Anomalies of some physical properties and electrochemical performance of lithium–lead–germanate glasses M. Rada b , E. Culea a, ⁎, S. Rada a, ⁎, A. Bot b , N. Aldea b , V. Rednic b a Department of Physics & Chemistry, Technical University of Cluj-Napoca, 400020 Cluj-Napoca, Romania b Nat. Inst. For R&D of Isotopic and Molec. Technologies, Cluj-Napoca, 400293, Romania abstract article info Article history: Received 5 July 2012 Received in revised form 22 August 2012 Available online 4 October 2012 Keywords: Lithium–lead–germanate glasses; FTIR and UV–VIS spectroscopy; DFT calculations The purpose of this paper was to approach the structure–properties interrelationship of lithium–lead–germanate glasses in order i) to understand the structural mechanism responsible for the germanate anomaly; ii) to determine some physical properties such as density, optical gap energy, refractive index; iii) to find electrochemical performance of the studied glasses. The studied homogeneous glass system has the xLi 2 O·(100 -x)·[7GeO 2 ·3PbO] composition where 0 ≤x ≤40 mol%Li 2 O. Our results show that the contracting effect of lithium ions causes the enhancement of density and reduction of molar volume of glass samples. By increasing lithium ions content up to 10 mol%, major changes in FTIR spec- trum of glass introduce lithium ions as network modifiers. Accordingly, the increase of lithium ions concentration up to 20 mol% turns on the modification in density, gap energy and refractive index values. By taking these under consideration, it would be noted that network modifying role of the lithium ions affects the properties (density, optical gap energy, refractive index) more than GeO 4 /GeO 6 ratio. The conductivity and electrochemical perfor- mances of the glass system with 20Li 2 O·80[7GeO 2 ·3PbO] composition were demonstrated. © 2012 Elsevier B.V. All rights reserved. 1. Introduction For many years the structure and properties of germanate glasses attracted the attention of glasses scientists because of their interesting and intriguing peculiarities. A large number of published works presenting many ideas concern on the correlation between structure and properties [1–3]. It is accepted that the addition of network modifiers to vitreous GeO 2 initially leads mainly to an increase in the coordination number of some of the germanium atoms from 4 to 6, rather than to the formation of non-bridging oxygen atoms as in silicate glasses. The structure of GeO 2 glass is a three-dimensional random network with the coordination number of four and with each oxygen being connected to two germanium atoms [4]. Adding alkaline-metal ions, such a network is able to incorporate the additional oxygen by changing the coordination number of germanium from four to six-fold. This change would induce the so-called germanate anomaly observable for several physical properties such as the refractive indices, densities, elas- tic constants and molar volumes. To understand the germanate anomaly effect in glasses much progress has been made in the last years. It is a generally accepted rule that the increase of coordination is accompanied by an increase in density just as observed in alkali-germanate glasses. This picture agrees with the absence of such changes in alkali-silicate glasses because the small Si +4 ions do not allow an increase of coordina- tion number of silicon without external stress. The density of a glass is a powerful tool to examine the structural compactness of a glass network. The alkali-germanate glasses show that the relationship can be very puzzling because the density increases and presents a maximum near 15 mol% alkali-metal ion. The original expla- nation for this modification is germanate anomaly, which supposes a change of the coordination number from tetrahedral to octahedral coor- dinated germanium. Spectroscopic investigations and diffraction experiment show the germanate anomaly in a rather indirect way. It is more clearly manifested by the density and by physical properties which depend on density. In brief, it seems to be necessary to reconsider the densities. Therefore, the structural modifications of binary germanate glasses through the germanate anomaly remain controversial in two respects. Firstly, there is an alternative viewpoint, according to which the forma- tion of higher coordinated germanium does not occur or does not play an important role in the germanate anomaly. Secondly, it is not clear whether the higher coordinated germanium atoms, if their existence is needed, are five or six-cordinated, or a mixture of both. The recent interest in lithium conductive glasses and related glass-ceramics derivatives comes from their electrical, mechanical and optical properties which lead to extensive technological applications, especially in vacuum ultraviolet optics, electronic devices, batteries [5,6]. On the other hand, lead oxide is known as a non-conventional glass former, since it can act as a glass former or as a glass modifier. Journal of Non-Crystalline Solids 358 (2012) 3129–3136 ⁎ Corresponding authors. E-mail addresses: eugen.culea@phys.utcluj.ro (E. Culea), Simona.Rada@phys.utcluj.ro, radasimona@yahoo.com (S. Rada). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2012.08.026 Contents lists available at SciVerse ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol