Influence of Fe2O3 on the physical, structural and electrical properties of sodium lead borate glasses

The influence of adding Fe2O3 at the expense of Na2O in sodium lead borate glasses on the structural, physical and electrical properties have been investigated. Results obtained from Fourier transform infrared (FTIR) spectra indicated that Fe2O3 plays an important role in converting three coordinated boron atoms [BO3] to four coordinated boron atoms [BO4]. The physical properties such as density and molar volume helped to evaluate the compact structure of the prepared glass samples due to presence of [BO4] groups. The increase of Fe2O3/Na2O replacements led to increasing the microhardness values and decreasing the thermal expansion coefficients of the studied glasses. The increase of Fe2O3/Na2O replacements generally decreased the AC conductivity. That decrease might be due to converting of the three coordinated boron atoms [BO3] to four coordinated boron atoms [BO4]. Dielectric constants of the samples might be an indication of the distortion in the coordinated boron atoms. The obtained experimental data indicated the internal structure of glass network and the change of the structure of the samples from three [BO3] to four coordinated boron atoms [BO4].


Introduction
Since the past several years, borate glasses have attracted much attention because of their electrochemical and optical applications, namely as solid-state batteries, optical wave guides and luminescent materials. Several previous works on borate glasses were devoted to study the structure, magnetic and electrical properties [1,2]. The glass materials, which consist of heavy metal oxides, such as lead, are more promising for photonics and optoelectronics. Depending on a kind of bond: ionic or covalent between lead and oxide, PbO plays the role as a modifier or a glass former. It is a function of PbO/B 2 O 3 ratio in the glass. In order to stabilize glass network, many elements have been added to the base matrix. It has been very well known that iron [3], vanadyl [4], BaO [5], titanium dioxide [6] and copper [7] additions affect structure and glass properties in PbO-B 2 O 3 system.
Lead borate glasses are good host matrix for rare-earth and transition-metal oxide. These glasses carry a great importance due to their promising  applications in the fields of laser technology, radiation shielding materials, luminescence materials, photonics and optoelectronic devices [8][9][10].
Lead borate oxide glasses are highly transparent in the visible and near-infrared regions and exhibit very good glass formation over a large compositional region [11]. Moreover, PbO-B 2 O 3 glasses have desirable characteristics against irradiation, since the naturally occurring stable boron isotope is a good absorber of thermal neutrons [12] and lead is known as a shielding material of γ-rays [13]. It is now well known that lead oxide (PbO) is unique in its influence on the glass structure and is widely used in glasses, because it enhances the resistance against devitrification, improves the chemical durability and lowers the melting temperature [14]. It can act as either a glass network former or a modifier, depending on its concentration in the glasses [15].
Due to their potential applications in various domains of modern technology, glasses containing transition-metal oxides have been the subject of intensive investigations [16], for example, the glasses containing Fe 2 O 3 .
The co-existence of iron ions in more than one valence state, i.e., Fe 2+ and Fe 3+ , is a general condition for semiconducting behavior of borate glasses with these ions [17]. Consequently, glasses containing Fe 2 O 3 are used in electrochemical, electronic, electro-optic and memory switching devices [18]. The addition of iron to borate glasses makes it electrically semiconducting and super paramagnetic [19].
The content of iron in diverse environments with different valence states that exist in the glass depends on the quantitative properties of modifiers and glass formers, size of the ions in the glass structure, their field strength, mobility of the modifier cation, etc. Hence, the correlation between the state and the position of the iron ion in the glass network and its physical properties is expected to be highly interesting [20].
The present work explores the dominant role of Fe 2 O 3 on structural, physical and electrical properties in Na 2 O-PbO-B 2 O 3 glass system. Fourier transform infrared (FTIR) spectroscopy has been used to study the structure features and their effects on the density, molar volume, microhradness, thermal expansion and electrical properties. Therefore, the aim of this paper is to investigate the effect of increasing Fe 2 O 3 at the expense of Na 2 O on the structural, physical and electrical properties of sodium lead borate glasses.

1 Preparation of glass
Analytically pure grade chemicals were used to prepare the following glass samples according to the molecular formula xFe 2 O 3 -(25-x)Na 2 O-25PbO-50B 2 O 3 (in molar percentage) where x = 0, 5, 10 and 15. Fe 2 O 3 , Na 2 CO 3 , Pb 3 O 4 and H 3 BO 3 were mixed thoroughly. Each batch of formulas was weighed 50 g and melted in porcelain crucible placed in an electric furnace at temperatures ranging from 1000 ℃ to 1100 ℃ depending on the glass composition. Melting was continued under normal atmospheric condition for 2 h after the last addition of the batch constituents. The melts were swirled for every 30 min to ensure homogeneity and cast in rectangular forms, and all the samples were immediately transferred to annealing furnace at 400-450 ℃. The furnace was switched off and left to cool to room temperature.

2 Density and molar volume measurements
Density  of all glass samples was calculated by employing the Archimedes principle using xylene and applying the relationship: where a W is the weight in air; b W is the weight in xylene; and b  is the density of xylene ( b  = 0.863 g/cm 3 ). The density data were used to calculate the molar volume M V given by M , where T M is the molecular weight of glass (in mol%);  is the density of glass [21].
The FTIR absorption spectra were recorded with JASCO FT/IR-300E spectrometer at room temperature in the range of 400-4000 cm 1 with a resolution of 4 cm 1 using the KBr pellet technique. In order to obtain good quality spectra, the samples were crushed in an agate mortar and the powders were pressed, and then the IR spectra were immediately measured.
Polished glass samples with dimension of 5 mm × 5 mm × 10 mm were used for linear thermal expansion measurements at a heating rate of 5 ℃/min by Linseis Thyristor-Stellglied 35V160V.23A analyzer. From these measurements, the thermal expansion coefficient  at 300 ℃ and both glass transition temperature g T and dilatometric softening temperature s T were determined.
The microhardness of the investigated samples was measured by using Vicker's microhardness indenter (SHIMADZU, HMV-2 Series, Japan). The eye piece on the microscope of the apparatus allowed measurements with an estimated accuracy of ±0.5 mm for the indentation diagonals. The specimens were cut using a low speed diamond saw, dry ground using 1200 grit SiC paper, and polished carefully using 6 mm, 3 mm and 1 mm diamond pastes to obtain smooth and flat parallel surfaces of glass samples before indentation. At least, six indentation readings were made and measured for each sample. Testing was made using a load of 100 g; loading time was fixed for all glass samples (15 s). The measurements were carried out under normal atmospheric conditions. The Vicker's microhardness value V H (kg/mm 2 ) was calculated using the following equation: where A is a constant value equal to 1854.5 (taking into account the geometry of squared based diamond indenter); p is the applied load (g); and d is the average diagonal length (mm).

3 Electrical measurements
Data were collected in the frequency range from 100 Hz up to 1 MHz using a Hioki 3522-50 LCR Hitester impedance analyzer contacted with one of the two electrodes shown in Fig. 1, which was a four electrode measurement. The measured parameters were the series and parallel capacitances and the series and parallel resistances at different frequencies (see Refs. [22][23][24] for more details). In Fig. 1, A and B are the current electrodes, and M and N are the potential electrodes. They represent thin copper discs. The current and potential electrodes were immersed in the agar-agar/copper sulphate gel compartment [23]. Samples were connected to non-polarizable electrodes (Cu/CuSO 4 ) [23]. The oscillation amplitude was set at 1 V (small signal). Electrical properties of a material can be expressed in either series or parallel configuration [25]. The measured parameters were the parallel capacitance and conductance ( p C and p G ) and the series impedance Z at different frequencies (see Refs. [22][23][24] for more details). Measurements were done at room temperature at atmospheric relative humidity of ~50% in an isolated chamber (desiccator).

1 Density and molar volume
The composition dependence of the density  of the present glass samples is shown in Fig. 2. It may be observed that density increases gradually with the increase of iron oxide content in the glass compositions. The relationship between density and composition of an oxide glass system can be expressed in terms of an apparent volume M V occupied by 1 g atom of oxygen. The molar volume M V of all the prepared glasses, is defined as the mean molecular weight T M of its constituents divided by its experimental density, as given by the equation . These values are also included in Table 1.   Figure 3 illustrates the infrared spectra of the present glass systems. All the glass compositions show bands at 460 cm 1 , 575 cm 1 , 664 cm 1 , 711 cm 1 , 987 cm 1 , 1225 cm 1 , 1349 cm 1 , 1631 cm 1 and 3420 cm 1 .

3 Thermal expansion properties
The thermal expansion coefficient  , glass transition temperature g T and dilatometric softening temperature s T were determined in glasses containing different amounts of Fe 2 O 3 at the expense of Na 2 O and are summarized in Table 2 and Fig. 4.
The present results reveal that the increase in the amount of Fe 2 O 3 at the expense of Na 2 O leads to the decrease of the thermal expansion coefficient value and the increase of both g T and s T values (Table 2).

4 Microhardness
The present results indicate that the addition of Fe 2 O 3 at the expense of Na 2 O leads to the increase of microhardness value of the investigated glasses (Table 1).

5 Electrical measurements
The conductivity decreases with the increase of Fe 2 O 3 at the expense of Na 2 O replacement (at the same frequency). Figure 5 shows the variation of conductivity with frequency for different samples containing various proportions of Fe 2 O 3 replacements. Slope for all samples is related to the increase of frequency. Figure 6 shows the variation of dielectric constant with frequency for different samples containing various proportions of Fe 2 O 3 replacements. The increase of Fe 2 O 3 replacements results in the increasing values of dielectric constants. Figure 7 shows the impedance plane with frequency for different samples containing various proportions of Fe 2 O 3 at the expense of Na 2 O. The equivalent circuit shown in Fig. 8 is assumed to represent the sample general structure.

Discussion
It is well known that pure B 2 O 3 glass is composed essentially of [BO 3 ] triangles forming three-membered (boroxol) rings. By adding the modifier oxide to B 2 O 3    Fig. 7 Imaginary impedance variation with real impedance for samples G 1 ( ),G 2 ( ), G 3 ( ) and G 4 ( ). Fig. 8 Equivalent circuit that represents the sample ( g C and R  are the capacitance and resistance of the sample at high frequency, respectively; i C is the interfacial capacitance and i R is the interfacial resistance).

1 Density and molar volume
As shown in Fig. 2, the density of glasses increases progressively with the decrease in molar volume due to addition of Fe 2 O 3 . The increase in density might be related to the replacement of Na 2 O having lighter molecular mass with Fe 2 O 3 having high molecular mass. The measured values of density and molar volume have been presented in Table 1. It shows how the compaction of glass network and higher molecular weight of Fe 2 O 3 help to increase the density of glass samples. Another reason for increase in density may be the formation of more [BO 4 ] groups. The presence of iron oxide modifies the structural network by providing more oxygen, making conversion of [BO 3 ] to [BO 4 ] units [28]. Hence, it leads to an increase in density of the glass samples. The molar volume also plays good role. A decrease in molar volume leads to decrease in bond length which is responsible for compaction of glass network [29].
In order to confirm the compaction of glasses due to the presence of iron oxide, the average boron-boron separation ‹d B-B › has been calculated [30]. The volume B M V corresponds to the volume that contains one mole of boron within the given structure and it can be stated as where M V is the molar volume; B X is the molar fraction of B 2 O 3 .
where A N is the Avogadro number.
The calculated values of the average boron-boron separation are shown in Table 1. It can be observed that when the Fe 2 O 3 content increases, the average boron-boron separation decreases. Consequently, the addition of Fe 2 O 3 leads to compaction of glass network which confirms the results in terms of density and molar volume [29].

2 Infrared transmission spectra
IR spectroscopy ranks among the rather sensitive methods of structure investigation of glasses. The application of this method makes it possible to obtain information on the structural role of oxides entering into the composition of glass, the change in the polymerization of the anion carcass with a change in the glass composition, and the coordination state of ions [31]. Figure 3 shows infrared transmission spectra in the 400-4000 cm 1 region for sodium lead borate glasses.  [32].
The infrared spectrum of glass sample G 1 free from iron is shown in Fig. 3. The spectra of these glasses have exhibited FeO 6 octahedral band due to 1  vibration at 575 cm 1 , which is present in our spectra starting with x = 5 [33,34]. Iron ions are glass modifier and enter the glass network by breaking up the B-O-B bonds, and the Fe 3+ and Fe 2+ ions occupy interstitial positions and introduce coordination defect with non-bridging oxygen ions. In this case, Fe 3+ and Fe 2+ ions are octahedrally coordinated [35].
All glasses show a peak at 664 cm 1 . This is attributed to the bending vibration of B-O-B linkage in the borate network, which is previously reported by many authors [36]. It may also due to the presence of Pb-O vibration of lead oxide [37]. It is also assumed that the band observed at 664-711 cm 1 is due to the B-O-B bending vibrations of [BO 3 ] triangles [38].
In sample G 1 , the band at around 1349 cm 1 has been assigned to the stretching of trigonal [BO 3 ] units in meta, ortho and pyro-borate groups [37,39]. The position of this band is shifting towards the lower wavenumber at higher content of iron oxide in glass samples G 2 to G 4 . The band centered at 987 cm 1 in glass sample G 1 is assigned to B-O stretching vibrations of tetrahedral [BO 4 ] units in different borate groups [40]. With the initial addition of Fe 2 O 3 (5 mol%) into the glass network, intensity of this band decreases in glass sample G 2 . At the same time, the intensity of the band due to [BO 3 ] structural units has been observed to increase at the expense of the band due to [BO 4 ] units. This observation suggests a gradual increase in the content of divalent iron ions in the glass network that act as modifiers [20].
For further addition of Fe 2 O 3 (10 mol% and 15 mol%) to glass samples, intensity of this band decreases (bands due to [BO 3 ] units). At the same time, it has been observed that intensity of the band due to the BO 4 structural units increases at higher content of iron oxide. The absorption band arises in the region of 850-1100 cm 1 indicating B-O bond stretching of the tetrahedra [BO 4 ] units [38,41].
The predominant absorption band around the 850-987 cm 1 is related to the symmetrical stretching vibration of [BO 4 ] units [42,43]. The transmission band at 1225 cm 1 is specific to the B-O stretching vibrations of [BO 3 ] triangular units with non-bridging oxygen atoms [44,45]. This band shifts toward smaller wavenumber (glass samples G 3 and G 4 ) because of the increasing coordination number around B atoms [46].
The observed transmission bands from 1400 cm 1 to 1600 cm 1 can be ascribed to the B-O stretching vibrations in [BO 3 ] units from metaborate, pyroborate, orthoborates and groups containing a large number of NBOs [42,45] which are disappeared with increasing iron oxide content.
So the presence of iron oxide modifies the structural network by providing more oxygen, which results in conversion of [BO 3 ] to [BO 4 ] units [29]. As a result, introduction of iron increases the cross linking of the glass matrix due to the increasing the number of tetrahedral [BO 4 ] groups in the glass system. The infrared transmission peak observed in the region of 2500-4000 cm 1 is attributed to water groups OH stretching vibrations [29].
The [BO 3 ] units in the iron sodium lead borate glasses prefer a coordination change to [BO 4 ] rather than producing NBOs. The four-fold boron atoms are favored in the investigated system as compared with the three-fold ones [47].

3 Average coordination number
The average coordination number is an important parameter to confirm the bridging or non-bridging oxygen bond. The average coordination number (m) is calculated with the help of the equation [29]: where ci n is the coordination of cation. The coordination numbers of boron, lead, sodium and iron cations are 4, 4, 1 and 6, respectively. The observed value of average coordination number of glass samples increases with the incorporation of iron oxide (Table 2). These results confirm that the Fe 2 O 3 introduction in glass matrix progressively increases the number of bridging oxygen which causes an increase of [BO 4 ] groups in glass network. As a result, the introduction of iron increases the cross linking of the glass matrix due to the increasing number of tetrahedral [BO 4 ] groups in the glass system.

4 Bond density
The addition of Fe 2 O 3 affects the number of bonds per unit volume and it can be calculated by the relation [29]: where A N is the Avogadro number; ci n is the coordination number of cation; and i X is molar fraction of different oxides. The obtained result (Table  2) shows the value of number of bonds per unit volume increases with the increase in iron content. It reveals clearly that iron ion plays the role of a modifier in the present glass system by creating more B-O bond vibrations of [BO 4 ] groups. All these results are in good agreement with the FTIR measurements.

5 Thermal expansion, softening temperature and glass transition temperature
Thermal expansion is a very important thermal property of glass. The thermal expansion of glass is controlled by the asymmetry of the amplitude of thermal vibrations in the glass. It decreases as the rigidity of the glass network increases [48]. An increase of the number of non-bridging bonds weakens the structure, which in turn increases the thermal expansion coefficient, whereas the change in coordination number of the network former cation may cause either its increase or decrease depending on the effect on glass structure [49]. The cation B 3+ is capable of changing its coordination number, which in turn influences the structure and properties of the investigated glasses [31]. The anomaly tendency of the thermal expansion correlates with the ratio of [BO 4 ] to [BO 3 ] structural unit formation [48,49]. In this study, the thermal expansion coefficient gradually decreases by increasing Fe 2 O 3 at the expense of Na 2 O (Fig. 4). While, both g T and s T values are increased. This could be explained on the basis that the Fe 2 O 3 introduction in glass matrix progressively increases the number of bridging oxygen which causes an increase of [BO 4 ] groups in glass network, leading to a tightening of the network as indicated from FTIR spectra, resulting in the decrease of thermal expansion coefficient and the increase of both g T and s T values.

6 Electrical properties of the studied samples
Electrical measurements of the studied glasses were conducted in the frequency range from 100 Hz up to 1 MHz at room temperature (~20 ℃). Generally, the values of conductivity and dielectric constant of the samples are very close to each other. That means that the structure of the samples does not change with the adding of Fe 2 O 3 at the expense of Na 2 O in sodium lead borate glasses (from the electrical point of view) [50]. PbO content is constant through all the batch compositions, so we did not study its effect on the structure. The conductivity decreases with the increase of Fe 2 O 3 at the expense of Na 2 O replacements (at the same frequency). Figure 5 shows the variation of conductivity with frequency for different samples containing various proportions of Fe 2 O 3 replacements. It can be noticed that the change of composition affects the electrical properties due to the increase of Fe 2 O 3 replacements of samples. The increase of Fe 2 O 3 replacements results in the decrease in the conductivity. The decrease of conductivity may be due to the remarkable increase in the cross linking of the glass structures which increases its strength, due to converting three coordinated boron atoms [BO 3 ] to four coordinated boron atoms [BO 4 ] from G 1 to G 4 . Figure 5 shows a slope for all samples which is related to the increase of frequency. Figure 6  That increase is found by the progressive substitution of Fe 2 O 3 at the expense of Na 2 O that results in the replacement of three coordinated boron atoms [BO 3 ] to four coordinated boron atoms [BO 4 ], i.e., capacity of capacitors will increase. Also, the increase in the dielectric constant may be due to the increase in its strength of the investigated glasses. Figure 7 shows the impedance plane with frequency for samples containing various proportions of Fe 2 O 3 at the expense of Na 2 O. Such impedance plane curves may be used as an indicator of the change in the internal structure of samples (continuous and broken paths or links). It is clear that the curves of impedance plane do not change greatly with the increase of proportions of Fe 2 O 3 contents (Fig. 7). As the continuous conducting paths or links decrease (by increasing of Fe 2 O 3 /Na 2 O replacements), the values of the imaginary part slightly decrease and the values of the real part decrease [50].
The equivalent circuit shown in Fig. 8 is assumed to represent the sample general structure. The conducting ions are completely insulated (blocked) from each other. Here, g C and R  represent the capacitance and resistance of the sample at high frequencies, respectively, where no space charges accumulate. i C is the interfacial capacitance (frequency independent) and i R is the interfacial resistance. The resistance decreases as the frequency increases at frequencies lower than the radio frequency range, where space charges are accumulated at interfaces. It is supposed that there is no direct conducting path between the two electrodes.

Conclusions
The FTIR study confirms that with the incorporation of iron oxide, [BO 3 ] groups get converted into [BO 4 ] groups. Change in the structure of glasses is also confirmed from the shifting of the bands and change in intensity with the addition of Fe 2 O 3 . The density of the glass samples increases with the increase of the Fe 2 O 3 content and that causes a corresponding decrease in the molar volume. It may be attributed to the compaction of glass structure and formation of tetrahedral [BO 4 ] groups.
The electrical properties of the studied samples are investigated in the frequency range from 100 Hz up to 1 MHz, and the effect of change in composition (Fe 2 O 3 at the expense of Na 2 O contents) on the measured properties is investigated. The increase of Fe 2 O 3 replacements generally decreases the conductivity and increases the dielectric constant of the studied glasses. The obtained electrical data indicate the internal structure of glass network and the change in the structure of samples from [BO 3 ] to [BO 4 ].
Open Access: This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.