The Germanate Anomaly in Alkaline Earth Germanate Glasses

Abstract

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 an...