Partitioning of gadolinium and its induced phase separation in sodium-aluminoborosilicate glasses

Phase separation in sodium-aluminoborosilicate glasses was systematically studied as a function of Gd2O3 concentration with transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) methods. Gadolinium-induced phase separation in the glasses can be consistently explained by proposing that Gd cations partition to the borate-rich environments and subsequent agglomeration of the Gd–borate moieties, or short-range ordered structural groups, in the glass. Agglomeration of the Gd–borate rich environments is further discussed within the context of excess metal oxides, [Na2O]ex or [Al2O3]ex=|Na2O–Al2O3|, and excess B2O3, [B2O3]ex, available for incorporating Gd cations. Results showed that agglomeration of the Gd–borate rich environments occurred at a much lower Gd2O3 concentration in the glass without [Na2O]ex or [Al2O3]ex and at a significantly higher Gd2O3 concentration in the glass with either [Na2O]ex or [Al2O3]ex. Assuming 1BO4:1Gd:2BO3 (based on literature-reported Gd–metaborate structure) as a local Gd–borate environment in glass, we introduced the saturation index of boron, SI[B]=Gd2O3/(1/3[B2O3]ex), to examine the glass susceptibility to Gd-induced phase separation for all three alkali-aluminoborosilicate systems. While our results have provided some insight to the glass structure, they also provide insight to the mechanism by which the metal oxide is dissolved into the melt. This appears to occur predominately through boron complexation of the metal oxide.