ABSTRACT

The structure and chemical durability in the vitreous part of the system 60P₂O₅-2Cr₂O₃-xFe₂O₃-(38 − x)Na₂O phosphate glasses (with 3 ≤ x ≤ 33 mol%) were investigated using various techniques such as IR spectroscopy, X-ray diffraction and Mössbauer spectroscopy. The presence of Cr₂O₃ and the increase of Fe₂O₃ at the expense of Na₂O in the glass network lead to a large number of covalent and rigid Fe-O-P and Cr-O-P bonds. The infrared and XRD spectra indicate a radical change of structure and show that the increase of the Fe₂O₃ content favors the depolymerization of the vitreous network towards pyrophosphate chains. The presence of Cr₂O₃ in the glass seems to favor the covalent Cr-O-P bonds linked to the most probable cyclic metaphosphate chains. However, when the Fe₂O₃ content increases (≥23 mol%), its impact on the glass network is stronger than that of Cr₂O₃. The infrared and XRD spectra indicate a radical change of structure and show that the increase in Fe₂O₃ content favors the depolymerization of the vitreous network to short pyrophosphate chains. The results of Mössbauer spectroscopy indicate the presence of both Fe (III) and Fe (II) ions which occupied more or less deformed octahedral sites. The growth of the glass transition temperature (Tg) with the increase of the iron oxide in the vitreous network, leads to an improvement of the glass rigidity. This explains the decrease of the ionic radius of the iron and the reinforcement of the interconnection of the chains of vitreous networks. The structure of sodium-chromium-iron phosphate glasses can be considered largely as pyrophosphate units linked to ferric and ferrous ions in octahedral or deformed octahedral coordination. The dissolution rate is 200 times lower than that of the silicate glasses.

Keywords:

Chemical Durability, Phosphate Glasses, Chromium and Iron Oxides, IR, XRD, Nuclear and Chemical Wastes

1. Introduction

Phosphate-based glasses are particularly interesting in the scientific and technological fields because they generally have low melting temperatures as well as high coefficient of thermal expansion [1] - [7] . These properties make them good candidates for many applications. It has been found that the introduction of oxides, such as PbO, ZnO, Al₂O₃, Fe₂O₃ and Cr₂O₃, results in the formation of PO-Pb, P-O-Zn, Al-O-P, P-O-Fe and P-O-Cr bonds, leading to improvement of the chemical durability of phosphate glasses. Iron phosphate glasses generally have good chemical resistance [7] - [14] . The synergy of phosphate glasses with certain types of nuclear waste has indicated the possibility of a form of waste with a lower corrosion rate than borosilicate glass. It has been suggested that the chemical durability of sodium-chromium-iron phosphate glasses is attributed to the replacement of P-O-P, Na-O-P bonds by Fe-O-P and Cr-O-P bonds. The presence of Fe-O-P bonds at higher concentrations in the phosphate network makes the glass more resistant to chemical attackers [11] [12] [13] [14] [15] . The purpose of the present paper is to study the effect of the composition on structural modifications, chemical durability, and iono-covalent character of metal-oxygen-phosphorus bonds by varying the Fe₂O₃ ratio along with a series of phosphate glasses with the composition 60P₂O₅-2Cr₂O₃-(38 − x)Na₂O-xFe₂O₃ (with 3 ≤ x ≤ 33; mol%). It follows that the structural change causes a depolymerization to short pyrophosphate chains, and leads to a strengthening of bonds, better than that cyclic metaphosphate chains or isolated short orthophosphate chains, by the formation almost certainly aggregates of crystallites in perfect equilibrium with the vitreous bath. This results in high chemical durability [7] [11] [12] [15] [16] [17] [18] [19] .

2. Experimental Procedure

The synthesis of phosphate glasses with the composition 60P₂O₅-2Cr₂O₃-(38 − x)Na₂O-xFe₂O₃ (with 3 ≤ x ≤ 33; mol%) was carried out by directly melting the mixtures with suitable proportions of (NH₄)H₂PO₄, Na₂CO₃, Cr₂O₃ and Fe₂O₃. The reagents were finely ground and then introduced into a porcelain crucible. They were initially heated at 300˚C for 1 h and then held at 500˚C for 1 h to complete their decomposition. The reaction mixture was then heated at 1060˚C - 1100˚C for 20 min. The homogeneous liquid obtained was poured into an aluminum plate, which had been preheated to 200˚C to avoid thermal shock. Pellets of ≈1 cm in diameter and 2 - 3 mm thick were obtained. The dissolution rate (D_R) of these glasses was evaluated based on the weight loss of the sample. The samples were immersed in Pyrex beakers containing 100 ml of distilled water and heated to 90˚C. The surface of the sample was constantly immersed in distilled water for 30 consecutive days. The IR spectra of the phosphate glasses studied were determined in the zone between 400 and 1600 cm⁻¹ with a resolution of 2 cm⁻¹. The samples were finely ground and mixed with KBr. Infrared spectroscopic analysis of the materials was performed using a Vertex Spectrometer 70 and recorded on a DTGS (triglycine deuterium sulfate) detector. The vitreous state was evident by its brilliance and transparency, and confirmed by XRD. The glasses S₁ and S₄ were annealed at 500˚C and 650˚C, respectively, for 48 hours. Structural determination was performed using XRD which allowed us to follow the structural evolution using a Pro MPD Panalytical X’Pert diffractometer. Differential scanning calorimetry was performed using DSC 121 type SETARAM, at a heating rate of 10˚C/min in a flowing Argon atmosphere (30 cm³/mn) with alumina crucibles. ⁵⁷Fe Mössbauer spectra were recorded at room temperature in transmission geometry using a conventional constant-acceleration spectrometer with a 57Co(Rh) source. The Mössbauer samples consisted of powdered glass, containing 5 mg Fe per cm². A α-Fe foil was used as a calibration of the spectrometer, and the values of the isomer shift (δ) of the measured samples are reported relative to that of α-Fe measured at 300 K. The Mössbauer spectra were fitted by means of the NORMOS program, assuming the superposition of two quadrupolar doublets with Lorentzian line.

3. Chemical Durability

As presented in Table 1 and Figure 1, the dissolution rate (D_R), defined as the weight loss of glass (expressed in g∙cm⁻²∙min⁻¹), showed an impressive decrease which was dependent on the Fe₂O₃ content of samples immersed in 100 ml of distilled water at 90˚C for 30 days [16] [20] . Figure 1 shows a gradual decrease

in the dissolution rate of the glasses from 1.34 × 10⁻⁵ to 2.32 × 10⁻¹⁰ g∙cm⁻²∙min⁻¹ when the Fe₂O₃ content was increased from 3 to 23 mol% to the detriment of Na₂O. Beyond 23 to 33 mol% of Fe₂O₃, the dissolution rate becomes practically constant.

4. X-Ray Diffraction and Differential Scanning Calorimetry Measurements

X-ray diffraction confirmed the vitreous nature of all samples studied [11] . The values of glasses transition temperature (Tg) increase from S₁ to S₄, when the iron oxide increases in glass network involving an improvement glass rigidity (see Table 2). The heat treatment of the S₁ and S₂ glasses at 540˚C and 630˚C, respectively, for 48 hours, gives the XRD patterns shown in Figure 2. In Figure 2(a), major metaphosphate phases occurred in the sample S₁, whereas the spectrum for the sample S₄, shown in Figure 2(b), indicates the total dominance of pyrophosphate groups [18] .

5. Infrared Spectra

The IR spectra of sodium-chromium-iron-phosphate glasses were obtained in a frequency range between 399 and 1600 cm⁻¹ using a Fourier-transform spectrometer. The IR spectra of these glasses are shown in Figure 3. The bands at 474 - 530 cm⁻¹ are assigned to skeletal deformation δske (P-O-P) [21] . The band at 759 - 777 cm⁻¹ is assigned to the symmetrical vibration mode ν_sym (POP) [12] , while the band at 883 - 945 cm⁻¹ is attributed to the asymmetric vibration νasym (PO-P) [11] [19] [22] . The band observed at 1074 - 1099 cm⁻¹ is assigned to stretching ν_sym (PO₃)/ν_sym (PO₂), and the region 1268 - 1278 cm⁻¹ was attributed to the stretching of ν_asym (PO₃) [11] [12] . It should be noted that with increasing values of x, the intensity of characteristic metaphosphate bands tended to decrease when the Fe₂O₃ content was increased to the detriment of Na₂O. This phenomenon appears clearly when the Fe₂O₃ content exceeds 23 mol%, when we observed the disappearance of the vibration band at 1268 - 1288 cm⁻¹ assigned to ν_asym (PO₃) and attributed to the metaphosphate groups, which became a simple shoulder, while the vibration of the ν_sym (PO₃)/ν_sym (PO₂) band, located at 1099 -

1070 cm⁻¹ and attributed to the majority pyrophosphate groups, became wider [9] [11] [23] [24] . So, as we can be seen clearly on the ternary diagram (Figure 4), the localization of S1, S2, S3, and S4 indicates the depolymerised structure from metaphosphate (S1) to predominant pyrophosphates groups (S4).

57Fe Mössbauer spectrometry and iron redox phenomenon: The 300 K Mössbauer spectra of sodium chromium iron phosphate glasses of composition (38 − x)Na₂O-xFe₂O₃-2Cr₂O₃-60P₂O₅ (with 8 ≤ x ≤ 38; mol%) are illustrated in Figure 5. They exhibit a central prevailing quadrupolar doublet with broadened lines and a small line at higher velocities. The first part is well described by means of at least four quadrupolar doublets, which can be unambiguously attributed to high-spin-state Fe³⁺ in octahedral sites. A quadrupolar splitting distribution can also be successfully used, which is consistent with a disordered local structure, as expected in a glassy system [12] . The second part of the Mössbauer spectra can be associated with a minor quadrupolar component, which is well described by two doublets, unambiguously ascribed to the presence of Fe²⁺. The refined values of the mean Mössbauer hyperfine parameters, such as isomer shift (δ), quadrupolar splitting (ΔE_Q) and fraction of Fe²⁺ are listed in Table 3. The values of the quadrupolar splittings suggest Fe²⁺ and Fe³⁺ with distorted octahedral coordination [12] [25] . This description allows us to determine the iron redox ratio, Fe²⁺/(Fe²⁺ + Fe³⁺), from the corresponding spectral area of each component, assuming the same value of the f-Lamb Mössbauer factor. It can be concluded that the increasing Fe₂O₃ content favours a reduction of Fe³⁺ to Fe²⁺ when melting in air. It seems, then, that the significant factor affecting the redox state of the iron in sodium chromium-iron phosphate glasses is the presence of ammonium phosphate, (NH₄)H₂PO₄, in a batch composition, which acts as a source of P₂O₅. As a matter of fact, reducing conditions were created during the melting process, which favours the reduction of Fe³⁺ to Fe²⁺ [12] . However, this

reduction does not exceed 10%. Some authors have suggested that the Fe²⁺/(Fe³⁺ + Fe²⁺) ratio is related more to the melting temperature than to the glass composition [12] [26] . The obtained values lie in the same range of about [Fe²⁺]/[Fe²⁺] + [Fe³⁺] = 5% - 10%.

6. Discussion

In this study, we prepared glass series with different percentages of phosphate oxides (Na₂O, Fe₂O₃, Cr₂O₃ and P₂O₅) as the basic constituent. In this series we substituted the Na₂O oxide with Fe₂O₃ oxide while keeping the percentage of P₂O₅ and Cr₂O₃ oxides constant. Both the presence of Cr₂O₃ and the substitution of Na₂O with Fe₂O₃ in the glass network led to considerable improvement in chemical durability. The presence of Cr₂O₃ oxide in the glass seems to favor covalent Cr-O-P bonds linked to the most probably cyclic metaphosphate chains [19] [27] . However, the infrared spectra and XRD indicate, both, a radical change in the structure and show that increasing Fe₂O₃ content from 3 to 33 mol% lead to the increase of O/P ratio and favors the depolymerization of the vitreous network by creating predominant short pyrophosphate chains [12] [28] . This depolymerization becomes more obvious when the Fe₂O₃ content reaches 23 mol%, and its impact on the glass network becomes stronger than that of Cr₂O₃, which leads to a strengthening of the chemical bonds by the covalent bond formation Fe-O-P at detriment of P-O-P and Na-O-P bonds, easily hydrated, and possibly Cr-O-P. On the other hand, the increase in the glass transition (Tg) when Fe₂O₃ increases in the phosphate network confirms an improvement in the glass rigidity [29] . This phenomenon explains the decrease of the ionic radius of the iron due, both, to the coordination bonds, translated by the hybridization phenomenon, and to the distorted sites occupied by the iron, which become smaller in the phosphate glass compared to the crystalline sites [30] [31] [32] . Therefore, the electron density becomes very important and leads to a reinforcement of the bonds between the irons and non-bridge oxygen atoms and thus to a reinforcement of the interconnection of the chains of vitreous networks [29] [30] . The results of the Mössbauer spectroscopy indicate a partial reduction of the Fe III iron ions in the state of the Fe II iron ions during the melting. The hyperfine parameters show that these occupied purely octahedral sites more or less deformed. The presence of FeII and FeIII enhances the covalent Fe-O-P band. The increase in the number of covalent Fe-OP bonds favors the formation of short pyrophosphate chains and leads to a strengthening of the bonds, better than those of cyclic metaphosphate or isolated orthophoqphatic chains, by the probably formation of aggregates of crystallites in perfect equilibrium with the vitreous bath [16] [23] [25] [27] . It follows that the structure of sodium-chromium-iron phosphate glass can be considered largely as pyrophosphate units linked to ferric and ferrous ions in octahedral or octahedral coordination [12] . The dissolution rate obtained was 200 times lower than that of the silicate glasses.

7. Conclusion

The structure and chemical durability of 60P₂O₅-2Cr₂O₃-xFe₂O₃-(38 − x)Na₂O phosphate glasses (with 3 ≤ x ≤ 33; mol%) were investigated using various techniques such as IR, XRD and Mössbauer spectroscopy. Increased Fe₂O₃ content in the glass composition leads to stronger bondings in the vitreous network. Both, the presence of FeII and FeIII reinforces the covalent Fe-O-P band and makes the vitreous network exceptionally rigid. The IR and DRX spectra indicate the formation of short pyrophosphate chains, which provides important chemical durability. These studied glasses have better chemical durability than silicate glasses and can be used as an alternative form in the vitrification of nuclear waste.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Cite this paper

Beloued, N., Makhlouk, R., Er-Rouissi, Y., Taibi, M., Sajieddine, M. and Aqdim, S. (2019) Relationship between Chemical Durability, Structure and the Ionic-Covalent Character of Me-O-P Bond (Me = Cr, Fe), in the Vitreous Part of the System 60P₂O₅-2Cr₂O₃-(38-x) Na₂O-xFe₂O₃ (with 3 ≤ x ≤ 33 mol%). Advances in Materials Physics and Chemistry, 9, 199-209. https://doi.org/10.4236/ampc.2019.910015

References