Properties and Structural Features of Iron Doped BABAL Glasses

The chemical durability, density and structure of the BABAL glasses with batch compositions (100-x)(0.30BaO·0.50B2O3·0.20Al2O3)·xFe 2O3 (1 < x < 10 mol%), were investigated using Mossbauer spectroscopy, electron paramagnetic resonance (EPR), X-ray diffraction, Raman and differential thermal analysis (DTA). The chemical durability for the glass of composition 27BaO·45B2O3·18Al 2O3·10Fe2O3 (mol%) at 90 °C in distilled water was 700 times lower than that of iron phosphate glass 40Fe2O3·60P2O 5 (mol%). The Mossbauer spectra indicate the presence of iron (II) and iron (III) in tetrahedral or octahedral coordination. The results obtained from the gef = 4.3 EPR line are typical of the occurrence of iron (III) occupying substitutional sites and the line gef = 2.0 is related to the association of two or more Fe ions found in the interstices (or holes occupied by the glass modifier cations) of the glass network. The paths of X-ray diffraction are typical for glasses based in borate glasses. The Raman spectra showed that the boroxol ring disappears with the increase of iron content, concomitant with the appearance of BO4 and tetraborate structural units. At these conditions, an increase of dissolution rate and clustering of iron ions is observed.


Introduction
As long been known 1 , the considerable practical interest of borate glasses stems mainly from the complex influence of boron oxide on the physical properties 2 . Some appropriate values of constants such as low melting point, high thermal expansion coefficients and several optical data make these glasses potential candidates for many technological applications such as optical 3 , medical use 4 , and solid state electrolytes 5 .
The local structure of iron doped aluminoborate glasses has been studied using 57 Fe Mössbauer, Raman spectroscopy and electron paramagnetic resonance (EPR) 6,7 . The boroxol ring is the main structural group present in borate glasses and the addition of modifier oxide like CaO gradually converts three-coordinated boron into four-coordinated boron units, resulting in the formation of diborate, tetraborate and pentaborate glass network superstructural units 7 .
Iron is one of the most pervasive impurities in borate and phosphate glasses. Some specific features of the EPR line width and shape of Fe 3+ ions in glass matrices depend on the glass composition and the concentration of paramagnetic ions 8 . The EPR spectra of most glasses containing iron exhibit the two well known resonances at the effective g values g ef ≈ 2 and g ef ≈ 4.3 that have been considered as a signature of the presence of Fe 3+ ions in a glassy host 9 . The X-band EPR spectra (~ 9 GHz) of most oxide glasses with low Fe 3+ concentrations show a g ef = 4.3 line, and in most cases the g ef = 2 line is also present. A low field feature (g ef ~ 9.7) is also observed in X-band EPR spectra of oxide glasses 10 . At low iron concentrations (< 1 mol%), only a sharp g ef ≈ 4.3 resonance is commonly observed 9 . As the concentration is increased, a broad line begins to grow at about g ef ≈ 2 11 . In many oxide glasses the intensity of the line with g = 2 increases much faster than that of g = 4.3 with increasing Fe 3+ ions concentration above 5 mol%. This result leads to the hypothesis of Fe 3+ clustering, which effectively contributes to the former broad line 12 . As the linewidth of the g ef ~ 4.3 resonance remains roughly constant, in practice the peak-to-peak amplitude of its derivative is proportional to the Fe concentration of the sample 13 , while the relative concentrations of the Fe 2+ and Fe 3+ ions are easily determined by Mössbauer spectroscopy.
Bogomolova and Henner 14 showed how the X-band the g ef ≈ 2 line width can be explained by a clustering of Fe 3+ ions. Nevertheless, the Fe 3+ concentration determines not only the number of EPR lines and their positions, but the line shapes as well. The theory predicts an increase of line width with spin concentration due to the dipolar broadening. The exchange interaction appears at a sufficiently high concentration of paramagnetic ions (molar contents > 5 mol%) 15 , when neighboring spins become highly coupled 14 .
The aim of the present work is to investigate the conditions of the chemical durability of BABAL glasses. Evidence of the presence of diads and microclusters of iron ions and structural studies of the glass matrix were provided by EPR, X-ray diffraction and Mössbauer spectroscopy. The chemical durability of BABAL glasses was determined and compared with those of the iron phosphate glasses.

Experimental Procedure
Samples of BABAL glasses with compositions (100-x)(0.30BaO·0.50B 2 O 3 ·0.20Al 2 O)·xFe 2 O 3 where x = 0, 1, 2, 4, 8 and 10 mol% (see Table 1) were produced by melting the batches in dense alumina crucibles at 1200 °C for 1 h. The melt was quenched in air by pouring into 1 × 1 × 5 cm 3 steel mold. The samples were transferred to an annealing furnace and held at 350 °C for 3 h. The densities of the samples were measured by the Archimedes method using distilled water at room temperature. A portion of the glass samples was crystallized on heating at 750 °C for 8 h in air. Room temperature X-ray powder dif-fraction (XRD) patterns for both glasses and crystalline counterparts were collected on a X-ray diffractometer (Scintag XDS2000). The chemical durability of the bulk glasses with approximate size of 1 × 1 × 1 cm 3 was evaluated by weight loss measurements from glasses exposed to deionized water at 90 °C for 2-8 days. Glasses were polished to 600 grit, finished with SiC paper, cleaned with acetone and suspended in glass flasks containing 100 ml of deionized water at 90 °C. Duplicated measurements were conducted for each glass and the average dissolution rate (DR), normalized to the glass surface area, and the corrosion time was calculated from the weight loss.
The Mössbauer spectra were obtained at room temperature on a spectrometer provided with a 50 mCi rhodium matrix cobalt-57 source. The amount of iron found at the sample holder was 4 mg/cm 2 . The velocity of the source was calibrated using a pure iron foil that was also used as a reference for the isomer shift value.
The Raman spectra of the glass samples were taken at room temperature using a Renishaw Raman system 3000, coupled to an Olympus metallurgical microscope. The spectra were excited at 6328 nm (He-Ne Spectra Physics mod. 127) and recorded in the 200-1800 cm -1 range. EPR spectra of powdered samples were taken with a Bruker EMX 10/12 homodyne, X-band spectrometer.

Chemical durability
The dissolution rates (DRs) of the BABAL glasses measured from the weight loss experiments conducted in distilled water at 90 °C are given in Table 1 and Fig. 1. Clearly DR varies with Fe 2 O 3 content: it decreases from 4.7 × 10 -6 g/cm 2 min . for glasses containing 0 mol% Fe 2 O 3 containing glass to about 1.0 × 10 -7 g/cm 2 min for glasses containing about 4 mol% of Fe 2 O 3 . Even though the samples with Fe 2 O 3 content between 2 and 4 mol% had the low- est chemical durability (see Table 1 and Fig. 1), there is an increase in DR for samples containing more than 4 mol% Fe 2 O 3 . As a comparison, the iron phosphate glass containing 40 mol% Fe 2 O 3 16 had a dissolution rate 100 times less than the dissolution rate for window glass and 700 times less than the dissolution rate for the BABAL glass containing 10 mol% of Fe 2 O 3 , as shown in the Fig. 2.
The pH of the solution after the corrosion tests for a BABAL glass with 2 mol% Fe 2 O 3 is shown in Fig. 3. The reduction in the pH values for the iron phosphate glass during the first four days is consistent with the observed relatively higher DR values as a result of corrosion in these glasses to be correlated with the amount of phosphoric acid formed from the release of phosphorus from this glass. However for the BABAL glass the higher pH values indicates the formation of considerably higher amount of basic compounds from the release of Ba and Al from interstitial (glass modifier) positions.

X-ray diffraction and density measurements
All of the compositions listed in Table 1 formed glasses, as no crystalline phases were detected by XRD. The XRD pattern of crystalline counterpart of a sample with x = 0 mol% is given in Fig. 4 which indicates that the major crystalline phase in the crystallized sample is the Ba 2 Al 2 B 8 O 17 17 .
As shown in Fig. 5, the density of the BABAL glasses at room temperature increased linearly from 3.12 to 3.41 g/cm 3 with increasing Fe 2 O 3 content (see Table 1). Figure 6 shows the X-band EPR spectra of BABAL glasses containing increasing amounts of 1 to 10 mol% of    Fe 2 O 3 . The intensities were progressively decreased in order to show the evolution of the relative line shapes and intensities at g = 4.3 from a) isolated ions in local tetrahedral (and eventual octahedral) sites 9 and b) at g = 2.0 from clusters of two or more Fe 3+ ions which interact among themselves.

EPR
No significant changes were observed in the line width and shape of the g = 4.3 EPR spectra, with the Fe 2 O 3 concentrations ranging from 1 to 8 mol%, indicating that the increase of Fe has occurred mainly by substitutions of Fe 3+ ions at the sites of the BO 4 tetrahedra of the glass forming network.
As the Fe 2 O 3 content reaches about 10 mol% there is observed a significant increase of the resonance at g = 2.0 to-gether with a strong line broadening which is also observed in the resonance at g = 4.3 before it is going to be completely obscured by the former which becomes dominant.
A similar evolution has been observed by H.O. Hooper et al. 18 in the EPR spectra of iron containing sodium borate glasses, with the disappearance of the g = 4.3 resonance at about the same relative amount of Fe 2 O 3 (8-0 mol%). Later on D.W. Moon et al. 19 have reported that in the glass system xFe 2 O 3 ·(100-x)[BaO·4B 2 O 3 ] at concentrations greater than 3 mol% Fe 2 O 3 , iron ions associated in pairs (diads) and groups of three (triads) became predominant, in addition to the isolated Fe 3+ ions. They assigned the shoulder at g = 6.0 to weak crystal field terms and the g = 2.0 resonance to clusters of more than one iron atom with antiferromagnetic interaction.
More recently, L. Cugunov et al. 20 , E.M. Yahiaoui 11 , and R. Berger et al. 9,21 carried out studies with computer simulations of the Fe 3+ resonance in borate glass. The experimental EPR spectra were well described by assuming that the environment of Fe 3+ ions are highly distorted and in addition to the orthorhombic distortion related to the g = 4.3 feature, an important part of Fe 3+ arises from axial or slightly rhombic distortions.

Raman
The Raman spectra for the lower concentration of Fe 2 O 3 are dominated by a narrow and intense band at 806 cm -1 (Fig. 7). The 806 cm -1 and 540 cm -1 line intensities decrease with the increasing concentration of Fe 2 O 3 and other bands centered at 580, 780 and 920 cm -1 appear. For higher concentration of Fe 2 O 3 the bands between 580, 780 and 920 cm -1 are predominating in the spectra.

Mössbauer
The room temperature Mössbauer spectra of the BABAL glasses with iron content > 2 mol% are shown in Fig. 8. The iron valence and hyperfine parameters, isomer shift δ and quadrupole splitting ∆E Q , calculated from the Mössbauer spectra are given in Table 2. It is found that some of the Fe(III) ions in the starting batch are reduced to Fe(II) ions    as all of the glasses contain varying amounts of Fe(II) as determined from the Mössbauer spectra. The isomer shift values for Fe(III) and Fe(II) ions range between 0.39 mm/s and 1.08 mm/s while the quadrupole splitting values range between 1.10 mm/s and 2.09, respectively. These values of isomer shifts correspond to tetrahedral coordination for both Fe(II) and Fe(III) in these glasses.

Discussion
The chemical durability for the (100-x) (0.30BaO·0.50B 2 O 3 ·0.20Al 2 O 3 )·xFe 2 O 3 glasses with iron content of 2 mol% is 700 times less than that iron phosphate glass containing 40 mol% of Fe 2 O 3 . In general the chemical durability for phosphate glasses improves with the increase in iron content 16 , however the decrease in the chemical durability of BABAL glasses for iron content > 4 mol% could be attributed to the formation of microclusters in substitutional sites in the glass network 22 . Figure 6 shows that the EPR lines g ef ≈ 2.0 and g ef ≈ 4.3 on the spectra these glasses are due to the diluted Fe 3+ ions in tetrahedral or orthorhombic coordination 9,23 and small clusters of exchange coupled Fe 3+ ions in substitutional sites of the glass network. Figure 6 also shows a low field feature, as a shoulder, in the g ≈10 region. Moreover, this shoulder seems to disappear in the samples with the higher iron concentration.
The Raman band around 806 cm -1 is attributed to boroxol rings, which is the main structural unit of the B 2 O 3 glasses 6,7 . The addition of Fe 2 O 3 converts tree-coordenated boron into four-coordinated boron units (see Fig. 7), resulting in the formation of tetraborate groups (780 cm -1 ). The band in 540 cm -1 is attributed to "loose" BO 4 groups and "loose" diborate groups 24 , and the band in the 920 cm -1 can be attributed to the formation of pentaborate groups 25 . These results are consistent with the investigation of Fe(II) and Fe(III) properties in calcium aluminoborate glasses 7 .
Mössbauer results indicate that even though there is no Fe(II) in the starting batch, the resulting glasses contain varying amounts of Fe(II) (ranging from 6 to 20%) which shows that some of the Fe(III) ions are reduced to Fe(II) ions during melting. The observed isomer shift values indicate that both Fe(II) and Fe(III) ions have oxygen neighbors in the tetrahedral coordination.

Conclusions
The BABAL glass corrosion rate in neutral environment is caused, respectively, by the increase in the pH of the solution (water) due to the attack of OHand by the reaction between H + and metallic ions (Fe 2+ and Fe 3+ ). EPR lines g ef ≈ 2.0 and g ef ≈ 4.3 on the spectra of these glasses can be due to the diluted state of Fe 3+ ions in tetrahedral or orthorhombic coordination and small clusters of exchangecoupled Fe 3+ ions in substitutional sites of the glass network.