The phosphate-based oxide glasses P2O5, compared to their silicate homologous, have important properties due to their low preparation temperatures. These properties, such as a low melting point, high coefficient of thermal expansion and optical properties, make these glasses potential candidates for many technological applications such as: medical field (biomaterials), solid electrolytes, vitrification of nuclear waste, etc.  - . The lead iron phosphate glasses used for the disposal of nuclear waste were produced in 1984 . The combination of chromium-doped phosphate glasses with different types of nuclear waste has shown that it is possible to have a waste form with a corrosion rate comparable to that of borosilicate glasses   . The previous work performed by our group demonstrated that the substitution of Na2O with lead oxide by more than 28 mol%, with the presence of 2 mol% of Cr2O3 in the vitreous lattice appears to be an unfavourable factor for chemical durability. The phenomenon has been explained by the approach of the boundary zone between the crystal and the glass by the continuous formation of groups of isolated phosphate    . Hence, the crystallites exceed a certain limit, and the equilibrium between the glass bath and these crystallites are not longer maintained; we notice, once, a few decrease in the chemical durability. The purpose of this work is to study the evolution of the dissolution rate as a function of the chromium oxide content in distilled water at 90˚C for the studied glasses of the series xCr2O3-(45-x)PbO-55P2O5 with (1 ≤ x ≤ 4; mol%). The study of the dissolution rate, carried out on these glasses, reveals an important chemical durability for the low contents of chromium oxides (2 mol%).
2. Experimental Section
The synthesis of chromium lead phosphate glasses of composition xCr2O3-(45-x)PbO-55P2O5 avec (1 ≤ x ≤ 4; mol%) was carried out by the direct fusion of mixtures of (NH4)2HPO4, Pb(NO3)2 and Cr2O3 in proper proportions. The reagents are finely crushed and then introduced into a porcelain crucible. They are heated to 300˚C for 1 h in the first instance and then to 500˚C for 1 h to complete their decomposition. The reaction mixture is then carried to 1050˚C ± 10˚C for 15 min. The liquid obtained is homogeneous. It is then poured on to an aluminium plate previously heated to 200˚C to avoid thermal shocks. The vitreous state was first evidenced from the shiny and transparency aspect, which was then confirmed from the X-ray diffraction (XRD) patterns. Samples S1, S2 and S4 were respectively annealed at 560˚C and 580˚C, for 72 hours. The chemical durability of these glasses was evaluated by the weight loss of the sample. The samples were polished by glass paper of silica carbon (CSI), cleaned with acetone and immersed in beakers of Pyrex containing 100 ml of distilled water and brought to 90˚C. The surface of the sample must be constantly immersed in the distilled water for 24 days. The density of the glass has been measured at ambient temperature using the Archimedes method. The glass is immersed in a solution of diethyl orthophtalate density, depending on the temperature, is known. The precision is 0.05 g/cm3. The density of the glass is given by the following equation:
ρ = Density
mair = Weight of glass measured in air
mortho = Weight of diethyl orthophthalate only
m ortho+glass = Weight of glass immersed in diethyl orthophthalate
ρortho = 1.11422 g/cm3
The infrared spectra of the phosphate glasses studied have been determined in the region between 1600 and 400 cm−1 with a resolution of 2 cm−1. The samples were finely ground and mixed with KBr (potassium bromide), which is transparent in the infrared, and whose role was to serve as a matrix.
Chemical composition of the departure mixture and some characteristics of the quaternary glasses, are summarized in Table 1.
3. Results and Discussion
3.1. Analysis of Chemical Durability of Series xCr2O3-(45-x)PbO-55P2O5
The chemical durability (DR) of the glass is strongly dependent on its composition, in the case of the glass of the xCr2O3-(45-x)PbO-55P2O5 composition series, the approximate durability analysis is carried out by measuring the dissolution rate (DR), which is defined as the weight loss of the glasses in g・cm−2・min−1. The DR values shown in Table 1 show a very low dissolution rate for low levels of chromium oxide (2 mol% of Cr2O3) introduced into the phosphate network to the detriment of PbO. The substitution of PbO by Cr2O3 for more than 2 mol% becomes an unfavourable factor for DR. Figure 1 shows the variation in the
Table 1. Compositions, density and chemical durability of xCr2O3-(45-x)PbO-55P2O5 with (1 ≤ x ≤ 4; mol%).
Figure 1. Chemical durability versus Cr2O3 for the series of composition xCr2O3-(45-x)PbO-55P2O5.
dissolution rate of the glasses immersed in distilled water at 900˚C for 24 days   . A progressive improvement of DR from 1.07 × 10−6 to 5.80 × 10−8 (g・cm2・min) was noted when the Cr2O3 content varied from 1 to 2 mol%, respectively. However, we noted an increase in the dissolution rate (DR) from 5.80 × 10−8 to 2 × 10−7 (g・cm−2・min−1) when the Cr2O3 content varies from 2 to 4 mol%, respectively.
3.2. Density and Molar Volumes
Density measurements allowed us to follow the evolution of the molar volume depending on the composition of the system xCr2O3-(45-x)PbO-55P2O5. The density measurements were completed at room temperature. As can be observed from the Figure 2, the variation in density versus Cr2O3 content (mol%) indicates a maximum value of x = 2. The molar volume of oxygen (VOM) and the radius of anions of oxygen rcal(O2−) in the glass have been determined from Equations (1) and (2), respectively
With M = molar mass, ρ = density, NA = Avogadro number; = number of oxygen atoms in the molecular formula. The value of the molar volume and the oxygen radius were calculated from the approximate hypothesis of close packing of oxygen anions O2−, having r(O2−) recapitulated for each composition in Table 2    . A detailed analysis of the data in Table 2 shows that the molar volume passes through a minimum when the Cr2O3 content reaches 2 mol%. However, the covalent radius value of the oxygen atom (O2−), calculated by the molar volume using the Equation (2) for each composition, seems to decrease slightly.
Figure 2. Variation of density versus Cr2O3 for the series of composition xCr2O3-(45-x)PbO-55P2O5Cr2O3 (mol%).
Table 2. Density and molar volume of system xCr2O3-(45-x)PbO-55P2O5.
3.3. Structural Approach by Infrared Spectroscopy
The infrared spectra for the xCr2O3-(45-x)PbO-55P2O5 glasses series (with 1 ≤ x ≤ 4) are shown in Figure 3. As can be seen from this figure, all the phosphate vibration bands of the treated sample are presented in the frequency range between 1600 and 399 cm−1  . The band of approximately 1244 - 1228 cm−1 is attributed to the asymmetric vibration modes uas (PO2) or the two non-bridging oxygen atoms linked to a phosphorus atom in the phosphate tetrahedron Q2 . The vibrations of the bands around 1070 - 1047 cm−1 are characteristic of the stretching vibrations υasy (PO3) and the terminal groups υs (PO2)   . The band about 912 - 894 cm−1 is attributed to the uasy P-O-P stretching vibrations      , while the band at 779 - 771
Figure 3. IR spectra of phosphate glasses of composition xCr2O3-(45-x)PbO-55P2O5, with (1 ≤ x ≤ 4; mol%).
cm−1 is assigned to the stretching vibration υsy P-O-P band of the transition oxygen atoms of phosphorus to a phosphate tetrahedron Q1    . Bands that appear between 500 and 466 cm−1 are attributed to the P-O-P vibration modes of the skeleton    . All characteristics of the phosphate vibrations show that the phosphate glasses of compositions xCr2O3-(45-x)PbO-55P2O5 (with 1 ≤ x ≤ 4) may have chains or rings of the metaphosphate groups, with some traces of pyrophosphate groups. In fact, when the Cr2O3 content (mol%) is equal to x = 2, the vibration bonds υas (PO2) assigned to metaphosphates groups tend to be the dominant characteristic of the spectrum. As shown in Figure 3, the band becomes more intense.
3.4. X-Ray Diffraction
The XRD pattern shown in the Figure 4 indicates that the local structure of chromium-lead phosphate glasses, evolved from ultraphosptate (O/P = 2.93) to nearby chains (cyclic metaphosphate O/P = 3, isolated orthophosphates O/P = 4) structures Cr(PO3)3, Cr2(PO3)6, Pb(PO3)2, Pb2(PO3)4, Pb3(PO4)2, Pb9(PO4)6, Pb3Cr(PO4)3 and some trace of Pb2P2O7. When the sample S1 was thermally treated at 560˚C, the amorphous phase partially disappeared and major Cr (PO3)3 [JCDDS FileN˚: 01-077-0672], Pb(PO3)2 [JCDDS. File N˚: 00-043-0335], Pb2(PO3)4 [JCDDS File N˚: 01-086-21] Phases occurred in the sample, with minor Pb3(PO4)2 [JCDDS File N˚01-070-1790] and Pb9(PO4)6 [JCDDS file N˚: 00033-0768] Phases. When the Cr2O3 content increased in the glass (S2), the heat treatment caused an increase in crystallization temperature at 580˚C, which
Figure 4. X-ray spectrum of the glasses of compositions S1, S2 and S4 devitrified respectively at 560˚C, 580˚C and 56˚C for 72 hours.
resulted in the disappearance of isolated orthophosphate phase and the formation of CrPO3)3, Pb2(PO3)4 and Pb(PO3)2. However, the S4 sample thermally treated at 560˚C, indicate the formation of metaphosphate and/or rings of metaphosphate phase Cr(PO3)3, Pb2(PO3)4 with the appearance of isolated orthophosphate phase Pb9(PO4)6 and Pb3Cr(PO4)3 [JCDDS File N˚: 00-047-0830] at the expense of metaphosphate phases   .
3.5. SEM Micrograph
SEM images in Figure 5 illustrate the morphology of the glasses considered in
Figure 5. SEM micrograph showing the visual structure of the samples glasses S2 (a) and S4 (b).
this work. The glass form of S2, shown in Figure 5(a), exhibits two phases, a vitreous phase and a crystalline phase. This last one indicates the formation of crystalline agglomerates particle having ring form that indicate, the crystallisation tendency is enhanced and major Cr (PO3)3 and Pb2(PO3)4 phases are crystallised in these glasses. The presence of the crystalline phase seems to explain the increase in chemical durability   . The SEM micrograph for the sample S4 indicates a radical change in the structure. There is formation of crystalline phase agglomerates of various sizes, ranging from some microns. This probably explains the structural change towards more short isolated orthophosphate chains as the Cr2O3 content increases in the glass network.
The structure and the chemical durability of the glasses series xCr2O3-(45-x)PbO-55P2O5 (with 1 ≤ x ≤ 4; mol%) have been investigated using various techniques such as density, X-Ray diffraction and IR. The measured properties indicate that the glasses series chromium-lead-phosphorus-oxygen network become stronger for x = 2. X-ray diffraction indicates that the samples S1, S2 and S4, respectively annealed at 560˚C and 580˚C for 72 h, contain metaphosphates, orthophosphates and certain traces of pyrophosphate phases in all the glasses. When the Cr2O3 content is equal to 2 mol%, X ray diffraction and IR spectra, both, confirmed the predominance of metaphosphate groups most probably cyclic. The bands at 1244 - 1228 cm−1, attributed to the asymmetric vibration modes νas (PO2), become more intense. The predominance of cyclic metaphosphate chains as Cr (PO3)3 and Pb2(PO3)4 have led to maximum chemical durability. Analysis of the density values also showed a maximum density for x = 2 mol%. The covalent radius values of oxygen calculated from Equation (2) indicate that the minimum value rcal (O2−) is observed for x = 2 mol% and therefore a relatively high reinforcement of the metal-oxygen-phosphorus (Cr-O-P) bond. Additionally, because of the big stability of the energy of field of ligand of d3systems in octahedral symmetry, the ions Cr3+ occupy almost exclusively the sites having this symmetry type   , while the Pb is estimated to be in the tetrahedral site forming PbO4 pyramids which are connected in phosphate tetrahedron by covalent links P-O-Pb  . Above 2 mol% Cr2O3, the glass structure changes relatively, it is found that the DRX spectra intensity of the ring metaphosphate phases decreases, whereas the appearance of the isolated orthophosphate phase becomes important when the Cr2O3 content reaches 4 mol%. The substitution of PbO with chromium oxide by more than 2 mol% in the vitreous lattice appears to be an unfavorable factor for chemical durability. The origin of this phenomenon is explained almost probably by the approach of the boundary zone between the crystal and the glass by the continuous formation of groups of isolated phosphate  . The decrease of chemical durability observed, can be also explained by the existence of the critical concentrations between the cation ions beyond which each ion seeks, by competition, to have a site that is appropriate to it by moving away from the other. This behavior leads to an increase in the glass volume and a relaxation of the structure   and consequently a decrease in density and chemical durability. On the other hand, the PbO oxide have à low melting temperature and can participated with P2O5 Oxyde, basis glass matrix, in the extend of the area glass. That explains probably the non evidence of the attributed band of orthophosphate isolated units in the IR spectra beside the X-ray diffraction spectra. Added, the elaboration method of the glasses (melting temperature, tempering speed, etc.) remains a significant factor in the deep understanding of the phenomenon .
The structure and the chemical durability of chromium lead phosphate glasses of composition xCr2O3-(45-x)PbO-55P2O5 avec (1 ≤ x ≤ 4; mol%) have been investigated using various techniques such IR, X-ray diffraction, density and SEM micrograph. The study of the dissolution rate carried out on the glasses, immersed in distilled water at 90˚C for 24 days, indicates a maximum chemical durability when the level of chromium oxide passes through 2 mol%. The analysis of the values of the density also showed a maximum density for x = 2 mol%. The covalent radius values of the oxygen indicate that the minimum values are observed for x = 2 mol%, and therefore, a relatively high reinforcement of the metal-oxygen-phosphorus (Cr-O-P) bonds. However, the increase in the Cr2O3 content in the vitreous network to the detriment of PbO beyond 2 mol%, is an unfavourable factor for both chemical durability and density. The increase in the Cr + Pb/P ratio leads to an increase in the number of metal-O-P bonds which cause a high tendency for crystallisation and confirms that the dissolution rate (DR) of the analyzed compounds is comparable to the values of borosilicate glasses and 40 times less than BaBal glasses which are used as alternative materials for the immobilisation of nuclear waste substances.
The authors wish to thank National Center for Scientific and Technical Research [Division of Technical Support Unit for Scientific Research (TSUSR) Rabat, Morocco] for their assistance to the realization of this work.