Properties and structural features of iron doped BABAL glasses

Properties and structural features of iron doped BABAL glasses

Signo Tadeu dos Reis^I*; Walter M. Pontuschka^II; Jinbo B. Yang^I; Dalva L A. Faria^III

^IGraduate Center for Materials Research, 301 Strautmanis Hall, University of Missouri-Rolla, MO65409, U.S.A.

^IIInstitute of Physics, University of São Paulo, C.P. 66318, 053890-970 São Paulo - SP, Brazil

^IIIInstitute of Chemistry, University of São Paulo, C.P. 26077, 05389-970 São Paulo - SP, Brazil

ABSTRACT

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 Mössbauer 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 Mössbauer spectra indicate the presence of iron (II) and iron (III) in tetrahedral or octahedral coordination. The results obtained from the g_ef = 4.3 EPR line are typical of the occurrence of iron (III) occupying substitutional sites and the line g_ef = 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.

Keywords: aluminoborate glass, structure, and chemical durability

1. Introduction

As long been known¹, the considerable practical interest of borate glasses stems mainly from the complex influence of boron oxide on the physical properties². 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 optical³, medical use⁴, and solid state electrolytes⁵.

The local structure of iron doped aluminoborate glasses has been studied using ⁵⁷Fe 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 units⁷.

Iron is one of the most pervasive impurities in borate and phosphate glasses. Some specific features of the EPR line width and shape of Fe³⁺ ions in glass matrices depend on the glass composition and the concentration of paramagnetic ions⁸. 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 Fe³⁺ ions in a glassy host⁹. The X-band EPR spectra (~ 9 GHz) of most oxide glasses with low Fe³⁺ 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 glasses¹⁰. At low iron concentrations (< 1 mol%), only a sharp g_ef» 4.3 resonance is commonly observed⁹. 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 Fe³⁺ ions concentration above 5 mol%. This result leads to the hypothesis of Fe³⁺ clustering, which effectively contributes to the former broad line¹². 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 sample¹³, while the relative concentrations of the Fe²⁺ and Fe³⁺ ions are easily determined by Mössbauer spectroscopy.

Bogomolova and Henner¹⁴ showed how the X-band the g_ef» 2 line width can be explained by a clustering of Fe³⁺ ions. Nevertheless, the Fe³⁺ 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 coupled¹⁴.

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.

2. Experimental Procedure

Samples of BABAL glasses with compositions (100-x)(0.30BaO·0.50B₂O₃ ·0.20Al₂O)·xFe₂O 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 cm³ 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 diffraction (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 cm³ 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/cm². 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.

3. Results

3.1 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 Fe₂O₃ content: it decreases from 4.7 × 10^-6 g/cm²min^. for glasses containing 0 mol% Fe₂O₃ containing glass to about 1.0 × 10^-7 g/cm²min for glasses containing about 4 mol% of Fe₂O₃. Even though the samples with Fe₂O₃ content between 2 and 4 mol% had the lowest chemical durability (see Table 1 and Fig. 1), there is an increase in DR for samples containing more than 4 mol% Fe₂O₃. As a comparison, the iron phosphate glass containing 40 mol% Fe₂O₃ 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 Fe₂O₃, as shown in the Fig. 2.

The pH of the solution after the corrosion tests for a BABAL glass with 2 mol% Fe₂O₃ 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.

3.2 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 Ba₂Al₂B₈O₁₇¹⁷.

As shown in Fig. 5, the density of the BABAL glasses at room temperature increased linearly from 3.12 to 3.41 g/cm³ with increasing Fe₂O₃ content (see Table 1).

3.3 EPR

Figure 6 shows the X-band EPR spectra of BABAL glasses containing increasing amounts of 1 to 10 mol% of Fe₂O₃. 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) sites⁹ and b) at g = 2.0 from clusters of two or more Fe³⁺ ions which interact among themselves.

No significant changes were observed in the line width and shape of the g = 4.3 EPR spectra, with the Fe₂O₃ concentrations ranging from 1 to 8 mol%, indicating that the increase of Fe has occurred mainly by substitutions of Fe³⁺ ions at the sites of the BO₄ tetrahedra of the glass forming network.

As the Fe₂O₃ content reaches about 10 mol% there is observed a significant increase of the resonance at g = 2.0 together 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 Fe₂O₃ (8- 0 mol%). Later on D.W. Moon et al.¹⁹ have reported that in the glass system xFe₂O₃·(100-x)[BaO· 4B₂O₃] at concentrations greater than 3 mol% Fe₂O₃, iron ions associated in pairs (diads) and groups of three (triads) became predominant, in addition to the isolated Fe³⁺ 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. Yahiaoui¹¹, and R. Berger et al.^9,21 carried out studies with computer simulations of the Fe³⁺ resonance in borate glass. The experimental EPR spectra were well described by assuming that the environment of Fe³⁺ ions are highly distorted and in addition to the orthorhombic distortion related to the g = 4.3 feature, an important part of Fe³⁺ arises from axial or slightly rhombic distortions.

3.4 Raman

The Raman spectra for the lower concentration of Fe₂O₃ 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 Fe₂O₃ and other bands centered at 580, 780 and 920 cm^-1 appear. For higher concentration of Fe₂O₃ the bands between 580, 780 and 920 cm^-1 are predominating in the spectra.

3.5 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 dand quadrupole splitting DE_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.

4. Discussion

The chemical durability for the (100x) (0.30BaO·0.50B₂O₃·0.20Al ₂O₃)·xFe₂O₃ glasses with iron content of 2 mol% is 700 times less than that iron phosphate glass containing 40 mol% of Fe₂O₃. In general the chemical durability for phosphate glasses improves with the increase in iron content¹⁶, 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 network²².

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 Fe³⁺ ions in tetrahedral or orthorhombic coordination^9,23 and small clusters of exchange coupled Fe³⁺ 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 B₂O₃ glasses^6,7. The addition of Fe₂O₃ 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" BO₄ groups and "loose" diborate groups²⁴, and the band in the 920 cm^-1 can be attributed to the formation of pentaborate groups²⁵. These results are consistent with the investigation of Fe(II) and Fe(III) properties in calcium aluminoborate glasses⁷.

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.

5. 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 OH^- and by the reaction between H⁺ and metallic ions (Fe²⁺ and Fe³⁺). 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 Fe³⁺ ions in tetrahedral or orthorhombic coordination and small clusters of exchange-coupled Fe³⁺ ions in substitutional sites of the glass network.

Acknowledgements

S.T. Reis thanks Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP) for a post-doctoral fellowship. The authors also thank Mr. Hannes Fischer and Mr. José Mário Prison for the analyses of the XRD and EPR.

Received: February 26, 2002

Revised: May 15, 2003

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Publication Dates

• Publication in this collection
  28 Nov 2003
• Date of issue
  June 2003

History

• Accepted
  15 May 2003
• Received
  26 Feb 2002