Anomalies of some physical properties and electrochemical performance of lithium–lead–germanate glasses

Journal of Non-Crystalline Solids 358 (2012) 3129–3136

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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 b

Department of Physics & Chemistry, Technical University of Cluj-Napoca, 400020 Cluj-Napoca, Romania Nat. Inst. For R&D of Isotopic and Molec. Technologies, Cluj-Napoca, 400293, Romania

a r t i c l e

i n f o

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

a b s t r a c t 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 xLi2O·(100−x)·[7GeO2·3PbO] composition where 0≤x≤40 mol%Li2O. 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 spectrum 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 GeO4/GeO6 ratio. The conductivity and electrochemical performances of the glass system with 20Li2O·80[7GeO2·3PbO] 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 GeO2 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 GeO2 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, elastic 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.

⁎ Corresponding authors. E-mail addresses: [email protected] (E. Culea), [email protected], [email protected] (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

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 coordination 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 explanation for this modification is germanate anomaly, which supposes a change of the coordination number from tetrahedral to octahedral coordinated 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 formation 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.

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The role is determined by its concentration and by the type of bond between lead and oxygen. Covalent bonding is associated with a network forming behavior, whilst an ionic bonding is related to glass modifier properties [7–9]. The aim of the present work was to investigate the compositional changes of lithium–lead–germanate glasses in significant spectral changes and to evaluate the structure–properties interrelationships, which reflect progressive structural variation of the glass network with increase of lithium ions content. Essential features of the germanate anomaly effect in ternare glasses, however, are still poorly understood. On the other hand, the purpose of this work is to determine, for this family of glasses, the optical band gap energies in view of applications for electrochemical cells. 2. Experimental details Glasses were prepared using reagent grade purity germanium (IV) oxide, lead (II) oxide and lithium carbonate of high purity in suitable proportion. The mechanically homogenized mixtures were melted in sintered corundum crucibles at 1000 °C in an electric furnace. The samples were put into the electric furnace direct at this temperature. After 10 min, the molten material was quenched at room temperature by pouring onto a stainless-steel plate. The samples were analyzed by means of X-ray diffraction using a XRD-6000 Shimadzu diffractometer, with a monochromator of graphite for Cu-Kα radiation (λ = 1.54 Å) at room temperature. The FTIR spectra of the glasses were obtained in the 350–1100 cm−1 spectral range with a JASCO FTIR 6200 spectrometer using the standard KBr pellet disc technique. The spectra were carried out with a standard resolution of 2 cm−1. UV-Visible absorption spectra of the powdered glass samples were recorded at room temperature in the 250–1000 nm range using a Perkin-Elmer Lambda 45 UV/VIS spectrometer equipped with an integrating sphere. These measurements were made on glass powder dispersed in KBr pellets. The validity of the band position is ±2 nm. The starting structures have been built using the graphical interface of Spartan'04 [10] and preoptimized by molecular mechanics. Optimizations were continued at DFT level (B3LYP/CEP-4 G/ECP) using the Gaussian'03 package of programs [11]. It should be noticed that only the broken bonds at the model boundary were terminated by hydrogen atoms. The positions of boundary atoms were frozen during a calculation and the coordinates of internal atoms were optimized, to model the active fragment flexibility and its incorporation into the bulk. The electrochemical properties were characterized using cyclic voltammetry using a VERSASTAT3 Potentiostat and V3Studio software. Discs of glasses were used as working electrode, platinum electrode as counter and calomel as reference electrode. All impedance spectra were obtained over the frequency range of 0.1–106 Hz by using a small sinusoidal excitation signal (10 mV amplitude). 3. Results The importance of the density for describing the structure of a glass is evident. The density of glass is mainly influenced by the molecular weight of glass components, the integration and the compactness of the glass network. When GeO2 is substituted by Li2O, the smaller molecular weight of Li2O contributes to the decrease of the density. Moreover, the formation of [GeO6] structural units improves the integration of the network and consequently increases the density. To investigate the role of Li2O and the mechanism for enhanced density, structural investigation was performed. Fig. 2 shows that the IR band situated at about 715 cm −1 is assigned to Ge–O–Ge stretching vibrations in [GeO6] structural units while the band centered at about 800 cm −1 is identified as due to vibrations of the Ge–O bonds in the [GeO4] structural units [12–15]. The bands located

at about 950 and 1050 cm −1 are assigned to the Ge–O–Ge and Ge–O stretching vibrations in [GeO4] structural units. In alkali-germanate glasses, the position of maximum of the IR band due to Ge–O–Ge stretching is shifted to lower wavenumbers indicating the change in the coordination number of germanium from four to six [15]. Study of the UV–VIS absorption data has proved to be a very useful method for elucidation of optical transitions and electronic band structure of the materials. In these cases, electromagnetic waves interact with electrons in the valence band, which are raised across the fundamental gap to the conduction band. In amorphous materials, the absorption coefficient (α) increases with the photon energy near the energy gap. So the band gap is a major factor determining the electrical conductivity of a solid. Fig. 3 plots the absorbance curves versus wavelength in the UV–VIS region for different glass samples doped with lithium ions. The measurements of optical absorption and the absorption edge are important especially in connection with the theory of electronic structure of amorphous systems. In graphs of the electronic band structure of solids, the energy band gap generally refers to the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. The gap band energy can be calculated from the curves obtained from UV–VIS absorption spectra, Fig. 4. These curves were plotted by the use of the relation:  n αhν ¼ α 0 hν–Eg

ð1Þ

where α0 is an energy-independent constant, n is a constant which determines the type of the optical transitions and can have different values, namely 2, 3, ½ or 1/3 corresponding to indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions, respectively. In our study, these curves represent the relation between the variation of (αhν) 2 versus hν which is used to calculate the optical band gap, Eg. The optical gap is determined by the intercept of the extrapolations to zero with the photon energy axis (αhν) 2 → 0. The width of mobility edge depends on the degree of disorder and defects present in the amorphous structure. In amorphous solids unsaturated bonds are responsible for the formation of these defects. Geometry optimization of local structure of the 40Li2O·60 [7GeO2·3PbO] glass network were carried out at the density functional theory calculations at the hybrid B3LYP level with the LANL2DZ basis functions (Fig. 6). Knowledge of the location of orbitals in glasses research is important (Fig. 7). One might need to know which part of the molecule is capable of absorbing a photon—normally that is where the HOMO is located. Once the photon is absorbed, the electron density migrates to other parts of the network, normally where the LUMO is located. Electrochemical performances of these glasses, when used as a working electrode, were examined in an electrochemical cell using silver nitrate solution as liquid electrolyte (Fig. 8) and were investigated by cyclic voltammetry (Fig. 9). 4. Discussion 4.1. Density, molar volume and oxygen packing density From Fig. 1, it is observed that changing the composition of lithium ions in the samples implies the variation of some physical properties, such as density, molar volume (Vm) and oxygen packing density (dO). It is observed that the oxygen packing density increases while molar volume decreases almost linearly with increasing lithium ion concentrations. It is well known that the change of molar volume is associated with the change of the glass structure. The decrease in molar volume is ascribed to a decrease in the number of non-bridging oxygens. This

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Fig. 1. Lithium oxide composition dependence on a) density, d, b) molar volume, Vm and c) the oxygen packing density, dO, for xLi2O·(100 − x)·[7GeO2·3PbO] glasses with x = 0–40% Li2O.

decrease in molar volume indicates that lithium ions have a contracting effect. This means that the lead–germanate glass network becomes more compact. Large volume contraction on adding lithium ions is due to the formation of [GeO6] structural units which are slightly larger in size than [GeO4] species and accordingly, increase the three-dimensional connectivity of the lead–germanate network. A simple inspection of the density data suggests that the gradual addition of lithium ions leads not only to a simple incorporation of these ions in the host glass matrix but also generates changes of the basic structural units of the glass matrix. The density increases along with the composition in the region between 0 ≤ x ≤20 mol%Li2O, after that it decreases by increasing lithium ions content up to 40 mol%. These findings indicate that lithium ion doping causes large modifications to the lead–germanate glass structure, probably by a large formation of octahedral germanium in the network which results in better packing and hence the increased density [16]. The decrease in density suggests an increasing formation of non-bridging oxygens and a loosely packed structure [17,18]. This behavior is a result of the creation of non-bridging oxygens which will break the bonds of the lead–germanate host glass, increasing the free space in the network [19]. 4.2. Infrared spectroscopy Fig. 2. shows infrared spectra of the ternare glasses in the xLi2O·(100 − x)·[7GeO2·3PbO] system where 0 ≤ x ≤ 40 mol% Li2O. The analyses of the IR spectra with smaller lithium ions content (0 b x ≤ 10 mol%) reveal that the accommodation of the network with the excess of oxygen ions is possible by the increase of the degree of polymerization of the lead–germanate network. In lead borate glasses it was found that the presence of lithium oxide can provide additional oxygen to the lead oxide, forming [PbO4] structural units, provided that the alkali cation remains in the vicinity of the formed [PbO4] structural units to keep the electron-neutrality [20]. On the other hand, it is known that in lead borate glasses, at low concentrations (15–20%), PbO acts as a modifier of the structural network having a high coordination number of 6 in [PbO6] structural units. For sufficiently large amounts of PbO (60% or higher), PbO acts as a glass forming agent for the vitreous network and is incorporated into the [PbO4] structural units. As a consequence, the Pb+2 cations which occupy interstices forming [PbO6] structural units, do not accommodate with the excess of oxygens and a [PbO6] donor–acceptor interaction with germanium ions facilitate a partial transformation of tetragonal-coordination germanium into the hexagonal coordination.

Fig. 2. FTIR spectra 0 ≤ x ≤ 40 mol% Li2O.

of

the

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xLi2O·(100 − x)·[7GeO2·3PbO]

system

where

For the sample with x = 10 mol% Li2O, a new IR band centered about 370 cm −1 appears indicating Pb–O stretching vibrations along with deformation modes of the germanium–oxygen network [19]. Then, the larger UV–VIS band situated at about 310 nm indicates the presence of Pb +2 cations. No non-bridging oxygens are formed because the gap energy will increase, in agreement with that in Section 4.4. A possible explication for the first formation of [PbO6] structural units and no of [GeO6] ones is that lithium ions have a pronounced affinity towards [GeO4] structural units. Therefore, in this stage, the number of [GeO6] structural units cannot be higher, because the modified [GeO4] structural units containing one or more Ge–O…Li + bonds are unable to accept an oxygen atom. This compositional evolution of the structure could be explained by considering that the excess of oxygen may be accommodated by the formation of some [PbO6] structural units and the formation of new [GeO4] structural units interconnected through Ge–O–Li bridges. For samples with x > 10 mol% Li2O, the bands situated over 950 cm−1 disappear and the band centered at about 790 cm−1 shifts towards higher wave numbers, to around 800 cm−1. These facts indicate that direct incorporation of Li2O into the lead–germanate network modifies the germanate network and the internal structure of glass network is rearranged. In turn, lead oxide is known as a non-conventional glass former since it can act both as a glass former and as a glass modifier. For higher Li2O concentration (x≥10 mol%), PbO acts as a glass former, whilst at low concentrations it acts as a glass modifier. The band situated at about 370 cm−1 seems not to have the same increase with increasing the Li2O concentration up to 10 mol%. This band was assigned to the Pb–O stretching vibration along with deformation modes of the germanium–oxygen network [21]. In brief, the larger differences of the IR bands position due to the addition of 10 mol% Li2O result from the network re-arrangements induced by lithium ions. Lithium ions break some of the Ge–O, Ge–O–Ge and Pb–O bonds, thus modifying the structure. For samples with 10 b x ≤ 20 mol% Li2O, a decrease in the relative intensity and a gradual disappearance of the bands situated over 950 cm−1 corresponding to Ge–O–Ge stretch in [GeO4] structural units occurs. This evolution indicates the decrease of the degree of germanate network linkage. The shift towards smaller wavenumbers can be related to the conversion of germanium ions from four- to six-fold coordination with increasing Li2O content. This interpretation is in accordance with the general notion that an increase in coordination number from 4 to 6 causes a decrease in Ge–O–Ge stretching frequency. No non-bridging oxygens are formed. This mechanism cannot explain the abrupt decrease of the gap energy (from 3.78 at 2.69 eV).

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An important question to be considered is if bridging oxygens are formed in the network whereupon how reveals the abrupt decrease of gap energy. We presume that the change of [GeO6] octahedral structural units is balanced by the localization of Li+ ions and formation of [GeO5] structural units (more thermodynamic stable than their analogues, [2]). The formation of six-fold coordination of the germanium ions reaches a limiting value (for x = 20 mol%Li2O). Then, for samples with x ≥ 30 mol% Li2O, some [GeO6] structural units were transformed to [GeO5] structural units. This hypothesis is supported by the FTIR data (a decreasing general trend of the bands towards smaller wavenumbers for sample with x = 40 mol%Li2O is observed), density data (for sample with x = 20 mol%Li2O, the density attains a maximum value, after it decreases) and gap energy calculations (for sample with x = 20 mol% Li2O, the gap energy attains a minimum value). As higher Li2O concentration (x > 20 mol%) is substituted into the lead–germanate network, new Ge–O–Li bonds occur in the glass network and difference in coordination number of lithium and germanium cations produces non-bridging oxygens, which result in a decrease of the density and an increase a gap energy. On the other hand, it is known that in the IR spectra of vitreous GeO2, in which the coordination number of germanium is four, the band situated at about 875 cm −1 is due to Ge–O–Ge stretching [22]. Some authors consider the bands situated at about 879 cm −1 and 887 cm −1 attributed to non-bridging oxygen ions [23]. In our case, the intensity of these bands decreases with the increase of lithium ions content up to 40 mol%. In conclusion, lead ions play the role of a modifier of the germanate network, but to a lesser extent than the lithium ions. This observation seems to be a general trend of lead oxide which to some extent has a glass network forming character since it favors covalent directed Pb–O rather than ionic Li +…..O −2 bonds. 4.3. UV–VIS spectroscopy A simple inspection of the UV–VIS spectra shows a strong absorption in all UV–VIS domains for samples with 10 ≤ x ≤ 20 mol% Li2O (Fig. 3). The absorption bands in UV–VIS region are generally interpreted in terms of π → π* transition type from the highest occupied molecular orbital (HOMO) to the lowest excited unoccupied molecular orbital (LUMO) [24,25]. The additional absorption bands situated in the range over 350 nm can be due to the generation of non-bridging oxygen ions centers [26–28].

Fig. 3. UV–VIS absorption spectra of the xLi2O·(100 − x)·[7GeO2·3PbO] system where 0 ≤ x ≤ 40 mol% Li2O.

For the sample with x = 10 mol% Li2O a strong absorption band in the ultraviolet region situated at about 310 nm can be observed. This band corresponds to the Pb +2 ions [29]. Accordingly, all properties of glasses will be strongly affected by long-range motions of lithium and lead ions showing large deviations from a simple additive behavior. This phenomenon is known as the mixed alkali effect [30] and occurs in all ionic conducting glasses, regardless of the types of ions that are mixed and the type of network constituents forming the disordered host matrix for the ionic motion. 4.4. Optical gap energy For the determination of nonlinear optical properties of materials, an accurate knowledge of absorption coefficient is necessary. According to Tauc [31], in many amorphous materials, variation of absorption coefficient with photon energy could be discussed in three different regions. The first region is known with the almost constant absorption due to exciton–phonon coupling [32]. The second region is associated with interband transitions and permits to calculate the optical band gap. The third region is an exponential one called Urbach region and illustrates the degree of crystallinity of the material. The gap energy can be calculated from the curves obtained from UV–VIS absorption spectra, Fig. 4. In the current study, the effect of lithium ions concentration on the glass network is given in Table 1. The gap energy values are lying within close limits around 1.98 eV to 3.78 eV for undoped and highly doped sample (x = 10 mol% Li2O). The variations in optical band gap values with the alkali content may be attributed to indirect influence of lithium ions on the band gap. It was believed that these variations may be due to the addition of higher proportions of the lithium ions that seems to cause progressive breakdown of the host network. Existence of lithium ions may cause the creation of energy levels in forbidden gap that could reduce gap of glass. As the concentration of lithium ions increases up to 20 mol%, the band gap is observed to decrease at 2.69 eV. This reduction is attributed to the structural changes occurring as a result of lithium increment in the forbidden gap (specific localized state in the deep energy levels) [33]. The mentioned reduction of the band gap associated with increasing lithium ion content can be attributed to reduction of average bond energy and fall of conduction band level. As a matter of fact, the lithium made new bands on the lattice between the germanium and oxygen ions. Bonding energy of samples has been decreased as a result of having less covalent and more ionic character. A probable reason for this reduction could be the creation of bonding oxygens in the structural units of the sample. Since there is a reverse relation between forbidden band of materials and its susceptibility and upon the fact that susceptibility is the static dielectric constant then, reduction of band gap reflects the tendency towards being a semiconductor material [28,34,35]. On the other hand, the observed shifts of the gap energy due to the lithium ions addition could be understood in terms of the variation in non-bridging oxygen ions concentrations [36]. In metal oxides, the valence band maximum mainly consists of O(2p) orbital and the conduction band minimum mainly consist of M(ns) orbital. The non-bridging oxygen ions contribute to the valence band maximum. When a metal–oxygen bond is broken, the bond energy is released. The non-bridging orbitals have higher energies than bonding orbitals [37]. In brief, for samples with 10 b x ≤ 20 mol% lithium ions, increase in concentration of the non-bridging oxygen ions results in the shifting of the valence band maximum to higher energies and reduces the band gap, in agreement with FTIR and density data. Thus, the lowering of band gap energy due to increase in the lithium ion content suggests that non-bridging oxygen ion concentrations increase with increasing lithium ion content that lowers the band gap energy. Observations of the variations of optical gap energy with increase in the modifier content (for samples with x = 10, 30 and 40 mol%

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Fig. 4. Plots of (αhν)2 versus hν for xLi2O·(100 − x)·[7GeO2·3PbO] glass samples.

lithium ions) can be attributed to the changes in the bonding that take place in the glass. A greater tendency to convert weak bonds into defects in presence of lithium ions could be demonstrated to values of the Urbach energy. Then, increase in the higher lithium ion content causes a decrease in non-bridging concentration that increases the band gap energy. Urbach energy, EU, is used to characterize the degree of disorder in amorphous and crystalline systems and corresponds to the width of localized states. The degree of crystallinity of the samples could be determined using the following equation:

facts. Accordingly, the comparative data from Table 1 show that the smaller values of the gap energy were attributed to the smaller values of the Urbach energy and the higher concentration of non-bridging oxygen ions in lead–germanate network. When the optical band gap energy of system attains maximum value, its Urbach energy is minimum and vice versa. Refractive index is one of the important properties in optical glasses. Regarding the refractive index, n, the listed values in the Table 1 confirmed that there is positive relation between the gap energy, Urbach energy and the refractive index.

α ¼ A expðhν=EU Þ

4.5. DFT calculations

ð2Þ

where EU is the Urbach energy which indicates the width of band tailing of localized states. Urbach energy can be calculated using least square fitting of ln(α) against hν curves in the tailing part of localized states as shown in Fig. 5. For samples with disordered microstructure, increase of absorption coefficient up to absorption edge occurs gradually with a low slope. Whenever the degree of crystallinity rises, the slope of this region will increase. The Urbach energy for samples with x = 15 (EU = 0.68 eV) and 20 mol% Li2O (EU = 1.60 eV) attains maximum values indicating that these materials would have greater tendency to convert weak bonds into defects and therefore, their gap energy will decrease. On the other hand, this is indicative of an increase in concentration of non-bonding oxygen ions and disorder in the glass network. The smallest Urbach energy values (EU = 0.17 eV) can be observed for samples with x =10 and 40 mol% Li2O and consequently, the optical band energy increases based on the above-mentioned

The optimized structures for the minima of 40Li2O·60[7GeO2·3PbO] glass network are depicted in Fig. 6. The study on structural modifications of the vitreous network and the equilibrium geometry can be summarized as follows:

Table 1 Optical gap energy (Eg), Urbach energy (EU) and refractive index (n) for different lithium ions contents. x (mol%)

d (g/cm3)

Eg (eV)

EU (eV)

n

5 10 15 20 30 40

5.48 5.58 5.61 5.63 5.59 5.57

2.81 3.78 2.80 2.69 2.94 3.00

0.44 0.17 0.68 1.60 0.45 0.17

2.45 2.21 2.45 2.48 2.41 2.39

Fig. 5. Plots of ln(alpha) versus hν for xLi2O·(100 − x)·[7GeO2·3PbO] glass samples.

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i) The sites of the tetra-coordinated germanium are subdivided into regular and irregular [GeO4] polyhedrons. In the regular polyhedrons, the Ge–O bonds have shorter interatomic distances (1.79–1.84 Å) than the covalent Ge–O bond length (1.85 Å). The [GeO4] polyhedrons situated in the vicinity of the [GeO6] and/or [GeO5] structural units have irregular geometry and the Ge–O bonds are subdivided into two groups: three bonds have shorter interatomic distances (1.79–1.81 Å) and one has longer interatomic distance (1.93 Å) but significantly shorter than the sum of the van de Waals radii (3.55 Å). The sites of penta-coordinated germanium atoms are formed by bonds with short, intermediate and long interatomic distances and the values of the angle O–Ge–O show the large deviation from trigonal bipyramidal geometry. The shorter bonds have interatomic distances in the range between 1.76 and 1.78 Å. Then the intermediate Ge–O bond lengths are in the range of 1.85 to 1.88 Å, while the longer bonds have interatomic distances of 1.94 Å. The Ge–O bonds of the hexa-coordinated germanium are subdivided into two groups: four bonds have shorter interatomic distances (1.79–1.85 Å) than covalent Ge–O bond length and two have longer interatomic distances (1.91 and 2.69 Å). Considering all these six bonds, the [GeO6] polyhedron is irregular with five Ge–O bond lengths ranging from 1.79 to 1.91 Å and with an oxygen atom remaining relatively far away from germanium cations. These bonds are too large to be accepted as a chemical bond in the usual sense. This shows that there is instability between the nonequivalent Ge–O bonds in the octahedron. The values of the O–Ge–O angles are deviated from the octahedral molecular geometry. In essence, there is a larger displacement of the germanium atom from the centre of the polyhedron, whose strong asymmetry depends on the manner of connection with the surrounding polyhedron. Accordingly, some [GeO5], [GeO4], [PbO3] structural units situated in the [GeO6] structural units are strongly deformed. ii) Lead site is coordinated to three and four oxygen atoms. The Pb–O distances are comparable to that of Pb–O bond distances from [PbO3] and [PbO4] structural units. For some [PbO3] structural

units situated in the vicinity of the [GeO6] and [GeO5] structural units, the coordination geometry is smaller symmetrically. iii) The Li–O bond lengths are subdivided into two groups: shorter (1.79–1.82 Å) and longer (1.90 Å in vicinity of the [PbO3] and [GeO5] structural units). Due to the irregular and random distribution of atoms in the glass structure, we assume that the [GeO6] structural units do not accommodate with the non-bridging oxygens, and that the [GeO5] polyhedrons are suitable neighbors for the structural units of the lithium and lead ions. The observations concerning these mechanisms show that the lead and lithium ions have an affinity pronounced towards structural units with non-bridging oxygens yielding the strong deformation of the [GeO6] structural units. Further the excess of oxygen can be accommodated in the host network by the formation of the [GeO5] entities. Fig. 7 shows relevant frontier molecular orbitals computed for full optimization geometry employing DFT procedure. The distribution of the electronic states of the HOMO gives the character of electron donor for the lead (II) and germanium (II) lone pair. The LUMO orbital gives the character of electron acceptor for the [GeO5] and [PbO4] structural units of the network. We can thus conclude that the deviation from trigonal bipyramidal and octahedral geometry is not the result of steric hindrance or packing effects, but is inherent to the nature of the glass network. There is a change transfer between the lead (II) or germanium (II) lone pair as a donor and the [GeO5] and [PbO4] structural units of the network with character of electron acceptor. These significant second order interactions could account for the narrowing of the angles. 4.6. Electrochemical cell Electrochemical properties of these glasses, when used as a working electrode, were examined in an electrochemical cell using a conventional liquid electrolyte. The electrochemical cell was constructed as follows: the reference electrode consisted of platinum and the working electrode was a glass disc electrode. The glass disc electrode with a diameter of 2–3 cm was prepared by quenching the melt in between

Fig. 6. The optimized structure of the 40Li2O·60[7GeO2·3PbO] glass network.

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Fig. 7. The distribution of the electronic states of the a) HOMO and b) LUMO of the 40Li2O·60[7GeO2·3PbO] glass network.

two steel plates. The composition of 20Li2O·80[7GeO2·3PbO] glass was selected because of its smallest gap band energy. The silver nitrate solution was used as electrolyte solution. The corresponding variations of the cell voltages, between the working and reference electrodes, are recorded as a function of time. The voltage cell profile a function of time is plotted in Fig. 8. The initial drop in the cell potential can be attributed to the formation of a low conducting AgNO3 layer at the cathode-electrolyte interface. As it can be seen, the cell voltage increases rapidly up to a nearly stationary state, after that it decreases. This suggests that there is a charge–discharge process. It can also be noted that there is reversibility in the electrochemical process [38]. These findings can be related to structural ordering process after the initial cycle, involving

transformations from short-range-ordered solid-solutions to longrange-ordered structures. Fig. 9 shows the results of cyclic voltammetry of glasses with x = 20 and 40 mol%Li2O performed between −2 V and 2 V with a scan rate of 100 mV in 10−5 M AgNO3. The voltammograms show that all samples have good reversibility in their electrochemical analysis [39]. The current intensity decreases with increases of the content of lithium ions.

5. Conclusions FTIR and UV–VIS spectroscopy, density measurements, DFT and optical band gap energy calculations, were utilized in order to study structural

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Acknowledgments The financial support of the Ministry of Education and Research of Romania-National University Research Council (CNCSIS, PN II TE 2012) is gratefully acknowledged by the authors. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Fig. 8. The corresponding variations of the cell voltages as function of the time.

changes produced in ternary glasses with xLi2O·(100−x)·[7GeO2·3PbO] composition where 0≤x≤40 mol%Li2O. The decrease in value of optical gap energy from 3.78 down to 2.69 eV with increasing concentration of lithium ions can be interpreted in terms of structural changes in the glass system. The incorporation of network modifiers increases the quantity of non-bridging oxygen ions. Since non-bridging oxygen ions are more easily excited than bridging oxygen, optical band gap energy decreases with addition of lithium ions. The non-bridging oxygen ions can be derived to the transformations of some [GeO6] to [O=GeO5] structural units. The smaller band gap energy and electrochemical performance of the glasses with 20Li2O·80 [7GeO2·3PbO] composition were demonstrated in this study.

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

Fig. 9. Cyclic voltammograms of xLi2O·(100 − x)·[7GeO2·3PbO] system where x = 20 and 40 mol% Li2O performed between −2 V and 2 V, for three cycles with a scan rate of 0.1 mV/s in 10−5 M AgNO3 aqueous solution.

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