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Advances in Condensed Matter Physics
Volume 2008 (2008), Article ID 937054, 7 pages
http://dx.doi.org/10.1155/2008/937054
Research Article

Structural and Physical Properties of Glasses

1Department of Electronic Science, Kurukshetra University, Kurukshetra 136 119, India
2Department of Applied Physics, S.D. College Ambala Cantt, Kurukshetra University, Ambala Cantt 133 001, India
3Department of Physics, Maharshi Dayanand University, Rohtak 124 001, India

Received 23 May 2008; Revised 13 August 2008; Accepted 22 October 2008

Academic Editor: Gayanath Fernando

Copyright © 2008 Virender Kundu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The structural and physical properties of -(40-x) - 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 . The absence of boroxol ring (806 ) in the present glass system suggested that these glasses consist of randomly connected and units. The conversion of to and to 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 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 F and 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 [35].

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 [69]. Infrared (IR) spectroscopy is one of the important techniques which are used to study the local arrangement in inorganic glasses. In oxide glasses, 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 () ions are triangularly coordinated by oxygen to form glasses easily. The main structural units of vitreous glasses are B triangles forming three member (boroxol) rings connected by B–O–B linkage [10]. Boroxol ring is a high-planar ring with a bond length  Å, whereas the B–O bond length for B tetrahedra was observed to be  Å [11]. In IR spectra of borate glasses, the boroxol ring has its characteristic absorption at 806  [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 B structural units to B 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 content above 50 mol% [13], and for  mol%, conduction becomes non-adiabatic [14].

Sufficient work regarding the structural and physical properties of different oxide glasses having Te and as glass former has been reported [1518], but relatively few work has been done on semiconducting oxide glasses (having Fe and V as TMIs) with 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 F in this glass system.

2. Experimental Details

2.1. Sample Preparation

Iron-doped vanadium borate glasses were prepared from analytical reagent grade powder of , and B 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 .

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 . 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 of each glass sample was calculated using formula [19] where is the molar fraction, and is the molecular weight of the th 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 characteristics.

3. Results and Discussion

3.1. FTIR Analysis

The infrared spectra of F-(40-) -60 glasses, with , 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  is due to the asymmetric stretching relaxation of the B–O band of trigonal B units. The second group of band lies between 800–1200  and is due to the B–O bond stretching of the tetrahedral B units. The third group of absorption bands is observed around 700  and is due to bending of B–O–B linkages in the borate network. Similar results have been reported for - glasses [22, 23].

937054.fig.001
Figure 1: IR transmission spectra of F-(40-) -60 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  is attributed to hydroxol or water groups [24] and it is due to hygroscopic nature of glass samples [25]. The peaks around 2270–2286  and 2342–2345  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  indicates the absence of boroxol ring formation [12], which suggests that the glass system under investigation consists of randomly connected B and B groups. The absorption peaks at 1192, 1445, 1537, and 1549  [27, 28] are related with the fundamental asymmetrical stretching vibration of the B–O bond of the trigonal B units. The absorption bands observed at 814, 883, and 1023  [20, 29] are assigned to the stretching vibrations of B–O bond of tetrahedral B units and are shifted toward lower wave numbers with increasing iron content. It is also observed (Figure 1) that on increasing F content, the absorption band at around 1445  shifts toward lower wave number (1445–1440 ) 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 B units. The decrease in intensity of vibrational bands (for  mol%) at 1445 and 1192  and clear appearance of band in the lower region at 814  suggest the formation of NBOs. A sharp absorption band observed at around 1192  may be attributed to triangular B–O stretching vibrations of B units [27]. On increasing the F contents, the frequency of this band remains almost the same, revealing the strong appearance of the triangular borate units (B). The absorption band observed at 621  is due to the bending of O–B–O linkage [30]. The shifting of this band toward higher wave number (621–646 ) with increasing F content indicates the formation of Fe group [30]. This possibility is more in the borate glasses in which boroxol rings are absent. The absorption peak observed at 704  is assigned to B–O–B bending vibration in borate network [31]. At low concentration of F, a weak absorption band observed at 814  originates from stretching vibration of V–O–V bridges [32] and becomes slight intense as the concentration of F increases. The weak absorption band appearing in all the samples at 883  is due to content. The shoulder at 1023  can be referred to stretching vibrations of B tetrahedra and is also due to higher content of in the glass system. It has been reported that the structure is built up by deformed V trigonal bonded zigzag chains. Each V group contains a short V=O bond (Vanadyl group), which shows its characteristic vibration band at around 1023  [33]. The appearance of this frequency band at 1020  at  (mol%) and the clear separation of the peak at 814  on increasing F contents suggest that this absorption is not only due to the presence of non-bridging and V=O bond, but also due to the presence of B 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  appears for low concentration of F. This peak, however, disappears as the concentration of F increases ( mol%) and with further increase in F content ( mol%), the increase in intensity of this peak indicates that the F acts as a glass modifier. The intensity of B structural units decreases on further increases of F contents, and clear appearance of peaks (814 ) in the lower region suggests that some B structural units are converted into some B tetrahedral units, which results in the formation of NBOs. On further addition of F contents (15–20 mol%), the absorption bands observed in the lower region, that is, at 1190, 881, 814, 706, and 646  remain almost unaffected, however, their intensities changes which indicate that F also acts as a glass former. These observations suggest that at low concentration ( mol%), F acts as a glass modifier and at high concentration ( mol%), F acts as a glass former. Therefore, in the present glass system, F acts both, as a glass modifier as well as a glass former. The results obtained are well consistent with already reported result [26].

tab1
Table 1: Peak frequencies from IR spectra for -(40-) -60 glass system.
tab2
Table 2: Electrical conductivity at different temperatures and the prefactor of Mott-Austin formula of F-(40-) -60 glasses.
3.2. Density and Molar Volume

The determined values of density “d” and molar volume “” 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 () is shown in Figure 2. It is observed that the density increases gradually with the increase in F content in the present glass system. The relationship between density and glass composition () can be explained in terms of an apparent volume occupied by 1 gm atom of oxygen. The value of 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 F 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 F 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].

tab3
Table 3: Chemical composition and physical properties of F-(40-) -60 glasses.
937054.fig.002
Figure 2: Variation of density and molar volume with F 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 F 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 F 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 F 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 -BaO- glasses [38]. The change in conductivity and activation energy may help to detect the structural changes as a consequence of increasing of F content and decreasing of boron oxide content. Generally, it is known that addition of F 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 where is a pre-exponentional factor, W is the activation energy, is Boltzmann constant, and is temperature in Kelvin. The values of log were determined from the intercept of the conductivity versus temperature curve (Figure 3). The observed values of log are found to be independent of F 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 “” of respective ions was calculated (in order to confirm the relation between the activation energy W and mean distance R) using the formula [14]: where , and 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 “” between the transition ions (assuming homogenous distribution of transition ion in glass volume) was calculated as follows: where “” is the concentration of total TMIs .

tab4
Table 4: Hopping parameters of F-(40-) -60 glasses.
937054.fig.003
Figure 3: Variation of log σ versus for studied glasses.
937054.fig.004
Figure 4: Effect of F content on dc conductivity at different temperatures.
937054.fig.005
Figure 5: Effect of F content on activation energy W and pre-exponential factor log .

The small polaron radii “” for the iron and vanadium ions have been calculated using [41] The calculated values of , and 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 F, 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  mol% of F, 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].

937054.fig.006
Figure 6: Variation of activation energy W with average ion separation R.
937054.fig.007
Figure 7: Variation of small polaron radius with iron content.

4. Conclusion

The structural and physical properties of F-(40-) -60 glass system, with (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 B and B groups. Addition of F produces NBOs in borate as well as in Vandate glass network along with the formation of V groups. The IR spectra of the present glass system indicate that F acts both, as a glass modifier ( mol%) as well as a glass former ( mol%). The density of all the glass samples increases with F content as a result of the increase in NBOs due to the conversion of trigonal B structural units into B tetrahedral unit. In the temperature range 310–500 K, the variation of log σ with is approximately linear. It was observed that the dc conductivity increases with increasing iron content, and ranges from to 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.

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