A linear relationship between the glass transition temperature and local distortion of calcium gallate, barium gallate, and calcium aluminate glasses

Abstract

The isomer shift in the 57Fe-Mossbauer spectra of xCaO.(90-x)Ga2O3.10Fe2O3 (x=40-60) and xBaO.(90-x)Ga2O3.10Fe2O3 (x=65-70) glasses decreases gradually from 0.28 to 0.16 mm s-1 with increasing CaO or BaO contents. This indicates a formation of nonbridging oxygen atoms in the GaO4 and FeO4 tetrahedra. Large quadrupole splitting (Δ) values ranging from 0.98 to 1.24 mm s-1 indicate a noticeable distortion of the GaO4 and FeO4 tetrahedra. Mossbauer parameters of xCaO.(95-x)Al2O3.5Fe2O3 glasses (x=55-60) are comparable to those of the xCaO.(90-x)Ga2O3.10Fe2O3 glasses, suggesting that the local structures are essentially the same with each other. The glass transition temperatures (T(g)) of the calcium gallate, barium gallate, and calcium aluminate glasses are 702-728, 580-583, and 750-773 °C, respectively. It is found that the T(g) of several oxide glasses is proportional to the Δ, being expressed by T(g)=aΔ+b, where ''a'' is 680 °C mm-1 s and ''b'' is -180 °C when the Fe3+ ions are present at substitutional sites of the network former (NWF). The ''T(g)-Δ rule'' is also applicable to the Fe3+ ions present at interstitial sites, in which case ''a'' and ''b'' are 35 °C mm-1 s and 260 °C, respectively. These findings indicate that the T(g) is primarily determined by the distortion of NWF-oxygen polyhedra.