A comprehensive investigation of Bi2O3 on the physical, structural, optical, and electrical properties of K2O.ZnO.V2O5.B2O3 glasses

The multi-component glass system has a composition of 10K2O–10ZnO–55 B2O3–(25–x)V2O5–xBi2O3 (x = 4, 5, 7.5, 9, 10 mol%) are synthesized by the melt-quenching method. Using X-ray diffraction examination, the amorphous phase in the material was confirmed. The physical characteristics of the produced compositions are examined using density (D) and molar volume (Vm). Calculations of physical properties showed that adding Bi2O3 from 4 to 10 mol% increased the glass density from 2.7878 to 3.3617 g cm−3 and decreased the molar volume from 40.4196 to 38.5895 cm3/mol. Studies of glass samples using the FTIR show bands of absorption for oxides in different structural groups. Octahedral [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\text{BiO}}}_{6}$$\end{document}BiO6], [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\text{BO}}}_{4}$$\end{document}BO4], and tetrahedral [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\text{BO}}}_{3}$$\end{document}BO3] structural units are observed in the present glass matrices. The cutoff wavelength (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\lambda }_{C}$$\end{document}λC), and optical band gap energy were determined using UV absorption spectra. The increase in non-bridging oxygens can be linked to the decrease in optical band gap energy (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${E}_{opt}$$\end{document}Eopt) (direct and indirect) and the increase in cutoff wavelength with an increase in Bi2O3 content. This is attributed to the existence of bismuth ions and the creation of non-bridging oxygens. Besides that, the values of optical parameters, viz., optical electronegativity, refractive index, and molar refractivity, are calculated. The metallization criterion values are less than 1 and the glass samples exhibit an increased tendency towards metallization. Both the conductivity and the dielectric constant increase with the rise in Bi2O3 content, however, the dielectric loss and the impedance reduce. The behavior and values of conductivity for the studied glasses reveal the semiconducting properties of all glass samples. These results suggest that the produced glass samples may be employed as amorphous semiconductors in electronics and memory switching devices.


Experimental details
The preparation of samples Five glass samples GBi1, GBi2, GBi3, GBi4 and GBi5 having chemical composition 10K 2 O-10ZnO-55 B 2 O 3 -(25-x) V 2 O 5 -x Bi 2 O 3 (where x varies from 4 to 10 mol%) were fabricated by using melt-quenching technique.The chemical compositions of different glass samples fabricated along with their labels are listed in Table 1.Highly pure analytical grade K 2 CO 3 , ZnO, H 3 BO 3 , V 2 O 5 and Bi 2 O 3 chemicals were used as starting materials.The well ground mixture of chemicals in appropriate weight ratios were taken in porcelain crucibles and melted in an electrical muffle furnace at temperature 1250 • C .The melt was poured on a preheated stainless steel plate.The quenched samples were annealed at 450 • C for 3 h and then left in the furnace to cool down to room temperature to reduce the internal stress.These samples were cut and then will undergo polishing and grinding process to analyze the glass samples for its characteristics.Images of the glass samples are displayed in Fig. 1.

Samples characterization
To confirm the amorphous nature of the prepared samples, X-ray patterns of the glass samples have been recorded by using a Rigaku Table-Top X-ray diffractometer with source Cu Ka radiation in the 2θ range 10°-80° at a scanning rate of 10 min.
. Images of all the investigated glass samples.
The tightness, rigidity and structural changes of the obtained glass samples can be investigated through the measurement of the density (ρ) of glasses was measured at room temperature based on Archimedes methods and can be calculated by the following equation: Where m 1 is the weight of the sample in the air, m 2 is the weight in distilled water, respectively, and ρ 0 is the density of water (= 0.9989 g/cm 3 ).The value of molar volume ( V m ) is related to the compaction of the glass network and can be calculated as follows: Where x i is the molar fraction, M i is the molecular weight of component {i}.Fourier transform infrared spectroscopy (FTIR) spectra of the glasses were recorded in the wavenumber range 400-4000 cm −1 using (Jasco-6100, Japan).The measurements were calculated using the KBr pellet technique.
Optical absorption measurements of the prepared samples were performed using a Cary series UV/Visspectrophotometer at room temperature in the range of 200-1100 nm.
The conductivity of the prepared samples was measured using Novocontrol Technologies, GmbH& Co. KG, high-resolution alpha analyser (0.1-20 MHz) in the temperature range 25-200 °C and stabilized with an accuracy of more than 0.1o Cusing Quattro temperature controllers employing pure nitrogen gas as the heating agent.The cell used was calibrated using standard materials (air, Trolitul and glass) with different thicknesses ranging from 1 mm up to 7 mm at 10 kHz with an LCR meter.Calibration curves were tested with two Teflon samples of different thicknesses, and it was found that the error in εʹ amounts to ± 2% and that the standard deviation amounts to 0.04.

X-ray diffraction
Using X-ray diffraction data, the glassy phase of the manufactured glass systems is displayed in Fig. 2. The XRD analysis demonstrated the complete amorphous nature of each glass sample and the absence of a uniform atom arrangement that would have been present in a crystal case.Due to variations in interatomic distance, glasses exhibit a wide range peak, as seen by the emergence of a broad hump in the range of 20 • -40 • for glass composition 23,24 .All of the glass samples are in the amorphous or non-crystalline phase, as demonstrated by this behavior.The ability of the borate glass networks to form glass was improved, and bulk glass samples were more transparent and clear as a result of the addition of Bi 2 O 3 [23][24][25] .

Density and molar volume characterization
Table 1 lists the density values (ρ) for each produced glass sample that was obtained.With the addition of bismuth oxide, the density values exhibit an increasing tendency in the following order: GBi1 < GBi2 < GBi3 < GBi4 < GB i5.This is because Bi 2 O 3 has a high molecular weight and density(465.96g/mol, 8.9 g cm −3 ) compared to V 2 O 5 (181.88 g/mol, 3.36 g cm −3 ), the density increased from 2.7878 to 3.3617 g cm −3 as expected with the substitution of V 2 O 5 with Bi 2 O 3

26
. In the meantime, the molar volume value and the density measurement typically behave in XRD patterns for all the compositions of glass samples (GBi1 -GBi5).
opposite directions.In contrast to the observed density, this investigation showed that the molar volume ( V m ) decreases in the order GBi1 > GBi2 > GBi3 > GBi4 > GBi5. Figure 3 shows the molar volume and experimental density of the produced glasses as a function of the Bi 2 O 3 content.Finally, it's possible that Bi 2 O 3 functions as a network modifier, forming non-bridging oxygen's (NBO's) atoms that alter the borate glass's structural composition.The concentration of non-bridging oxygens in the glass network increases when the bismuth oxide replaces the vanadium oxide, converting the [BO 3 ] structural units into [BO 4 ] structural units [27][28][29] .

Fourier transforms infrared spectroscopy studies (FTIR)
In order to investigate how the interactions between the different atoms in the samples affected their structure, infrared spectroscopy was employed.Table 2 displays the band positions and peak assignments of the FTIR spectra that were obtained for each produced glass within a 4000-400 cm −1 spectral range, as depicted in Fig. 4.
The glass network contains a variety of links and vibrational modes.Three basic groups are present in the vibrational modes of borate glass: (600-800) cm −1 , (800-1200) cm −1 , and (1200-1600) cm −1 .In triangular BO 3 structural units, the bending vibrations of the B-O-B and the stretching vibrations of the B-O bond are often associated with the first and third absorption regions.However, the second region is caused by the stretching vibrations of the tetrahedral BO 4 structural units 30 .
The well-defined peaks in the infrared spectra located at 470 cm −1 are due to the vibration in the local symmetry of highly distorted BiO 6 polyhedral units and/or BiO 3 units and/or bending of BO 4 units 31,32 .Another IR peak at 542 cm −1 may be attributed to Bi-O and Bi-O-Bi stretching vibrations of [BiO 6 ] octahedral structural units and/or bending vibration of the V-O-V bond 33,34 .The BiO 3 polyhedra vibration band does not show in the IR absorption 35 .Therefore, the bismuth structure that exists in the glasses is solely attributed to the [BiO 6 ] octahedral units.
In the borate network, the absorption band detected at 699 cm −1 is connected with B-O-B bending vibrations of BO 3 groups 36,37    units in tri-, tetra-and penta-borate groups 41,42 .Absorption peaks at around 1267 cm −1 are produced by the B-O stretching vibrations of trigonal [BO 3 ] units from boroxol rings containing non-bridging oxygen atoms 43,44 .
Trigonal [BO 3 ] units in the meta-, pyro-, and ortho-borate groups have asymmetric B-O stretching vibrations, which are linked to the absorption band found at 1378 cm −145, 46 .
However, the absence of the distinctive 800 cm −1 boroxol ring band, which is typically present for borate networks, suggests that there are no boroxol rings in the borate network.As a result, BO 3 and BO 4 structural groups make up the majority of the glass samples 47 .In these compositions, bismuth is expected to function as a network modifier.The BO 3 triangle's structure, however, changed as the content of Bi 2 O 3 increased to produce the BO 4 tetrahedral, which is close to the energy needed to break B-O-B bridges and form non-bridging oxygen and forms different kinds of structural units 48 .

UV-visible analysis
One effective method for examining the electrical structures of amorphous semiconductors is the examination of optical absorption spectra 49 .The UV-visible absorption spectra of the glass samples with different Bi 2 O 3 contents are shown in Fig. 5 in the wavelength range of 200-1100 nm.The bandgap, oxygen deprivation, surface roughness, and impurity centres are some of the variables that affect absorbance 50 .A straight line was drawn to determine the cut-off wavelength ( C ), and after the line crossed the wavelength axis, the cut-off wavelength was selected 51 .
The studied samples exhibited an increase in absorbance in the visible region upon increasing the Bi 2 O 3 content.Tetravalent V 3+ ions are exactly attributed to the absorption band at 597 nm.It is believed that V 3+ ensures three spin-allowed absorption transitions in tetrahedral and octahedral coordination.In oxide glasses, V 3+ cause absorption bands that represent the transitions from 3 T 1g (F) to 3 T 2g and 3 T 1g (P) states, respectively 52 .
It has been observed that as the amount of Bi 2 O 3 in borate glass structures increases, the optical absorption cut-off wavelength shifts from a lower wavelength to a higher wavelength value.As indicated in Table 3, the optical cut-off wavelength of the glasses under study has been moved from 472 to 521 nm.Because of the gradual formation of NBOs in the glass networks, the altered behavior of the absorption cut-off wavelength can be linked to reduced glass structure stiffness.Glass networks are degraded because non-bridging oxygen electron bonding is less tightly bound than bridging oxygen bonding 53 .Consequently, a decrease in the optical band gap energy would result from the breaking down of the BO's bond and a shift in the absorption edge to a longer wavelength.
Glass's band gap energy is determined by analyzing its UV absorption edge.To get the absorption coefficient α (ν) close to the spectrum edge, use Eq.(3) 54 : where d represents the glass sample's thickness and A its absorbance.Davis and Mott 55 report that optical absorption of amorphous materials occurs above the exponential tail with a larger value of α (ν), following a power law expressed by Eq. ( 4): where hυ is the incident photon energy, α (ν) is the optical absorption coefficient, B is constant, n is the index that is defined by the type of electronic transitions that occur during the absorption process, and E opt is the optical band gap energy between the valence band and the conduction band.The value of n can be either n = 1/2 and n = 3/2 for direct allowed and direct forbidden transitions or n = 2 and n = 3 for indirect allowed and indirect forbidden transition.
Plotting (αhυ) 0.5 and (αhυ) 2 vs the photon energy (hυ), Eq. ( 4) was used in this work to calculate the indirect and direct allowable optical energy band gap, or E opt , for glass samples.It is possible to calculate the optical energy band gap by extrapolating the linear portion of the observed curves to lower energy.Table 3 provides a summary of the relationship between E opt values and Bi 2 O 3 content for both direct and indirect transitions, as illustrated in Figs. 6 and 7, respectively.
In borate glass systems, the optical energy gap (E opt ) takes values between 2.7984 and 2.5230 eV in the case of a direct transition (Fig. 6), and ranges from 2.3627 to 2.1643 eV for an indirect transition (Fig. 7).Essentially, the changes in structure within the networks of borate glass are causing the optical band gap to decrease, as previously determined by researchers 56,57 .
By producing a concentration of NBO, the replacement of Bi 2 O 3 , which acts as a glass modifier, would disrupt the regular structure of borate glass networks, making the glass structure more random 58 .However, it is also thought that because Bi 2 O 3 elements are highly polarizable and easily deformed by cations, as cation concentrations increase, the bridging oxygen will form a bond with Bi 3+ ions and the glass networks will gradually break down 59 .The concentration of non-bridging oxygens (NBOs) is often increased by an increase in the network modifier concentration, and states originating from NBOs are easier to excite than ones originating from bridging oxygen atoms.As a result, the optical band gap reduces 60 .
An essential parameter that indicates the degree of disorder in amorphous materials is the Urbach energy (∆E).Following the empirical Urbach rule, the relationship between Urbach energy (∆E) and absorption coefficient α (v) is given 61 : where B is constant and ∆E is Urbach energy, which corresponds to the width of the band tails of localized state.The relation can be rewritten as:   3, and the values of ΔE were computed by taking the reciprocals of the slopes of the linear portion in the low photon energy region of ln(α) versus hν plot (not shown).Also, the tails are affected by the disorder level and the structure of the sample 62 .

Some other optical parameters
The Dimitrov-Sakka relation can be used to calculate the refractive index from optical band gap energy 63 .
where E opt is optical band gap energy and n is the refractive index.Because glasses are amorphous by nature, most indirect transitions occur as a result of the electrons' undefined momentum.For this reason, the refractive index is only determined via indirect bandgap energy.Table 4 shows that there is a slight increase in refractive index with increasing Bi 2 O 3 content, ranging from 2.5939 to 2.6686.Since non-bridging oxygens are more polarizable than bridging oxygens, this kind of increase may be explained by an increasing amount of these oxygens.The  www.nature.com/scientificreports/glass structure is changed by the non-bridging oxygens, making the molecular packing denser.The reason for this denser packing is that more network modifiers are occupied at intestinal sites.Given that a glass system's refractive index and density are closely correlated, a glass with a higher density will also have a higher refractive index 64 .
Lorentz-Lorentz provides the correlation between molar refractivity ( R m ) and molar volume 65 .
Molar refractivity values have opposite trends in the optical energy and its values decrease from 26.5270 to 25.8976.Also, molar refractivity is essential for understanding and predicting a material's conduction behavior.
Glass is determined to be metallic or insulator by calculating the metallization criterion (M), which takes into consideration the ratio of R m /V m and can be stated as follows 66 .
Herzfeld's metallization theory 67 specifies the criteria for classifying solids as either non-metallic ( R m /V m < 1) or metallic ( R m /V m ≥ 1) depending on their characteristics.The calculated values of M are listed in Table 4.If metallization criterion reaches to 1 means the materials are becoming insulators, instead if it reaches to 0 the materials becoming conductors 66 .The glasses under investigation show a greater tendency towards metallization as determined by the criterion of small metallization ( R m /V m is large).The obtained optical band gap energy measurements are in agreement with the results of the metallization criteria 68 .
The refractive index was used to compute the dielectric constants and optical dielectric constants of the prepared samples, as indicated by the following expressions: The empirical formulas were used to calculate characteristics like electronegativity (χ) and optical polarizability ( α 0 ) 69 .
One property of oxide glasses called electronegativity shows how strongly an ion may bind electrons.There is weaker bonding across ion networks as a result of the ions' reduced electronegativity, which causes them to attract adjacent oxide ions less strongly 68 .The values of the optical polarizability of the prepared glasses increased from 2.9284 to 2.9766 due to a decrease in electronegativity (χ).These parameter values are listed in Table 4.

Ac-conductivity
Studying the behavior of alternating-current conductivity (σ ac ) of the prepared glasses is very important to determine the extent of the glasses to conduction under the effect of an electric field.Ac-conductivity of different glass compositions GBi1, GBi2, GBi3, GBi4, and GBi5 over the frequency region 10 −1 -10 6 Hz at room temperature are shown in Fig. 8a.
The frequency (f) dependence of Ac-conductivity (σ ac ) is usually expressed by the following Jonscher relation Eq. ( 14) and Almond-West formalism Eq. ( 15) 70,71 : Where ω represents the frequency and equals to 2πf and called as the angular frequency, s is the frequencyexponent which have values (0 < s ≤ 1), σ dc is the dc-conductivity, A is a constant, and ω H is the crossover fre- quency which indicates the frequency at which the frequency-independent region separates from the dispersion conductivity region.Figure 8a shows a delay in the values of σ ac with decreasing the frequency due to the presence of free charge carriers at the electrode surface that causes electrode polarization (EP) 72 .At very low frequency values, the conductivity attains nearly constant value which is attributed to the dc-conductivity (σ dc ) which originated from the jumping of ions to the adjacent vacant site or from the diffusion of the ionic charge carriers 73 .In our samples, the reason of this conduction is mainly due to the electron transfer through V 4+ -O-V 5+74 .The data of Fig. 8a was non-linearly fitted by Almond-West formalism and the parameters of the fitting were listed in Table 5.As listed in the table, the values of σ dc are ranging from 10 −6 -10 −9 S cm −1 , which in agreement with the behavior of glasses contains transition metal where the electronic conductivity of these glasses is predominant 74 .The estimated values of s are used to define the mechanism by which the charge transferred [75][76][77] .As shown from Table 5, s < 1 indicates that the conduction occurs through hopping of charges between two potential barrier sites 78,79 .
The conductivity increases with increasing the frequency which indicates the semiconductor character of the examined samples, it also increases as the amount of Bi 2 O 3 in borate glass structures increases due to the presence of two oxidation state of Bi 3+ and Bi 5+ that share in the jumping process where one of them plays as a donor and the other as acceptor, respectively 80 .The presence of Bi in the structure of glass containing transition metal (V) can lead to decrease the bond distance in V-O-V that leads to increase in the V 5+ /V 4+ ratio [81][82][83] .In addition to the production of the tetrahedral BO 4 increases by increasing the Bi 2 O 3 content that results in increasing the donner Bi 3+ and the formation of non-bridging oxygen as discussed in the IR results.Also, the presence of shift in the wave number of [VO 4 ] towards longer wave numbers indicates its change to the trigonal bipyramids [VO 5 ] groups 40 .
It is worth to mention that the values of conductivity for all the samples are ranging from ~ 10 −8 at low frequency to ~ 10 −2 at high frequency that specifies the semiconductor character of the samples.The s values were drawn as a function of the Bi 2 O 3 content in borate glass as shown in Fig. 8b, where s increases with increasing Bi content till 7.5% then decreases but still its value > GBi1, this behavior is due to the formation of NBO with the increase in Bi 2 O 3 content, while the decrease of s value for Bi 2 O 3 content > 7.5% may be because of the disturbance in the NBO in glasses 84 .

The permittivity and dielectric loss
To identify the stored energy in the studied glasses under the effect of electric current, the real part of the dielectric constant (permittivity) (ε') was measured.
The frequency dependence of ε' for the studied glasses is shown in Fig. 9a, it is noted that ε' is affected by both the composition of the glasses and the frequency of the electric field, it increases with increasing the Bi 2 O 3 content due to the increase of both polarizability of glass and nonbridging oxygen (NBO) 85 .For all the studied glass compositions, it attains high value at low frequency due to the presence of different kinds of polarization such as the space charge and the dipole polarizations 86 .As the glass is amorphous, therefore there is a defect in its bulk interface that results in transferring the space charges at the presence of an electric field.Therefore, the predominant polarization of glass in low frequency values is space charge polarization 87,88 .Then, a gradual decrease in ε' was observed with raising the frequency due to the dielectric relaxation phenomenon that happened because of the instability of the localization of charge carrier localization under the electric field effect 89 .At frequency > 10 4 Hz, unchanged ε' value is achieved indicating the independence of ε' on the electric field.
To recognize the dissipated of energy in the studied glasses, the tangent loss (tanδ) was calculated from the dielectric loss (ε'') and ε' as the following equation: Figure 9(b) shows the change in tanδ with frequency for the studied glass samples, which looks like the change of ε' with frequency.At low frequency, tanδ have high values that decreases gradually as the frequency increases till 100 Hz, after that it reaches nearly constant, then a relaxation peak is observed at nearly 31.6 kHz which may be dipolar relaxation.It was also noted that tanδ decreased with increasing Bi 2 O 3 content and GBi2 achieves the highest value of dielectric constant and the lowest value of dielectric loss.

Impedance measurements
The measurement of impedance for the studied samples is represented by the Nyquist plots that give how can the real part of impedance changed with the imaginary parts at room temperature are shown in Fig. 10.This relation can help in understanding the role of the microscopic elements of the material, such as the grain, electrode effect, and relaxation process 90 .Inclined lines tend to bend at the x-axis to shape as semi-circles that interrelated to the capacitance and resistance of the bulk were observed in the figure.The angle by which the line is inclined decreases with increasing the Bi 2 O 3 content that means the semicircle radius reduces that indicates the increase in the conductivity of the bulk with Bi 2 O 3 concentration rising.This behavior is coincidence with the conductivity measurements.As the semicircle is asymmetric (depressed), therefore a deviation from Debye relaxation occurs that may be due to different factors such as the dipole groups formation, a defect in the atomic distribution and formation of nonpolar clusters 91 .Grain orientation, defect in the atomic distribution of the grain boundaries and the stress strain in the glass materials are from the factors that causes this nonideal behavior.However, the presence of one semicircle reveals that the glass system conducting behavior comes mainly from the grains rather than the grain boundaries 92 .

The electrical modulus analysis
To investigate the relaxation process and to understand the response of the bulk, the variation of the real part of electric modulus Mʹ, and its imaginary part M″ with the frequency was investigated as in Fig. 11a,b.Low values of M' were observed at low frequency, then a gradual enhancement in Mʹ occurred and went to higher values with rising the frequency, then accomplished maximum value at f > 20 Hz.This behavior demonstrates the dispersion of the relaxation processes along all the studied frequency range 93 .The mobility of the charge carriers is the reason for the increase in M′, where the effect of the electric field on their mobility is restricted 94 .
The behavior of changing M″ with frequency (Fig. 11b) shows an indication of a peak at low frequencies and its position changed to lower values of frequency as the Bi 2 O 3 content increases which directs the involvement of dc-conductivity 95,96 .Another peak with lower height is observed at high frequency and its height reduces with high Bi 2 O 3 content (inset Fig. 11b).Control in the charge carriers occurred between the two peaks.
. The combination of the V=O vibration of the [VO 5 ] vanadium group and the stretching vibration of the B-O bond in the [BO 4 ] tetrahedral units is responsible for the absorption peak at 930 cm −138,39 .

Figure 3 .Table 2 .
Figure 3. Density and molar volume as a function of Bi 2 O 3 content in glass samples.

Figure 4 .
Figure 4. FTIR spectra of present glasses as a function of Bi 2 O 3 mol%.

Table 1 .
Chemical composition, density and molar volume of the prepared glasses.

Table 3 .
Cutoff wavelength (λ c ), optical band gap energy E opt (direct), optical band gap energy E opt (indirect) and Urbach energy (∆E) of the prepared glasses.

Table 4 .
Optical parameters of the studied glasses.

Table 5 .
DC conductivity (σ dc ), crossover frequency (ω H ) and frequency factor (s) of the glasses synthesized in the system.