New insights into the structure and physical properties of sodium and potassium germanate glass via the phase diagram approach

Sodium and potassium germanate glasses and their corresponding crystals are investigated by X-ray diffraction, Raman spectroscopy, and Fourier transforms infrared spectroscopy. Highly coordinated species of Ge are found in germanate glass. In the K2O-GeO2 glass system, three Ge structural units (4-, 5-, and 6-fold) coexist in the glass structure when the content of K2O ranges between 11 and 20 mol%, while only two Ge species (4- and 6-fold) are left with a further increase of K2O content. It is interesting to note that K2O·8GeO2 crystal, which is prepared by the solid state sintering method or crystallized from potassium germanate glass (K2O < 20 mol%), is a congruent melting compound in the K2O-GeO2 phase diagram. Furthermore, the density and refractive index of sodium and potassium germanate glasses are predicted, and a reasonable explanation of abnormal germanium is proposed according to the phase diagram approach. The results might provide new insight into the mechanism of germanate anomaly.