributed to the overall reduction in tetrahedrally coordinated network units due to the failure of Ca²⁺ ions in charge compensation of the [GaO₄]⁻ tetrahedron. Among different network modifying oxides, BaO in combination with CaO (mixed alkaline earth) containing compositions have proved to yield clear glass formation with improved tetrahedral networking. This has been explained though the basicity equalization concept and also due to efficient charge compensation of the [GaO₄]⁻ tetrahedron requiring lower field strength cations like Ba²⁺. The effective phonon energy of this low germanium calcium barium gallate (LGCBGa) glass is found to be around 650 cm⁻¹, with an extended infrared transparency up to around 7 μm, which is superior to most of the low phonon energy oxide glass systems, making it promising for photonic and mid-infrared luminescence applications. Furthermore, the observed dark yellow to brown coloration in the glass has been attributed to the precipitation of metallic nanoparticles, and this mechanism has been discussed in detail.