As can be seen clearly from the ternary diagram (Figure 1) and taking into account the oxygen/phosphate ratios shown in Table 1. The location of S₀, S₅, S₁₀, S₁₅ and S₂₀ indicated that the structure of meta-phosphate chains (S₀ and S₅) evolved to predominant short pyrophosphate groups (S₁₅ to S₂₀).

The chemical durability of the studied glasses series was approached using the measurement of the dissolution rate, which was defined as the weight loss of the glass expressed in g·cm⁻²·min⁻¹. The values of the dissolution rate (D_R), as a function of the Fe₂O₃ molar percentage, are grouped in Table 1 and presented in

Figure 2. The typical results obtained for the samples, after their immersion in 100 ml of distilled water and heated at 90˚C for 24 consecutive days, showed a remarkable chemical durability as the Fe₂O₃ content increased in the vitreous network to the detriment of PbO content. Figure 3 shows the pH of the aqueous solution after the corrosion tests. Due to the high chemical durability of glasses containing more than 10 mole % Fe₂O₃ the pH decreases slightly from its initial values. However, there is a relatively significant decrease in the pH of solutions for immerged glasses with a less Fe₂O₃ content than 10 mol%. The decrease in pH between 5.2 and 7.2 is attributed to the formation of phosphoric acid whose pH generally, varies between 5 and 11, due to the degradation of glass by water, which plays an amphoteric role [14]. The decrease in pH is consistent with the large values of D_R. The visual appearance of glassy compounds emerged in distilled water at 90˚C for 24 days shows that glasses which have low Fe₂O₃ content such as S0 and S5 are relatively corroded, while S₁₅ and S₂₀ glasses are practically intact and keep their original appearance [15].

Figure 4 shows the DSC curves of iron-lead phosphate glasses. The results of the glass transition temperatures (T_g) as a function of the Fe₂O₃ content are grouped in Table 2 and presented in Figure 5. The T_g evolution indicated that the substitution of lead oxide by iron oxide led to an almost linear increase in T_g. The analysis of these results indicated a significant improvement in T_g when the Fe₂O₃ content increased, this increase characterizes a strengthening of the vitreous networks [8] [14] [16] [17]. This behavior has been confirmed by previous work [14]. For the S₀ sample (45PbO-55P₂O₅) the two endothermic peaks around 70˚C and 193˚C are due to the dehydration of the sample while the endothermic peaks which appear in the interval 376 - 470˚C are tuned to the glass transition temperatures of different samples.

The density measurements made it possible to follow the evolution of the molar

volume according to the composition of the glass along the xFe₂O₃-(45-x) PbO-55P₂O₅ system, which was measured at room temperature. As can be deduced from Table 3 and from the curve in Figure 6, we notice an increase in the density of the glass from 3.86 to 4.35 (g/cm³) when the Fe₂O₃ content varies, respectively, from 0 to 20 mol%. This increase leads to reinforce the vitreous network bonds by replacing the hydrated P-O-P bonds by covalent and rigid Fe-O-P bonds. The glass density is given by the follow equation:

ρ = m glass [ m glass + ( m ortho − m ortho + glass ) ] ρ ortho

with:

ρ = Density

m_glass = Weight of glass measured in air

m_ortho = Weight of diethyl orthophthalate only

m_ortho+glass = Weight of glass immersed in diethyl orthophthalate

ρ_ortho = 1.11422 g/cm³

The molar volume (V_OM) and the oxygen anionic radius r_cal (O²⁻) in the glass were determined from Equations (1) and (2), respectively:

V O M = M / ρ N A ⋅ N 0 (1)

r c a l = V O M 3 / 2 (2)

with M = molar mass.

ρ = density.

N_A = Avogadro’s number.

N₀ = number of oxygen contained in the molar formula.

Data analysis in Table 4 showed that, both, the molar volume and the anionic radius value of relative oxygen r_cal (O²⁻), calculated by Equation (2), decreased when the Fe₂O₃ content increased at the expense of PbO. [13] [16]. It should also be noted that the anionic radius of oxygen r_cal (O²⁻) decreased when the composition of the glass tended to pass from ultra and meta-phosphate domains (S₀, S₅) (O/P = 2.9, 3) to pyrophosphate domain (S₂₀) (O/P = 3.27), such a decrease in r_cal (O²⁻) highlights a strengthening of the Fe-O-P bond in the vitreous network which is interpreted by a shortening of the interatomic distances due to the purely covalent character coordination bonds [14] [17] [18].

The infrared spectra of the glass series xFe₂O₃-(45-x) PbO-55P₂O₅ are shown in Figure 7. All phosphate vibration bands of the treated samples were situated in the frequency domain between 1600 - 399 cm⁻¹ [17] [18]. The band between 466 - 500 cm⁻¹ was attributed to the δ (P-O-P) skeleton vibration mode [17] [18]. We also noted that the 1210 - 1228 cm⁻¹ band attributed to the ν_as (PO₂) vibration mode of the meta-phosphate groups (Q²) becomes a simple shoulder when the molar content of Fe₂O₃ reached 20 mol%, while the band located between 1000 - 1060 cm⁻¹ moved towards high frequencies giving a wide band attributed to the vibration mode ν_as (PO₃) of pyrophosphate groups (Q¹) [19] [20]. The band observed at 1000 cm⁻¹, when x = 0, can be attributed to the isolated ortho-phosphate group PO 4 3 − (Q⁰) [19]. The band located at 755 - 766 cm⁻¹, attributed to the ν_s (P-O-P) vibration mode, moved towards high frequencies when the Fe₂O₃ oxide content increased and indicated the formation of non-bridge oxygens of the di-phosphate groups (Q¹) to the detriment of the meta-phosphate groups (Q²).

To study the amorphous state of the prepared glasses, X-ray diffraction analyses were carried out on all the prepared samples. The results obtained in Figure 8,

confirmed the amorphous character which reflects a disordered structure. As expected, the analysis by X-ray diffraction of annealed glasses S₀, S₁₀ and S₂₀ respectively, at 580˚C and 620˚C for 72 hours, are presented in Figure 9. The analysis

of these spectra indicated the appearance of peaks mixture linked to meta-phosphate, ortho-phosphate and pyrophosphate phases. On the other hand, it indicated a radical change in the structure, when the iron oxide content increased in the vitreous network, showing that the studied glasses structure evolved from the meta-phosphate (presumably cyclic) and the isolated ortho-phosphate phases to majorities pyrophosphate phases while we moved from S₀, S₁₀ to S₂₀. The x-ray diffraction spectra obtained from the S₁₀ sample tended to crystallize as a mixture of meta-phosphate, ortho-phosphates and pyrophosphates with majorities of meta-phosphate and/or rings meta-phosphate phases. However, in the S₂₀ sample, we noticed that the meta-phosphate phases disappeared and the main phases called pyrophosphates such as PbFe₂(P₂O₇)₂ and Pb₂P₂O₇ were mostly intense in the sample [21] [22].

The scanning electron microscopy (SEM) micrographs presented in Figure 10 illustrated the morphology of the studied glasses. The micrographs of Figures 10(a)-(c) showed the existence of two phases, a vitreous phase and a crystalline phase. It also indicated the formation of agglomerates in the crystalline phase of different sizes, ranging from a few microns to several tens of microns, as shown by the different images attributed respectively to samples S₀, S₁₀ and S₂₀ [7] [17] [19] [21]. A deep of SEM micrograph analysis of the glasses considered in this work indicated the glass form of S₀ shown in Figure 10(a), exhibited the presence of an overload crystalline phases with different form and size [12] [23] [24]

[25]. When the Fe₂O₃ content increased in the glass, the number of crystallites decreased as seen in the S₁₀ (Figure 10(b)) and S₂₀ (Figure 10(c)) samples, respectively. Hence, SEM analysis confirmed a relatively large homogenous vitreous phase with the coexistence of crystalline particles in the S₂₀ sample which has the maximum Fe₂O₃ content. Some different crystalline phases were identified by XRD and it seems that a decrease of crystallization tendency was enhanced and PbFe₂(P₂O₇)₂, Pb₂P₂O₇, phases were crystallized in the last sample (S20) with some traces of Pb₃Fe(PO₄)₃ phases. This explains that the structure changes towards more short pyrophosphates at the detriment of metaphosphates and shorter isolated orthophosphate chains as the Fe₂O₃ content increases in the glass network [17] [19] [21] [26] [27].

Like all transition elements, iron was reputed to have at least two oxidation degrees under normal atmospheric conditions. Therefore, as expected the starting iron(III) oxide has been reduced during the elaboration of the title phosphate glasses. Indeed the batch melting is realized in a rather reduced atmosphere which is created by the decomposition of the starting compound (NH₄)₂HPO₄ which gives rise at high temperatures, to a gas mixture of the type (NH₃₍g) +H₂ + H₂O(g)) [16] [17] [22] [28]. Analysis of the results (Figure 11) shows that all the glasses studied underwent a reduction at the time of melting, from the Fe(III) state to the Fe(II) state. The study of this reduction shows that it decreases when the Fe₂O₃ content exceeds 10 mol%. This reduction does not exceed, in general under normal atmospheric conditions, 10%. However, in the case of iron-lead-phosphate glasses, the reduced Fe(III) rate reaches higher values for low iron oxide content, this can be explained by the oxidation in atmospheric air of a part of PbO into PbO₂ leading to a greater reduction of Fe(III) to Fe(II) [7] [14] [17].