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  -  . 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  -  . 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      . 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         .
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 (DR) 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 (DR), 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   . Figure 1 shows a gradual decrease
Table 1. Compositions, calculated O/P ratio, DR and transition temperature (Tg) of the glasses series 60P2O5-2Cr2O3-xFe2O3-(38 − x) Na2O with (3 ≤ x ≤ 33; mol%).
Figure 1. Dissolution rates (DR) of the glass series 60P2O5-2Cr2O3-(38 − x)Na2O-xFe2O3 versus Fe2O3 (mol%).
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  . 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  .
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)  . The band at 759 - 777 cm−1 is assigned to the symmetrical vibration mode νsym (POP)  , while the band at 883 - 945 cm−1 is attributed to the asymmetric vibration νasym (PO-P)    . The band observed at 1074 - 1099 cm−1 is assigned to stretching νsym (PO3)/νsym (PO2), and the region 1268 - 1278 cm−1 was attributed to the stretching of νasym (PO3)   . 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 νasym (PO3) and attributed to the metaphosphate groups, which became a simple shoulder, while the vibration of the νsym (PO3)/νsym (PO2) band, located at 1099 -
Figure 2. X-ray diffraction spectra for the samples S1 and S2 partially crystallized, respectively, at 500˚C and 650˚C for 48 h.
Figure 3. IR spectra of the series 60P2O5-2Cr2O3-(38 − x)Na2O-xFe2O3; x = 3, 13, 23 and 33 mol%.
Table 2. Glass compositions expressed in terms of quaternary systems.
1070 cm−1 and attributed to the majority pyrophosphate groups, became wider     . 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  . 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 (ΔEQ) and fraction of Fe2+ are listed in Table 3. The values of the quadrupolar splittings suggest Fe2+ and Fe3+ with distorted octahedral coordination   . 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+  . However, this
Figure 4. Localization of the investigated glasses compositions in the ternary diagram of (Cr2O3∙P2O5)-(Na2O∙P2O5)-(Fe2O3∙P2O5). Table 1 gives the corresponding compositions in the quaternary system Na2O-Fe2O3-Cr2O3-P2O5.
Figure 5. Room temperature Mössbauer spectra of the series of 60P2O5-2Cr2O3-xFe2O3-(38 − x)Na2O glasses.
Table 3. Mössbauer characteristics of the series of 60P2O5-2Cr2O3-xFe2O3-(38 − x) Na2O glasses.
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   . The obtained values lie in the same range of about [Fe2+]/[Fe2+] + [Fe3+] = 5% - 10%.
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   . 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   . 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  . 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    . 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   . 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     . 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  . The dissolution rate obtained was 200 times lower than that of the silicate glasses.
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.