Effect of composition on the properties of glasses in the K ₂O-BaO-MgO-SiO₂-Al₂O₃-B ₂O₃-MgF₂ system

Twenty five glasses in the(1-Z)BaO:ZK₂O:(6 - X)MgO:XMgF ₂:(3 - Q)Al₂O₃:QB₂O ₃:8SiO₂ system have been prepared and the effect of systematic changes in composition (Z=0, 0.25, 0.5, 0.75 and 1.0, X = 2, 2.5 and 3.0 and Q = 0, 0.5 and 1) on molar volume (MV), fractional glass compactness (C), microhardness (μH_V), glass transition temperature (T _gmid) and coefficient of thermal expansion (α) determined. As potassium is substituted for barium (increasing Z), increases in MV and α, and decreases in C, μH_V and T_gmid arise which are attributed to the replacement of O_NB-Ba-O_NB ionic bridges by two O_NB-K terminations (O_NB = non-bridging oxygen). When fluorine is substituted for oxygen, reductions in μH_v and T_gmid and increases in MV and α values occur. These property changes occur because of reduced crosslink densities associated with the replacement of ≡Al-O-Si≡ crosslinks by ≡Al-F terminations. In general, the substitution of aluminium by boron results in decreases in MV, μH_V and T_gmid values and increases in α values. These effects are attributed to boron assuming a tri-coordinated state in the glass, giving rise to reduced crosslink densities, together with the release of modifier cations (Mg, K and Ba) from their charge balancing role for four-coordinated aluminiums leaving them free to cause greater network disruption.