Germanium Anomaly And Its Related Issues

Germanate glass is a class of heavy metal oxide glass with excellent transparent performance in a wide infrared wave band because of its longer infrared cut-off wavelength. As a result, germanate glass is a kind of ideal infrared optical window material. The current research to germanate glasses were focused on germanate anomaly. They put forward different theoretical models about the inner restructuring process, but had not formed a recognized, credible conclusion. As a basic research, getting to know the structure of germanate glass as well as the restructuring process is of great significance for the future theoretical research and applications.In this paper, the binary alkali metal germanate glasses were prepared by conventional melt-quenching method. The relationships between chemical composition, micro-structure and the inner restructuring process were researched by DTA,DSC,Density,XRD,FTIR and Raman. Based on the research of the binary alkali metal germanate glasses, the forming and the structure of binary alkaline earth metal germanate glasses and mixed alkali germanate glasses were studied in this paper. The main conclusions of this paper are listed as below.Pure crystalline state germania GeO2 (a-cristobalite type) has a 3-D network structure, formed by corner-sharing between six-mumbered [GeO4] rings. All of the germanium atoms are four-coordinated. Density measurements clearly show that the present glasses do exhibit the germanate anomaly. As introducing alkali metal oxides to GeO2 network, alkali metal oxides melt to form R+ and O2-. On the one hand, alkali metal cations result in breaking the Ge-O-Ge bonds and forming Ge-O-R. So there are a few of NBOs in the structure. On the other hand, with progressively increasing R2O, the O/Ge ratio increases and most of O2-get involved in transition from [GeO4] to [GeO6] coupled with conversion of high-membered rings(4 or 6-membered [GeO4] rings) to 3-membered rings composed of [GeO4] and [GeO6]. The cations occupy interstitial holes in the network structure. The [GeO6] and 3-membered rings which are dominant in the structural rearrangement densify the network and suppress the effect of NBOs. So the density of glasses increases. When the concentrations of alkali metal oxides are high enough, the number of NBOs increases to a certain point and begins to be dominant in the network which causes the decreasing of the density of germanate glasses. Beyond the anomalous point the high coordination number [GeO6] and 3-membered rings continue to be formed. Their existence has a certain tightening effect of the network which slow the rate of the density of germanate glasses increasing. Before the anomaly, the field strength per unit volume of Li+, Na+ and K+ has a greater positive impact on the density while the relative atomic mass per unit volume of Rb+ and Cs+ plays a more important role in increasing the density.The "germanate anomaly" exists in the relationships between density and mole%R2O,the density maxima versus the type of alkali metals,the content of alkali metal oxides correspond to the density maxima versus the type of alkali metals. Based on the analysis of the parameters of all alkali metal ions, the field strength per unit volume of alkali cation is the prominent structural change responsible for changes in the structure of xR2O-(1-x)GeO2 (R= Li, Na, K) series glasses. For the potassium and rubidium, the relative atomic mass per unit volume of alkali cation has the greater impact on the density of alkali germanate glasses. With the content of R2O increasing, the six-membered [GeO4] ring converts to three-membered ring composed of [GeO4] tetrahedra and [GeO6] octahedral in the xR2O-(1-x)GeO2 (R= Li, Na, K, Rb, Cs) series glasses. There are still six-membered [GeO4] rings at the anomaly points in the xR2O(1-x)Ge02 (R= Li, Na, K) series glasses. What is the difference in the xR2O-(1-x)GeO2 (R=Rb, Cs)series glasses is that the six-membered [GeO4] rings do not exist at the anomaly points.The six-membered rings are not completely transformed into three-membered rings and some of them are broken by the O-. Beyond the anomaly point, the high coordination number Ge and three-membered rings continue to be formed, but not play a major role. Their existence has a certain tightening effect of the network, meanwhile delay the Raman activity of Ge-O-. Of course there is a maximum amount of the high coordination number Ge. The binary alkaline earth metal germanate glasses also exhibit the germanate anomaly in which the restructuring process are similar to the alkali metal germanate glasses. There is a platform in the germanate anomaly curve of the mixed alkali germanate glasses which is due to the formation of one to one couple between alkali metal ions.