T-g and FTIR of (2.5-x)CaO. x/3M2O3. 2SiO(2) (M = Y, La, In, Al, Ga) glasses

A comparative study of the effect of substituting oxides of (M = Y, La, In, Al, Ga) to CaO in 2.5CaO2SiO(2) glass in the narrow compositional range of the binary CaO-SiO2 system in which homogeneous glasses can be obtained (0.4 < CaO/(CaO + SiO2) < 0.55). A plot of glass transformation temperature, T-g, vs the ionic field strength, Z/r(2), where Z and r are the charge and the radius of the cation, is useful in discussing the role of oxides in the glassy structure. It is hypothesized that, in the composition range, studied Al2O3 and Ga2O3 act as network forming oxides, while La2O3, Y2O3 and In2O3 act as network modifying oxides. FTIR spectra agree with this hypothesis and with the expectations based on the criteria reported in the literature, in particular with the one proposed by McMillan that the network modifier cations have an ionic field strength Z/r(2) < 5 A(-2). (C) 1999 Elsevier Science B.V. All rights reserved.