The Germanate Anomaly in Alkaline Earth Germanate Glasses

A report is given of the first detailed investigation of the relation of structure to the germanate anomaly in alkaline earth germanate glasses. Pulsed neutron diffraction and high-energy X-ray diffraction measurements on calcium germanate glasses (22 to 41 mol % CaO, roller-quenched) show that the average Ge–O coordination number, nGeO, and average bond length, rGeO, exhibit broad maxima at ∼28 mol % CaO. Maxima in the mass density, atom number density, and glass-transition temperature also occur at a similar composition and are characteristic of the germanate anomaly. The anomalies occur at a higher modifier oxide content for alkaline earth (Ae) germanate glasses than for alkali (A) germanate glasses, attributed to the higher bond strengths within Ae-O polyhedra compared to those in A-O polyhedra. Consequently, the stabilization of [GeO6] octahedra and/or bridges between higher-coordinated [GeOm] units (m = 5, 6) is a feature of AeO–GeO2 glasses, in contrast to the existence of only [GeO4] tetrahedra and [GeO5] units proposed in alkali (Cs) germanate glasses. At high CaO content, nGeO behaves as in thallium germanate glasses because of the very similar metal–oxygen bond strengths within [CaO7] polyhedra and [TlO3] units. The composition dependence of the atom number density at low modifier content follows a common trend for Ca, Sr, and Ba germanate glasses because modifier cations fill germanate network cages. Our results show that the germanate anomaly for calcium germanate glasses is related to both a packing effect and a structural change in the germanate network; there is not a simple dichotomy whereby only one of these factors is of significance for the germanate anomaly.