Glasses in the system xV2O5·20Li2O·(80−x) [0.6B2O3:0.4ZnO] (where 10≤x≤50) have been prepared by a simple microwave method. Microwave synthesis of materials offers advantages of efficient transformation of energy throughout the volume in an effectively short time. Conductivity in these glasses was controlled by the concentration of transition metal ion (TMI). The dc conductivity follows Arrhenius law and the activation energies determined by regression analysis varies with the content of V2O5 in a non-linear passion. This non-linearity is due to different conduction mechanisms operating in the investigated glasses. Impedance and electron paramagnetic resonance (EPR) spectroscopic studies were performed to elucidate the nature of conduction mechanism. Cole–cole plots of the investigated glasses consist of (i) single semicircle with a low frequency spur, (ii) two depressed semicircles and (iii) single semicircle without spur, which suggests the operation of two conduction mechanisms. EPR spectra reveal the existence of electronic conduction between aliovalent vanadium sites. Further, in highly modified (10V2O5mol%) glasses Li+ ion migration dominates.
The conductivity in glasses has been of interest for long time because of their potential technological applications. Use of glasses as electrolyte and electrode materials has given a boost to the study of ion transport in glasses and search for new glassy materials [1–5]. The mechanism of electrical conductivity in ion-electron conducting glasses is a challenging problem [5–8].
In recent years many glassy materials have been synthesized as binary or ternary systems using network forming oxides such as B2O3, P2O5, TeO2 etc and alkali or silver oxides as modifiers by melt quenching method . Microwave synthesis of materials is a new technology undergoing rapid developments due to potential advantages it offers such as reduced processing time, energy efficiency and products with enhanced properties. The only requirement of this method is that at least one of the components used for synthesizing materials should be a microwave susceptor [1,2].
Alkali borate glasses have been extensively studied over the past two decades to elucidate the nature and relative concentration of various borate units constituting the glass network . B3+ atoms in these glasses are both in trigonal and tetrahedral state. The concentration of these borate species in the glass structure is however determined by the nature and the content of the modifier oxide. In glasses containing B2O3 and V2O5 the coordination number and connectivities of both borate and vanadate species vary in a complex manner as a consequence of modification . Further, modification is understood to be the reaction of oxide ion (O2−), which results in structural changes, by creating non-bridging oxygens (NBOs). These NBOs constitutes anionic sites with different binding energies in comparison to those oxygens localized in boron tetrahedra . Horopanitis et al.  pointed out that, the Li+ transport in lithiated boron oxide glasses increases with Li2O concentration, not only due to Li+ ion concentration but also due to structural modification. Ion conducting glasses with high Li+/Na+/Ag+/Cu+ concentration are called fast ion conductors (FIC) and they are promising glassy electrolyte for the solid state batteries [5,11,12].
Glasses containing transition metal oxide (TMO) such as V2O5, Fe2O3, CuO, MoO3, WO3, CoO, etc. are known to exhibit semiconducting property and hence these glasses have been studied extensively from the cathode point of view of batteries [1,13]. The existence of relative proportions of low and high valence states of transition metal ions (TMIs) is responsible for the electronic conduction in these glasses [6,14]. It is expected that TMO added to alkali modified glass, results in mixed conduction [6,15,16].
EPR spectra of V2O5 containing glasses originate from V4+ paramagnetic centers whose outer electronic structure 3p6, 3d1 enables unpaired magnetic moments of 3d1 electrons to interact with the electromagnetic field in the microwave range. Whereas, the electronic structure of V5+ is 3p6, which has total electron spin zero. Since the V4+ ion has electronic spin s=1/2 and nuclear spin of 51V is I=7/2, one should expect interactions between corresponding magnetic moments resulting in the hyperfine structure . Gupta et al.  pointed out that, long range electron spin–spin interactions between V4+ ions and the spin–orbit coupling cause an anisotropy of the g-factor and the broadening of the individual lines. In glasses, only orientation averaged spectra can be observed, which can lead to additional reduction of hyperfine structure lines. It was seen in V2O5–TeO2 glasses that the disappearance of hyperfine structure lines at higher contents of V2O5 is due to super-exchange interaction of V4+OV5+ chains . In this study we used impedance and EPR spectroscopic studies to analyze conduction mechanisms in microwave synthesized xV2O5·20Li2O·(80−x) [0.6B2O3:0.4ZnO] glasses.2Experimental
Glasses were prepared by microwave heating technique using xV2O5·20Li2O·(80−x) [0.6B2O3:0.4ZnO] (where 10<x<50) glass system. Analar grade vanadium pentoxide (V2O5) lithium carbonate (Li2CO3), orthoboric acid (H3BO3) and zinc oxide (ZnO) were used as starting materials. An appropriate quantity of weighed chemicals were mixed and thoroughly ground to homogenize the mixture and kept in a silica crucible inside a domestic microwave oven operating at 2.45GHz and at a tunable power level up to a maximum of 850W. When microwaves were switched on, complete decomposition of H3BO3 to B2O3, water and Li2CO3 to Li2O, carbon dioxide was achieved in 2–3min. Within 6–8min of microwave exposure a good homogeneous melt was obtained, which was immediately quenched between brass blocks. The silica crucible was found to remain clean and unaffected during the short duration of melting. The glass was annealed in a muffle furnace for 1h at 150°C to remove thermal strains that could have developed during quenching. The samples were preserved in a sealed desiccator containing CaCl2.
Glass transition temperature (Tg) of the samples was extracted from the thermograms recorded using Differential Scanning Colorimeter (Perkin Elmer DSC-2). For the electrical measurements, the annealed samples were thoroughly polished and coated with silver paste on both sides, which serve as electrodes having a thickness of about 0.1cm and diameter of about 0.8cm were used. The resistance of the sample was calculated by applying a dc field of 2V and measuring the current through it using a digital electrometer (ECIL EA-5600). The conductivity of the sample was calculated using the relation:d is the thickness of the sample and A is the area of the sample. Temperatures of the samples were measured using a chromel–alumel thermocouple placed very close to the sample holder. The measurements were repeated with changed polarity of the applied voltages.
Capacitance (Cp) and conductance (G) of the samples were measured as a function of frequency using a Hewlett-Packard HP 4192A impedance-gain phase analyzer from 100Hz to 10MHz in the temperature range 323–405K. A home built cell assembly (having two terminal capacitor configuration and spring loaded silver electrodes) was used for the measurements. The temperature was controlled using Heatcon (Bangalore 560090, India) temperature controller with an accuracy of ±1K in the entire range of measurements. The temperature of the sample was measured using Pt-Rh thermocouple positioned very close to the sample.3Results and discussion
The X-ray diffraction spectra of the annealed glasses did not show any sharp peaks (Fig. 1), indicating that the samples are amorphous. The method used to extract glass transition temperature (Tg) and liquidus temperature (TgLiquid) is indicated in the DSC thermograms of heat capacity as shown in Fig. 2. Also, a thermogram of heat flow versus temperature of VBZ-1 glass is shown in Fig. 2 inset. Codes, composition, glass transition temperature (Tg), glass transition width (ΔTg), activation energy (Edc), are listed in Table 1. The Tgs are decreasing with increasing V2O5 mol%. This decrease in Tg can be explained on the basis of the structural changes occur due to network modification. A simple borovanadate network consists of a continuous random network formed by [VOO3/2]0 and [BO3/2]0 structural units. Boron is 3-connected and 3-coordinated, while vanadium is 3-connected but 4-coordinated. Therefore, the random network structure consists of BOB and BOV linkages in B2O3 rich glasses while BOV and VOV linkages are present in V2O5 rich glasses. Only BOV linkages are expected to be present when the concentrations of these two glass formers (B2O3 and V2O5) are in equal proportion [19,20]. The variation of Tg with V2O5 mol% can be rationalized on the basis of network connectivities, such that, as V2O5 increases the stronger BOV and BOB linkages (bond dissociation energy of BO is 715kJ/mol) are replaced by VOV linkages (bond dissociation energy of VO is 617.6kJ/mol) . Further, the glass transition width (ΔTg=TgLiquid−Tg, where TgLiquid is the liquidus temperature and Tg is glass transition temperature) marginally decreases with increasing V2O5 mol%. According to the empirical criterion proposed by Moynihan  the glass formers displaying widths ΔTg>30 are classified as “Strong” glass and those with ΔTg<30 are termed as fragile . As can be seen from Table 1, ΔTg is much less than 30, hence the investigated glasses are “fragile”.
Code, composition, glass transition temperature (Tg), glass transition width (ΔTg), activation energy (Edc) of V2O5·Li2O·[0.6B2O3:0.4ZnO] glass system.
|Code||Composition (mol%)||Tg (K)||ΔTg (K)||Edc±0.002 (eV)|
The dc conductivity of the same composition from different batches prepared under identical conditions showed agreement within 5% error and dc conductivity on the same samples in different runs are within 2% error. Fig. 3 represents the variation of log (σ) as a function of inverse temperature for different V2O5 concentrations. It is depicted from Fig. 3 that the conductivity values lie in the range 1.548×10−9–3.784×10−6S/cm when temperature varied from 313 to 423K. Further, Fig. 3 shows that all the samples follow Arrhenius law:Edc) . Fig. 4 shows the variation of log (σ) with V2O5 concentration and Fig. 4 inset represents the variation of activation energy (Edc) with V2O5 concentration. As can be seen from Fig. 4, the variation is non-linear, indicating that different conducting mechanisms operating in the investigated glasses. Activation energy, Edc=0.83±0.002eV for x=10mol%, which is comparable with those of ion conducting glasses [24,25]. While for x=30–50mol%, Edc is in the range 0.5±0.002–0.41±0.003eV. These values are comparable with the activation barriers of electronically conducting glasses . Also, the non-linear variation of activation energy was reported by Garbarczyk et al.  and Jozwiak et al.  in their studies on silver–vanadate–phosphate and Li2O–V2O5–P2O5 glass system, respectively. In order to elucidate the observed variations in σdc and Edc, impedance spectroscopy and EPR studies have been carried out.3.2Impedance spectroscopy
The impedance spectra of all the investigated glasses depend considerably on their chemical composition. The characteristic features of these spectra follow the nature of conduction mechanism. The experimental spectra (complex impedance representation) can be classified into three types (i) single semicircle with a low frequency spur, (ii) spectra consisting of two depressed semicircles and (iii) single semicircle without spur . The inclined straight line (spur) at the low frequency region could be the effect of mixed electrode and electrolyte interface. The magnitude of inclination in the straight line is related to the width of the relaxation time distribution . The impedance spectra consisting of two semicircles represent mixed conduction while glasses exhibiting a single semicircle without spur are electronically conducting. Garbarczyk et al.  reported the simulated impedance spectra characteristic for ion conduction, mixed conduction and electronic conduction with the equivalent electrical circuits used to generate the spectra [8,28–30].
Conductance (G) and capacitance (Cp) were directly measured (for x=10mol%) from the impedance bridge and used to compute the real and imaginary parts of impedance using the relations given by Ross Macdonald . Impedance plots of VBZ1 glass is shown in Fig. 5, which is used in dc conductivity determination. Values of Z′ (bulk resistance) corresponding to the intersection of low frequency side of the high frequency arc were used for the purpose. The dc conductivity was calculated using Eq. (1) and the values lie in the range of 1.9×10−7–1.1×10−5S/cm (for x=10mol%). These values are comparable with those (measured using four-probe method) presented in Fig. 3. Impedance spectra of glass with x=20mol% showing two depressed semicircles, characteristic of mixed conduction are shown in Fig. 6. For x≥30mol%, single semicircles without any spur are shown in Fig. 6 inset.
As can be seen from Fig. 4, isothermal conductivity at 313K and activation energy (Edc) varies nonlinearly with V2O5 mol%. This can be attributed to the enhanced interactions between polarons and mobile ions . At lower x=10mol%, there is a reduction in the electronic component of conductivity due to the disruption of the glass network and the increased population of Li+ ions. Eventually, there is an enhanced population of Li+ ions in the network structure compared to trapped polarons at x=10mol%. Thus, the ionic conductivity dominates. At x=20mol%, the interaction between electron and cation is maximum, suggesting a kind of transition from predominantly ‘ionic’ to ‘electronic’ conductivity. Hence, x=20mol% is considered to be the cross-over composition. In the present study, we identified two regimes viz. highly modified ‘ion’ conducting (x=10mol%) and least modified ‘electronically’ conducting regimes. As reported in the literature, alkali ion transport is characterized by high activation barrier . Further, if electronic contribution to conductivity is significant as ionic contribution , thenσelectronic/σionic varies from glass to glass, since Li+/([V4+]+[V5+])=Li+/Vtotal=rc varies.
This ratio plays a pivotal role in non-linear variation of conductivity. A similar trend is seen in glasses containing both alkali oxide and TMO's [33–36]. The other possible explanation for the observed conductivity is that polaron percolation paths are blocked by alkali ions in alkali rich glasses [33–36]. More importantly modifier to network former ratio can be effective, it is a measure of disruption of the glass network. If the ratio is higher, the glass network becomes more depolymerized. In such a case the electron conduction paths are discontinuous . In V2O5 rich glasses conductivity increases while the activation energy decreases (see Fig. 4). This can be well explained using Austin–Mott's relation . Conductivity in such glasses is characterized by an electron transfer process represented by V4+OV5+. The increase in σdc with V2O5 concentration can be attributed to (i) a decrease in VOV distance resulting in a larger overlap of d-orbital wave functions and (ii) increase in the redox ratio, C=[V4+]/([V4+]+[V5+]). The dependence of conductivity on composition is clearly reflected in EPR study.3.3EPR spectroscopy
EPR spectra of the investigated glasses are shown in Fig. 7. As can be seen from Fig. 7, a strong absorption line arises from the fact that at high V2O5 content, most of the vanadium ions are in the V4+ state. The absence of hyperfine structure (hfs) points to interaction between vanadium centers via V4+OV5+ super-exchange mechanism. Generally, such glasses exhibit electronic conductivity . However, the mechanism of conduction in glasses with high concentration of V2O5 has been suggested as the transfer of an electron from V4+ site to a V5+ site. Structural groups formed in V2O5 rich glasses, provide the path for the conduction of electrons . The increase of the electronic conductivity in V2O5 rich glasses can be explained by considering the decrease in the average distance between the TMI sites. According to Mott's polaron theory, the dc conductivity rapidly varies with site spacing and redox ratio. The observed correlation between the concentration of V2O5 and the appearance of hyperfine structure (hfs) can be justified, taking into account that two nearest aliovalent vanadium centers can exchange an electron via bridging oxygen. The least modified network is characterized by a strongly cross-linked network. In such a network, the conditions for electron hopping via V4+OV5+ bonds are more favorable than highly disrupted network. An illustration of transfer of electron from V4+ site to neighboring V5+ site is shown in Fig. 7b. At x=20mol%, the EPR spectra consists of V4+ line with a weak but visible superimposed hfs, which indicates the cross-over from non-hfs regime to hfs regime. A similar cross-over point is seen at this composition in the dc conductivity studies. Further, in highly modified glasses (x=10mol%) ion (Li+) transport dominates the polaron conduction.4Conclusion
Lithium–zinc–boro-vanadate glasses were synthesized by microwave method by varying V2O5 concentration from 10 to 50mol%. Glasses were characterized by XRD and DSC studies. As the concentration of V2O5 increases the Tg decreases suggesting that the rigidity of the network decreases while the fragility of glass itself increases. The dc conductivity study reveals a non-linear variation in conductivity and activation energy. These nonlinearities are attributed to different conduction mechanisms due to the presence of both alkali oxide and TMO. Conductivity is predominantly ionic for glasses with low V2O5 content while predominantly electronic for glasses containing higher content of V2O5. Conductivity transition occurs at the concentrations of alkali ion and TMI are nearly equal.Conflicts of interest
The authors declare no conflicts of interest.
The authors are grateful to Professor K.J. Rao, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore for encouragement and many helpful discussions.