Optical basicity and refractivity of germanate glasses

The acid-base properties of germanate glasses cannot presently be compared quantitatively with those of silicate, borate, and several other glasses because the optical basicity of the parent oxide, Delta(GeO2), has not yet been unequivocally established. The usual method of measuring optical basicity, relying on probe ions such as Tl+ or Pb2+, is prevented by the restricted ultraviolet transparency range of germanate glasses. An alternative method is used, based on refractivity data and the correlation between oxide polarizability and theoretical optical basicity known to operate in other glass systems. Data for alkali metal germanate systems (up to approximately 0.4 mole fraction alkali metal oxide) yield the value Delta(GeO2) = 0.61 and basicity moderating parameter for Ge4+, gamma, of 1.64. These values are supported by calculations based on the far-infrared rattling frequencies, v(eff), of alkali metal ions in germanate glasses xM(2)O-(1 - x)GeO2, M = K, Rb. The latter approach has led to the establishment of a new optical basicity scale for the germanate glasses, A = (nu(eff)/nu(0) + 1.94)/4.64, nu(0) = nu(eff)(x-->0). This new far-infrared basicity scale gives Delta(GeO2) = 0.63. GeO2 therefore has a basicity which is close to that of Al2O3 (Delta = 0.60) but greater than Of SiO2 (0.48). The previously established data concerning the ratio of 6-fold coordinated germanium in the K2O-GeO2, system allows calculation of gamma = 2.5 (+/-0.5) for 6-fold coordinated Ge4+.