Structural and Physical Properties of Fe_𝟐O_𝟑-B_𝟐O_𝟑-V_𝟐O_𝟓 Glasses

Abstract

The structural and physical properties of 𝑥Fe₂O₃-(40-x) B₂O₃-60V₂O₅(0≤𝑥≤20) glass system have been investigated. The samples were prepared by normal melt-quench technique. The structural changes were inferred by means of FTIR by monitoring the infrared (IR) spectra in the spectral range 600–4000 cm⁻¹. The absence of boroxol ring (806 cm⁻¹) in the present glass system suggested that these glasses consist of randomly connected BO₃ and BO₄ units. The conversion of BO₃ to BO₄ and VO₅ to VO₄ tetrahedra along with the formation of non-bridging oxygen's (NBOs) attached to boron and vanadium takes place in the glasses under investigation. The density and molar volume of the present glass system were found to depend on Fe₂O₃ content. DC conductivity of the glass system has been determined in the temperature range 310–500 K. It was found that the general behavior of electrical conductivity was similar for all glass compositions and found to increase with increasing iron content. The parameters such as activation energy, average separation between transition metal ions (TMIs), polaron radius, and so forth have been calculated in adiabatic region and are found consistent with Mott's model of phonon-assisted polaronic hopping.

1. Introduction

Borate glasses are generally insulating in nature, and the addition of transition metal oxide such as Fe₂O₃ and V₂O₅ makes these glasses semiconducting [1, 2]. These semiconducting glasses have been extensively studied owing to their potential applications as optical and electrical memory switchings, cathode materials for making solid state devices, and optical fiber [3–5].

Structure of the borate glasses has been studied by various physical and chemical methods including Raman and Infrared spectroscopy, Brillouin experiments, NMR, and neutron scattering investigations [6–9]. Infrared (IR) spectroscopy is one of the important techniques which are used to study the local arrangement in inorganic glasses. In oxide glasses, B₂O₃ is a basic glass former because of its higher bond strength, lower cation size, smaller heat of fusion, and trivalancy of boron. In these glasses, boron (B³⁺) ions are triangularly coordinated by oxygen to form glasses easily. The main structural units of vitreous B₂O₃ glasses are BO₃ triangles forming three member (boroxol) rings connected by B–O–B linkage [10]. Boroxol ring is a high-planar ring with a bond length 1.36±0.005 Å, whereas the B–O bond length for BO₄ tetrahedra was observed to be 1.47±0.01 Å [11]. In IR spectra of borate glasses, the boroxol ring has its characteristic absorption at 806 cm⁻¹ [12]. It has been observed that the structure of these glasses depends on the nature of the network formers as well as the network modifier. It has been reported that addition of a network modifier in borate glasses could produce the conversion of the triangular BO₃ structural units to BO₄ tetrahedra with coordination number of 4, which are incorporated in more complex cyclic groups such as diborate, triborate, tetra or pentaborate, and the formation of NBO atoms [8, 9].

Transition-metal oxides (TMOs) glasses exhibit semiconducting properties due to the existence of TMIs in more than one valence state [1]. The electron-phonon interaction in these glasses is strong enough to form small polaron, and the electrical conduction process occurs by the hopping of small polarons between different valence states as proposed by Austin and Mott [2]. Hoping conduction in these glasses was generally known to be adiabatic for V₂O₅ content above 50 mol% [13], and for V₂O₅<50 mol%, conduction becomes non-adiabatic [14].

Sufficient work regarding the structural and physical properties of different oxide glasses having TeO₂ and P₂O₅ as glass former has been reported [15–18], but relatively few work has been done on semiconducting oxide glasses (having Fe and V as TMIs) with B₂O₃ as a network former. The objective of present paper is to study the structural and physical properties of iron-boro-vandate glasses to shed some light on the role of the Fe₂O₃ in this glass system.

2. Experimental Details

2.1. Sample Preparation

Iron-doped vanadium borate glasses were prepared from analytical reagent grade powder of Fe₂O₃,V₂O₅, and H₃BO₃ of high purity which are thoroughly mixed, in appropriate proportions. The batch materials were dry mixed and melted in porcelain crucibles placed in an electrically heated muffle furnace at 1473 K for about two hours, until a bubble-free liquid was formed. The melts were quickly cooled at room temperature by pouring and pressing between two stainless plates. As obtained glass samples were polished and finally cut into desired size (≈10mm×10mm×1mm).

2.2. FTIR Measurements

The vibration spectra of the glass system were obtained at room temperature using FTIR spectrophotometer model ABB Bomen (MB-Series) in the range 600–4000 cm⁻¹. The measurements were performed directly on glass pallets obtained as above.

2.3. Density Measurements

The density “d” of the glasses was determined at room temperature using Archimedes principle with Xylene as an inert immersion liquid.

The molar volume 𝑉_𝑀 of each glass sample was calculated using formula [19] 𝑉_𝑀=𝑥_𝑖𝑀_𝑖𝑑, (1) where 𝑥_𝑖 is the molar fraction, and 𝑀_𝑖 is the molecular weight of the 𝑖th component.

2.4. D.C. Conductivity Measurements

The conductivity measurements were carried out by using Keithely electrometer (Model 617) in the temperature range of 310–500 K. Silver paste electrodes were deposited on both faces of the polished samples. The absence of the barrier layers at the contacts was confirmed by linear 𝐼−𝑉 characteristics.

3. Results and Discussion

3.1. FTIR Analysis

The infrared spectra of 𝑥Fe₂O₃-(40-𝑥) B₂O₃-60V₂O₅ glasses, with 𝑥=0,5,10,15, and 20 mol%, are shown in Figure 1. The vibrational modes of the borate network are seen to be mainly active in three infrared regions which are similar to those reported earlier [20, 21]. The group of bands that occur at 1200–1600 cm⁻¹ is due to the asymmetric stretching relaxation of the B–O band of trigonal BO₃ units. The second group of band lies between 800–1200 cm⁻¹ and is due to the B–O bond stretching of the tetrahedral BO₄ units. The third group of absorption bands is observed around 700 cm⁻¹ and is due to bending of B–O–B linkages in the borate network. Similar results have been reported for V₂O₅-B₂O₃ glasses [22, 23].

Figure 1 

IR transmission spectra of 𝑥Fe2 O3-(40-𝑥) B2 O3-60V2O5 glasses.

The absorption peaks assigned in IR spectra of glasses under study are listed in Table 1. The absorption peak observed in all glass samples at 3204–3217 cm⁻¹ is attributed to hydroxol or water groups [24] and it is due to hygroscopic nature of glass samples [25]. The peaks around 2270–2286 cm⁻¹ and 2342–2345 cm⁻¹ are attributed to –OH group [26]. The absorption peak around 1722–1730 is due to H–O–H bending [22]. In the present glass system, the absence of peak at 806 cm⁻¹ indicates the absence of boroxol ring formation [12], which suggests that the glass system under investigation consists of randomly connected BO₃ and BO₄ groups. The absorption peaks at 1192, 1445, 1537, and 1549 cm⁻¹ [27, 28] are related with the fundamental asymmetrical stretching vibration of the B–O bond of the trigonal BO₃ units. The absorption bands observed at 814, 883, and 1023 cm⁻¹ [20, 29] are assigned to the stretching vibrations of B–O bond of tetrahedral BO₄ units and are shifted toward lower wave numbers with increasing iron content. It is also observed (Figure 1) that on increasing Fe₂O₃ content, the absorption band at around 1445 cm⁻¹ shifts toward lower wave number (1445–1440 cm⁻¹) with noticeable decrease in intensity. In the present glass system, the shift of the vibrational band from higher to lower wave number is ascribed to the increase in the bond length of B–O groups and the formation of BO₄ units. The decrease in intensity of vibrational bands (for 𝑥≤10 mol%) at 1445 and 1192 cm⁻¹ and clear appearance of band in the lower region at 814 cm⁻¹ suggest the formation of NBOs. A sharp absorption band observed at around 1192 cm⁻¹ may be attributed to triangular B–O stretching vibrations of BO₃ units [27]. On increasing the Fe₂O₃ contents, the frequency of this band remains almost the same, revealing the strong appearance of the triangular borate units (BO₃). The absorption band observed at 621 cm⁻¹ is due to the bending of O–B–O linkage [30]. The shifting of this band toward higher wave number (621–646 cm⁻¹) with increasing Fe₂O₃ content indicates the formation of FeO₄ group [30]. This possibility is more in the borate glasses in which boroxol rings are absent. The absorption peak observed at 704 cm⁻¹ is assigned to B–O–B bending vibration in borate network [31]. At low concentration of Fe₂O₃, a weak absorption band observed at 814 cm⁻¹ originates from stretching vibration of V–O–V bridges [32] and becomes slight intense as the concentration of Fe₂O₃ increases. The weak absorption band appearing in all the samples at ≈883 cm⁻¹ is due to V₂O₅ content. The shoulder at 1023 cm⁻¹ can be referred to stretching vibrations of BO₄ tetrahedra and is also due to higher content of V₂O₅ in the glass system. It has been reported that the V₂O₅ structure is built up by deformed VO₅ trigonal bonded zigzag chains. Each VO₅ group contains a short V=O bond (Vanadyl group), which shows its characteristic vibration band at around 1023 cm⁻¹ [33]. The appearance of this frequency band at 1020 cm⁻¹ at 𝑥=10 (mol%) and the clear separation of the peak at 814 cm⁻¹ on increasing Fe₂O₃ contents suggest that this absorption is not only due to the presence of non-bridging V-O⁻¹ and V=O bond, but also due to the presence of BO₄ tetrahedra. Similar results have also been observed in potasium-boro-vanadate-iron glasses [34]. From Figure 1, it can be clearly inferred that a weak absorption band corresponding to 814 cm⁻¹ appears for low concentration of Fe₂O₃. This peak, however, disappears as the concentration of Fe₂O₃ increases (𝑥=5 mol%) and with further increase in Fe₂O₃ content (𝑥=10 mol%), the increase in intensity of this peak indicates that the Fe₂O₃ acts as a glass modifier. The intensity of BO₃ structural units decreases on further increases of Fe₂O₃ contents, and clear appearance of peaks (≈814 cm⁻¹) in the lower region suggests that some BO₃ structural units are converted into some BO₄ tetrahedral units, which results in the formation of NBOs. On further addition of Fe₂O₃ contents (15–20 mol%), the absorption bands observed in the lower region, that is, at 1190, 881, 814, 706, and 646 cm⁻¹ remain almost unaffected, however, their intensities changes which indicate that Fe₂O₃ also acts as a glass former. These observations suggest that at low concentration (𝑥≤10 mol%), Fe₂O₃ acts as a glass modifier and at high concentration (𝑥=15-20 mol%), Fe₂O₃ acts as a glass former. Therefore, in the present glass system, Fe₂O₃ acts both, as a glass modifier as well as a glass former. The results obtained are well consistent with already reported result [26].

Table 1 

Peak frequencies from IR spectra for 𝑥Fe₂O₃-(40-𝑥) B₂O₃-60V₂O₅ glass system.

Table 2 

Electrical conductivity at different temperatures and the prefactor of Mott-Austin formula of 𝑥Fe₂O₃-(40-𝑥) B₂O₃-60V₂O₅ glasses.

3.2. Density and Molar Volume

The determined values of density “d” and molar volume “𝑉_𝑀” of the glass samples are presented in Table 3. In general, the density of glass system is explained in terms of a competition between the masses and sizes of the various structural units present in glass. In other words, the density is related to how tightly the ions and ionic groups are packed together in the structure. The variation of density as a function of glass composition (𝑥) is shown in Figure 2. It is observed that the density increases gradually with the increase in Fe₂O₃ content in the present glass system. The relationship between density and glass composition (𝑥) can be explained in terms of an apparent volume 𝑉_𝑀 occupied by 1 gm atom of oxygen. The value of 𝑉_𝑀 has been calculated from the density and composition using the formula reported earlier [19], and its composition dependence is shown in Figure 2. It is observed that molar volume decreases monotonically with the increase of Fe₂O₃ content which indicates that the topology of the network is significantly changed with composition. On the other hand, these trends can be explained rather simply due to the replacement of a lighter cation (B) by a heavier one (Fe). As observed in IR spectra, the addition of Fe₂O₃ causes increase in NBOs, which in turn randomizes the structure and, therefore, glass structure becomes relatively more open. These results are found consistent with the results reported earlier [35, 36].

Table 3 

Chemical composition and physical properties of 𝑥Fe₂O₃-(40-𝑥) B₂O₃-60V₂O₅ glasses.

Figure 2 

Variation of density and molar volume with Fe2O3 content.

3.3. DC Conductivity

The temperature dependence of dc conductivity “σ” for the different glass compositions is shown in Figure 3. It is observed that conductivity increases smoothly with increasing temperature, indicating temperature dependence activation energy “W” which is characteristic of small polaron hopping (SPH) conduction mechanism in TMO glasses [37]. As shown in Figure 3, the logarithmic conductivity in the temperature range (310–500 K) exhibits a linear dependence on reciprocal temperature. The composition dependence of dc conductivity at particular temperature (Figure 4) indicates that the conductivity increases with increasing Fe₂O₃ content. The activation energy calculated from the slop of the graphs (Figure 3) is listed in Table 3. It is clear from Figure 5 that the activation energy decreases with increasing Fe₂O₃ content. The composition dependence of dc conductivity at 400 K (Figure 4) and activation energy (Figure 5) indicate that the variation of “σ” as well as “W" with composition is much faster for the present glass system than those for the traditional vandate glasses. It is observed that as the concentration of Fe₂O₃ increases, the activation of electrical conduction decreases and electrical conductivity increases. The low value of activation energy and high value of electrical conductivity are similar to those of V₂O₅-BaO-B₂O₃ glasses [38]. The change in conductivity and activation energy may help to detect the structural changes as a consequence of increasing of Fe₂O₃ content and decreasing of boron oxide content. Generally, it is known that addition of Fe₂O₃ in borate glasses increases the conductivity as result of increasing of NBO ions [39]. It is clear from Figures 4 and 5 that the magnitude of conductivity is higher for those compositions which have lower activation energy. This result is in consistent with the small polaron hopping theory [40]. According to this theory, the conduction process at higher temperature is considered in terms of optical phonon assisted hopping of small polaron between localized states. The dc conductivity in adiabatic region is given by 𝜎=𝜎₀𝑊exp(−𝑘𝑇), (2) where 𝜎₀ is a pre-exponentional factor, W is the activation energy, 𝑘 is Boltzmann constant, and 𝑇 is temperature in Kelvin. The values of log 𝜎₀ were determined from the intercept of the conductivity versus temperature curve (Figure 3). The observed values of log 𝜎₀ are found to be independent of Fe₂O₃ content (Figure 5) which confirms the adiabatic SPH for the present glass system. The present glass system consists of two types of TMIs, that is, Fe and V. The density “𝑛” of respective ions was calculated (in order to confirm the relation between the activation energy W and mean distance R) using the formula [14]: 𝑛=2[(𝑑𝑊_𝑡𝑀_𝑤)𝑁_𝐴], (3) where 𝑑,𝑁_𝐴,𝑊_𝑡, and 𝑀_𝑤 are the measured density of the sample, Avogadro’s number, weight fraction, and molecular weight of the respective ions (Fe & V), respectively. The calculated values of iron and vanadium ions are listed in Table 4. The average distance “𝑅” between the transition ions (assuming homogenous distribution of transition ion in glass volume) was calculated as follows: 1𝑅=(𝑁)^1/3, (4) where “𝑁” is the concentration of total TMIs (𝑛_Fe2O3+𝑛_V2O5).

Table 4 

Hopping parameters of 𝑥Fe₂O₃-(40-𝑥) B₂O₃-60V₂O₅ glasses.

Figure 3 

Variation of log σ versus 1000/𝑇(K−1) for studied glasses.

Figure 4 

Effect of Fe2O3 content on dc conductivity at different temperatures.

Figure 5 

Effect of Fe2 O3 content on activation energy W and pre-exponential factor log 𝜎0.

The small polaron radii “𝑟_𝑝” for the iron and vanadium ions have been calculated using [41] 𝑟_𝑝=12(𝜋)6𝑁^1/3. (5) The calculated values of 𝑁,𝑅, and 𝑟_𝑝 are tabulated in Table 4. The variation of activation energy W with average distance R, as shown in Figure 6, suggests that there is a prominent positive correlation between activation energy and average site separation. It is evident from Figure 7 that the small polaron (SP) radius decreases with the increase in Fe-ion concentration. From the above observations, it is concluded that on addition of Fe₂O₃, conductivity increases, while the activation energy decreases due to the decrease in respective ion separation. These results are in good agreement with those reported by El-Desoky [36]. However, in the present investigation for sample 𝑥=10 mol% of Fe₂O₃, there are some minor changes observed in the density and electrical parameters as compared to our earlier investigation [42]. These changes may be attributed to difference in sample preparation conditions [11].

Figure 6 

Variation of activation energy W with average ion separation R.

Figure 7 

Variation of small polaron radius with iron content.

4. Conclusion

The structural and physical properties of 𝑥Fe₂O₃-(40-𝑥) B₂O₃-60V₂O₅ glass system, with 0≤𝑥≤20 (mol%), have been studied. No boroxol ring formation was observed in the structure of these glasses which suggest that glasses under study consist of randomly connected BO₃ and BO₄ groups. Addition of Fe₂O₃ produces NBOs in borate as well as in Vandate glass network along with the formation of VO₄ groups. The IR spectra of the present glass system indicate that Fe₂O₃ acts both, as a glass modifier (𝑥≤10 mol%) as well as a glass former (𝑥=15-20 mol%). The density of all the glass samples increases with Fe₂O₃ content as a result of the increase in NBOs due to the conversion of trigonal BO₃ structural units into BO₄ tetrahedral unit. In the temperature range 310–500 K, the variation of log σ with 𝑇⁻¹ is approximately linear. It was observed that the dc conductivity increases with increasing iron content, and ranges from 5.435×10⁻⁴ to 4.194×10⁻²(Ωm)⁻¹ at 400 K. The conduction in the present glass system was confirmed to be a result of primarily adiabatic hopping of small polarons between TMIs.