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Short-Range Structural Insight into Lithium-Substituted Barium Vanadate Glasses Using Raman and EPR Spectroscopy as Probes

We present a corroborative study of the structural characterization of lithium-substituted barium vanadate glasses using Raman and electron paramagnetic resonance (EPR) spectroscopy. Investigation of the thermal and physical properties of these glasses showed a gradual increase in the concentration of nonbridging oxygen. Raman and EPR analysis gave an insight into the changing structure of the glasses. Both the spectroscopic techniques confirmed that vanadium is present in the glasses as distorted VO6 octahedra. From the analysis of both spectroscopic techniques, it is proposed that the lithium ion prefers to occupy planar positions of the VO6 octahedra, thus reducing the tetragonal distortion and making the environment around the network-forming unit in the glass matrix more homogeneous as we increase the lithium content. The concentration of V4+ showed a non-monotonic variation with an increase in Li2O as indicated by Raman studies and confirmed by EPR, which indicates a structural change in the distorted VO6 octahedra.


Introduction:
There has been renewed interest in vanadium oxide and its intercalates over the last few decades due to their potential use in many technological applications especially advanced battery materials [1][2][3][4]. The unique properties of vanadium compounds are attributed to the variable oxidation states of vanadium (+3,+4,+5). In battery applications, this allows the insertion and removal of ions such as lithium in vanadium oxide compounds [5]. However, the structural framework of vanadium in any compound is decided by its coordination polyhedral with oxygen which in turn decides its stability, particularly during redox reactions [5,6]. So, it is important to understand the structure of vanadium-oxygen coordination polyhedral in any given compound and how it changes with the change in the composition of the compound. The addition of a varying amount of network modifiers in a glass compound changes its structure and properties. These short to medium range structural changes can be probed using different spectroscopic techniques as they are powerful tools for the structural characterization of glasses [7]. Raman and EPR spectroscopies are widely used to study the short range structural and coordination changes in glasses.
Raman is widely used to characterize the structural changes in vanadate glasses because the measured Raman peak positions and intensities are remarkably affected by the sample composition and the EPR of these glasses shows rich hyperfine structure due to the presence of vanadyl VO 2+ ions. These techniques were able to estimate the changes in the specific coordination of network former(V2O5) and estimate bond lengths up to a few angstroms [12,13].
Both these non-destructive spectroscopic techniques have been widely used to determine the structural changes in binary and ternary vanadate glasses [14][15][16][17][18]. Specific studies have explored the effect of variation in Li + ion concentration on the structure of vanadate glasses using these techniques [14,19,20]. However, vanadate glasses with a combination of alkalineearth oxides and alkali oxides have not been extensively studied. Also, in most of the studies, these spectroscopic techniques are used as stand-alone characterization methods. In the few reports where a combination of these techniques is used, the results of one technique are not used to support and enhance the observations from the other [17,21,22].
Our present work aims at studying the structural changes in V2O5-BaO-Li2O glasses as a function of increasing the concentration of Li2O. In our samples, we have kept the TMO content constant and varied the amount of Li2O at the expense of BaO. A detailed study of structural changes in these types of oxide glasses with constant TMO content and a variable amount of alkaline-earth oxide and alkali metal oxide has not been done so far. We were able to probe the short-range structural changes in these glasses via a corroborative study of Raman and EPR spectroscopy.
Homogenous samples weighing 5g were synthesized using the conventional melt-quench technique [23]. The starting materials weighed in appropriate proportion were put in porcelain crucibles and melted in a muffle furnace at 800 o C for 30 min. The melt was then quenched between brass plates to prepare glass. The samples were then annealed at 100 o C for 8 hours and left to cool down slowly to room temperature to relieve any mechanical stress. The glass transition temperature (Tg) was measured using Differential Scanning Calorimeter (DSC 2920 TA Instruments) by heating the samples at a constant rate of 5 o C/min. with an accuracy of ± 0.05 C. (The cumulative DSC graph is given in supporting material).
The density of prepared samples 'ρ' was measured using the Archimedes principle with Xylene as the immersion liquid. The density of Xylene used was 0.861 g/cm 3 as mentioned by the manufacturer. All measurements were done for 3 different sets of samples to take care of the random errors. The molar volume and the error in molar volume for each glass sample were calculated from the density data.
The average cross-link density gives a quantitative analysis of thermal properties in terms of structural parameters. So, these properties were further investigated using the average crosslink density of the glasses ( ̅ ). Average cross-link density can be calculated from few physical parameters but the variation in it gives an insight into the variation in glass transition temperature [24]. The calculated values of molar volume and ̅ are tabulated in Table 1.
(Details of calculation of molar volume and average cross-link density are given in supporting material) The Raman spectra were recorded at room temperature using a LabRAM HR Raman spectrometer from Holmarc with a CCD camera. The spectral resolution of the instrument was 0.4 cm -1 . The data was obtained in the spectral range from 200-1100 cm -1 . This data was then baseline corrected and smoothened to 100 points using the Shavitzky-Golay method. The temperature and frequency dependence of the Raman scattered data was then removed to obtain a reduced Raman intensity ( ) by applying temperature-frequency correction [25] ( ) = ( )( 0 − ) −4 (1) Here ( ) is the measured experimental Raman intensity, 0 is the frequency of laser excitation line in cm -1 , is the Raman shift in cm -1 and is the temperature factor given by Bose-Einstein formula as = 1 − exp (− ℎ ) (where is the room temperature in K). The spectral intensity was then scaled between 0 and 1 to obtain the final reduced intensity. The data thus obtained was again corrected for baseline and de-convolved using Gaussian line shape between the spectral range of 700-1100 cm -1 to obtain the various V-O bond stretching vibrations for structural analysis of the various glass samples as the short-range structure of the glasses is reflected by a series of bands in this high frequency region.
The EPR spectra of powdered samples were obtained using JEOL, JES-FA200 ESR spectrometer operating in 9.4 GHz range in the X-band (measured with an accuracy of 10 -6 GHz) with a modulation frequency of 100 kHz. The spectra for these glasses were obtained at room temperature. The magnetic field was swept with a width of ± 150mT with 350mT as the center field. The experimental error involved in measurement was ± 0.0001mT. Each of the samples was weighed and the intensity of each sample was normalized by the weight of the corresponding sample.

Thermal properties:
From the DSC of all the studied glass samples, we determined the glass transition temperature is substituted by lithium oxide (as shown in Figure 1(b)). The gradual increase in Li2O content, which is more ionic compared to both V2O5 and BaO tends to create more non-bridging oxygens (NBOs) which softens the glass network.

Density and Molar Volume:
A perusal of Table 1 reveals that both density and molar volume decrease linearly as the Li2O content increases in the present glass system. A decrease in density may be attributed to the lowering of the total molecular weight of the glass because a lighter component (Li2O, molecular weight =29.87 g/mol) replaces a heavier component (Barium oxide 153.32 g/mol).
The increase in the NBOs causes a decrease in the molar volume and the smaller ionic radius of Li + (76 pm) as compared to Ba + (138 pm) also contributes to this decrease.
The average cross-link density showed a decrease from 3.76 to 3.46 with Li2O content increasing from 10 mol% to 25 mol% as shown in Table 1. The decrease in the average cross-link density correlates with the decrease in Tg [24]and it also indicates the creation of more NBOs.
Both the thermal and physical analysis of the glass system under study has hinted at an increase in NBOs. This could result in structural changes in the glasses. To investigate the short to medium range structural changes in these glasses, we carried out Raman and EPR analysis. * Tg for each sample listed here was measured at a scan rate of 5 o C/min.

Raman Spectroscopy
We have fitted Gaussian peaks to the spectra in the 700-1100 cm -1 range to a set of 5 bands, out of which four bands correspond to V-O bond stretching vibrations which are denoted by where ( =1,2,3,4). Peaks below 800cm -1 correspond to bending vibrations and will not be discussed here. The Raman spectra and the deconvolution of various V-O bonds along with a sketch of the VO6 polyhedral structure that the V2O5 assumes in these types of glasses are shown in Figure 2. The various V-O bond vibrations suggest that vanadium in these glasses is present in the form of distorted VO6 octahedra [8,26]. The fitted peaks are attributed according to Hardcastle et al. [8]: the highest frequency band at 991 cm -1 ( 1 band) is due to V 5+ =O bond stretching vibrations [8,9], the next band at 961 cm -1 ( 2 band) is assigned to stretching vibrations of V 4+ =O bond [9,19,26], the next band around 925 cm -1 ( 3 band) is for O-V-O stretching vibrations within the VO6 structure [26] and the vibration band at 830 cm -1 ( 4 band) is due to bridging asymmetric stretching vibrations of V-O-V [26] that extend over to the nearest neighboring VO6 polyhedron (as shown in Figure 2c). corresponds to an increase in the length of this bond. However, this bond length increases from VBL20 to VBL25, which could be due to a change in the surrounding environment of VO6 octahedra as Li + is being substituted. The area under each peak shows the number of species corresponding to that particular bond.   shows a non-monotonic variation where the bond becomes more homogenous up to VBL20 but then the FWHM increases which shows that the bond environment becomes nonhomogeneous.
The V-O stretching frequencies ( ) can be related to their bond lengths ( ) using the exponential relation as applied by Hardcastle [8]: where and are fitting parameters. The best fit to the experimental values was obtained with the values of and as 21349 and -1.9176 respectively [8]. So, the above equation becomes: Equation (3) gives in Ȧ if is in cm -1 . Using equation (3), the most probable bond length can be obtained. These values for V-O-V are summarized in Table2. As can be seen from the observed values, the bond length keeps increasing with increasing lithium concentration. This type of estimation of bond lengths was previously performed by Gajanan et al. [12] and was supported by the DFT simulations by Saetova et al. and Saiko et al. [27,28]

EPR Spectroscopy
The EPR spectra in glasses were first evaluated by Sands [29]. Siegel and many others studied the spectra of VO 2+ in glasses [11,16,30]. They considered that V2O5 is present in the glasses as VO6 octahedra which is tetragonally distorted as shown in Figure 5(b).
The obtained EPR spectrum is attributed to the presence of V 4+ in the glass samples as this species of vanadium is EPR active due to the presence of a single unpaired electron [13]. 51 V in V 4+ has an outer shell electronic configuration of 3p 6 3d 1 . So, in vanadyl ion (VO 2+ ) with electron spin S=1/2 and nuclear spin I=7/2, the total number of hyperfine components are (2I+1) and its rich hyperfine structure consists of 8 lines. However, due to g anisotropy, some of the components overlap and hence not all the lines are well resolved in the observed spectra.
Corresponding to the various values (like ±1/2, ±3/2, ±5/2 and ±7/2), the magnetic field values were obtained from the first derivative spectra (shown in Figure 5(a) The spin Hamiltonian of the axially symmetric field can be used to evaluate the spectra of these glasses. It is evaluated to obtain the components of tensor and hyperfine coupling tensor and is given as [16] = || + ⊥ ( where is Bohr magneton, || and ⊥ are the parallel and perpendicular principal components of the tensor. The first approximation to the values of || (0), ⊥ (0), || and ⊥ were obtained from the least-square straight-line fits to equations: The values of || (0) ⊥ (0) were obtained from the intercepts and || and ⊥ were obtained from the negative slopes of the linear fits to equations (7) and (8). The second order terms of in equations (5) and (6) were calculated by applying these initial values. Then by substituting the values of these second order terms in (5) and (6) were well resolved in all spectra and used for determining the parameters in equation (5). All the perpendicular components except components are least anisotropic, so the difference between the observed and calculated values is high [31]. So, they were constantly found to add greater error in the fitted parameters and hence were excluded from the calculations. Values of various obtained parameters are listed in

Table3.
For the vanadyl VO 2+ to exist in tetragonally distorted octahedral symmetry in the glasses, the conditions, || < ⊥ < and || > ⊥ should be satisfied [33]. A perusal of Table 3 shows that these conditions are satisfied in our glass samples. Here, is free electron parameter with value 2.0023.  [31]. A larger tetragonal distortion results in a greater value of this ratio and vice-versa [13]. The concentration of the total number of vanadium ions (given in Table1) was obtained from density data (The detailed calculation is given in supporting material). The total number of V 4+ species was obtained by finding the area under the normalized intensity spectrum by doubly integrating it [13]. Figure 7(a) shows that while the total number of vanadium ions increases slightly with an increase in Li2O concentration (though TMO content is constant) but the normalized number of V 4+ species (obtained by dividing the total number of V 4+ ions by the total number of vanadium ions) shows non-monotonic variation. It increases up to VBL20 and then decreases from VBL20 to VBL25. This observation supports the non-monotonic variation in the peak area of V 4+ =O as obtained from Raman spectra. This shows that the environment around V=O changes from VBL20 to VBL25 due to an increase in Li + ions. As we increase the concentration of Li2O, the number of NBOs increases but with a large concentration of Li2O, more lithium atoms may occupy positions near to the V=O bond as shown in Figure 6.
Since lithium is electropositive, so its presence close to the V=O bond pulls the electron towards oxygen. This increases the concentration of V 5+ in VBL25 glass, thereby reducing the concentration of V 4+ from VBL20 to VBL25.

Conclusions:
In this communication, we have evaluated the effect of systematic substitution of the lithium ion on the structure of barium vanadate glasses.
The drop in the glass transition temperature with a rise in Li2O content in the glasses at the expense of BaO relates directly to the continuous increase in the number of NBOs and the decrease in the average cross-link density. The collapsing skeletal structure as indicated by the decreasing molar volume also indicates an increase in the non-bridging oxygens.
Raman analysis which is a non-destructive tool to probe the structure supports the above observation. The gradual stretching of the V-O-V bond proves that more and more NBOs are formed. The stretching in the planar direction and substitution of Ba 2+ with Li + affects axial V=O bond whose bond length reduces a little. So, the distorted VO6 octahedron gets stretched in the direction perpendicular to the axial bond with the addition of more Li2O which reduces the tetragonal distortion of the VO6 octahedra. This result is supported by the EPR analysis, where the ratio || ⊥ continuously decreases from VBL10 to VBL25. The decrease in this ratio shows that the tetragonal distortion of the VO6 octahedra decreases with increasing Li2O content in the planar direction. So, as we replace BaO in the glasses with Li2O, the environment around VO6 octahedra which is the basic building block in our glasses becomes more and more homogenous.
However, the concentration of V 4+ shows a non-monotonic variation with an increase in Li2O content similar to the non-monotonic variation in the peak area of V 4+ =O bond in the Raman study which indicates an increase in the number of lithium atoms sitting near the double bond.
Thus, we show that EPR and Raman analysis corroborate perfectly to envisage the structural changes in the studied lithium substituted barium vanadate glasses.