Chemical durability and structural approach of the glass series (40-y) Na 2O-yFe 2O 3-5Al 2O 3-55P 2O 5-by IR, X-ray diffraction and Mössbauer spectroscopy

IOP Conference Series: Materials Science and Engineering

Chemical durability and Structural approach of the glass series (40-y) Na 2 O-yFe 2 O 3 -5Al 2 O 3 -55P 2 O 5 - by IR, X-ray diffraction and Mössbauer

Spectroscopy

To cite this article: S Aqdim et al 2012 IOP Conf. Ser.: Mater. Sci. Eng. 28 012003

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Chemical durability and structural approach of the glass series (40-y) Na ₂ O-yFe ₂ O ₃ -5Al ₂ O ₃ -55P ₂ O ₅ - by IR, X-ray diffraction and Mössbauer Spectroscopy

S. Aqdim

, El. H. Sayouty

, B. Elouad

, J. M. Greneche

a

Laboratoire de Physique de l’Etat Condensé, Université Hassan II Ain Chock, Faculté des Sciences Casablanca B.P. 5366 Maârif, km 8 route d'El Jadida, Casablanca- Morocco.

b

Laboratoire de Chimie Minérale, Département de Chimie, Université Hassan II Ain Chock, Faculté des Sciences-Casablanca B.P. 5366 Maârif, km 8 route d'El Jadida, Casablanca-Morocco.

c

Laboratoire d'Elaboration, Analyse Chimique et Ingénierie des Matériaux (LEACIM), Université de la Rochelle, avenue Michel Crépeau, 17042 La Rochelle Cedex 01, France.

d

Laboratoire de Physique de L'Etat Condensé, UMR CNRS 6087, 72085 Le Mans Cedex 9. France

E-mail: said_aq@yahoo.fr; hassayout@yahoo.fr; belouadi@univ-lr.fr;

jean.marc.greneche@univ-lemans.fr

Abstract.

The relationship between the composition and structure for the glasses of general composition (40-y)Na

O-yFe

O

-5Al

O

-55P

O

(5≤y≤20), has been studied. The chemical durability and density of these glasses increase with increasing Fe

O

content. The dissolution rate (D

), calculated from the weight loss in distilled water at 90°C for up to 20 days was ≈ 3.10

g/cm

/mn which is 30 times lower than that of window glass. The structure and valence states of the iron ions in the glasses were investigated using, X-ray diffraction, 57Fe Mössbauer spectrometry, potentiometric analysis, and infrared spectroscopy. Both Mössbauer spectrometry and potentiometric analysis allow to estimate both Fe

and Fe

contents in all these glasses. X-ray diffraction indicates that the local structure of iron phosphate glasses is related to the short range structures of NaFeP

O

. Infrared spectra indicate the formation of P–

O–Fe bonds in the pyrophosphate glasses that replace P–O–Na bonds. The presence of a small content of Al

O

in the glass seems to play a role as a network modifier. The addition of Fe

O

to Al

O

in phosphate glasses favours the enhancement of the formation of pyrophosphate units because iron ions have stronger effect on the depolymerization of metaphosphate chains than the aluminium ions. Finally, the I.R spectra indicate that the presence of P-O-Fe bands of these glasses containing more than 15 mol%Fe

O

is consistent with their good chemical durability.

1 To whom any correspondence should be addressed.

MATERIAUX 2010 IOP Publishing

IOP Conf. Series: Materials Science and Engineering 27 (2012) 012003 doi:10.1088/1757-899X/27/1/012003

1. Introduction

Phosphate glasses have recently gained great interest due to their potential application in various technological fields: optics, vitrification of high level nuclear wastes, etc [1-20]. However this class of materials should face a serious limitation resulting from their high sensitivity to the ambient moisture [21-23]. Since our early work [22] on iron phosphate glasses we have recognized the significant wet corrosion resistance of some compositions, as confirmed by many authors [2, 10, 22]. Various studies have shown that the combination of P

O

with oxide like ZnO, Al

O

and more specifically Fe

O

can give rise to glass compositions with high chemical durability [1, 11-17]. Such corrosion resistance was attributed to the change of chemical bonds in the vitreous state, as the bridging oxygen P-O-P bonds tend to be replaced by P-O-M bonds (with M= Zn, Al, Fe, etc). The presence of Fe originates two possible valence states, Fe

and Fe

the equilibrium of which in phosphate glasses depends upon the melting atmosphere. In addition, the chemical durability has been found to independent of the Fe

/Fe

ratio in some phosphate glasses [14]. It is thus important to follow the evolution of the concentration of Fe valence states which can be easily and accurately estimated by using transmission

57

Fe Mössbauer spectrometry.

The aim of the present study is to investigate the structural change and the durability modification versus composition as the ratio of Fe

O

is varied along the series of phosphate glasses (45-y)Na

O- yFe

O

-5Al

O

-55P

O

with 5≤y≤20.

2. Experimental section

In the absence of any redox phenomenon, the investigated glasses of the (40-y)Na

O- yFe

O

-5Al

O

- 55P

O

series belong to quaternary system Na

O-Fe

O

-Al

O

-P

O

. As it is shown in Fig. 1, these materials can be located on one line of pseudo ternary system 9Na

O.11P

O

-9Al

O

. 11P

O

- 9Fe

O

.11P

O

which represents the planar section of the Na

O-Fe

O

-Al

O

-P

O

tetrahedral diagram, corresponding to a constant molar fraction of P

O

(P

O

) =0,55. The chemical compositions of the present glasses are listed in Table 1. The glass samples were prepared from the appropriate mixture of the starting compounds Na

CO

, Al

O

, Fe

O

and (NH

)

HPO

. The weighted mixture of about 10 g were thoroughly in an agate mortar before to be submitted to moderate heat treatments between 300°C and 500°C in order to get an homogeneous system before the glass preparation. The melt was achieved for about 15 min at 1070 ±10°C while the quenching down to room temperature allows to obtain glassy pellets with approximate sizes of 5-10mm diameter and 1-3mm thickness. The vitreous state was first evidenced from the shiny and transparency aspect and then confirmed from XRD patterns.

For the chemical durability analysis, pieces of 0.9x0.9x0.3 cm

were prepared after polishing using a 400-grit polishing paper and then finished with SiC paper; they were then immersed into a flask filled with 100ml of distilled water at 90°C for 20 days. The dissolution rate (D

) was then determined from the weight loss during the aqueous treatment at 90°C. The specific mass (density) was determined using the picnometry with both powdered and monolith glasses.

57

Fe Mössbauer spectra were recorded at 300 K and at 77 K using a conventional constant acceleration transmission spectrometer with a

Co(Rh) source and a bath cryostat. The Mössbauer sample consists of powdered glass containing 5mg Fe/cm2. An α-Fe foil was used as calibration sample while the values of isomer shift (IS) are quoted to that of α-Fe at 300K. The Mössbauer spectra were fitted by means of the MOSFIT program, assuming the superposition of quadrupolar doublets with lorentzian lines. Indeed, the interest of using Mössbauer spectrometry lies on the distinction of Fe valence states and consequently the estimation of the iron redox ratio Fe

/(Fe

+Fe

) from the spectral area. The values of this ratio were then compared to those obtained by potentiometric method in dosing both iron Fe

and iron Fe

with dichromate solution after complete digestion of the glass sample employing concentrated H

SO

. The experimental error in the value of Fe

/(Fe

+Fe

) was

MATERIAUX 2010 IOP Publishing

IOP Conf. Series: Materials Science and Engineering 27 (2012) 012003 doi:10.1088/1757-899X/27/1/012003

2

found about ± 2. Infrared (IR) spectra were measured for each glass between 400 and 1400 cm

using mX-1 and NIC-3600 FTIR spectrometers. The analysed samples were prepared by pressing a mixture of about 2mg of glass powder with 100 mg of anhydrous KBr powder.

3. Results and discussion

3.1. Structural approach by Infrared spectroscopy

The infrared recorded spectra for glass series (40-y) Na

O-5Al

O

-yFe

O

-55P

O

are illustrated in Fig 1. The analysis of these spectra shows a clear evolution from the metaphosphate to the pyrophosphate modes. As a matter of fact with increasing of iron (III) oxide, the vibrational bonds of the pyrophosphate tend to be the dominant feature of the spectrum. As can be evidenced in Fig 2, the band (s PO

) at 1050cm

appears as the only vibrational mode in the glass. This band is characteristic in the pyrophosphate glasses since it is related to s PO

. However in the region of 500-700cm

, it is still possible to observe the vibrational mode related to P-O-P bonds the number of which tends to decrease when the glass composition is changed from the metaphosphate to the pyrophosphate domains. Such behaviour results from the depolymerisation of the metaphosphate into pyrophosphate compounds.

Indeed the depolymerization phenomenon corresponds to the increase break of the P-O-P bonds.

Figure 1. Fig.1: Localization of the investigated glass compositions G

, G

55

, G

and G

55

within the ternary diagram (9Na

O.11P

O

)-(9Al

O

.11P

O

)-(9Fe

O

.11P

O

). Table 1 gives the corresponding compositions within the quaternary system Na

O-Fe

O

-Al

O

-P

O

The IR spectrum of G

55

is very similar to that of the metaphosphate composition [10, 14, 25] as it exhibits all the modes characteristic of a metaphosphate family ((PO

) = 550cm

, s (P-O-P) = 725cm

, and as (PO

) = 1198 cm

) in addition to the diphosphate modes as (PO

) = 1070 cm

;

s(PO

) and s (P-O-P) =755 cm

[26, 27]. Furthermore, the structure medication deduced from the vibrational spectroscopy is totally compatible with localization of the analyses compounds (G

55

, G

, G

, G

55

) inside the ternary diagram given in Fig. 1. As a matter of fact, G

55

is located in the olygophosphate domain close to the metaphosphate line. Therefore it is normal that its structure contain mainly the metaphaphate units. Indeed when the molar fraction of Fe

O

is increased, the composition (Fig 1. and Table 1) of the glass becomes closed to the pyrophosphate line. For G

for example, the dominant structure units are expected to be those of pyrophosphate. This is the reason why its infrared spectrum contain mainly a large vibration band assigned to s (PO

) = 1060cm

. Since the number of P-O-P has decreased between G

55

and G

55

, it is normal to observe a decrease of the corresponding intensity s (P-O-P)  700cm

.

MATERIAUX 2010 IOP Publishing

IOP Conf. Series: Materials Science and Engineering 27 (2012) 012003 doi:10.1088/1757-899X/27/1/012003

Table 1. Glass starting compositions in terms of basic oxides (see Fig 1 for the position inside the ternary system with constant P

O

ratio)

Sample Starting glass Composition (mol %)

ratio[Fe

]/

[Fe

+ Fe

],%

[O/P]

ratio *

(D

) (g/cm

/mn)



(g/cm

)

Na

O Fe

O

Al

O

P

O

± 1 20 days ± 0.02

G

35 5 5 55 6,5 3.09 (5± 0,2) x10

2.33

G

30 10 5 55 6,6 3.18 (9± 0,2)x10

2.63

G

25 15 5 55 8,2 3.27 (2.5± 0,2)x10

2.72

G

20 20 5 55 7,5 3.36 (3.± 0,2)x10

2.81

3.2.

Fe Mössbauer Spectrometry and iron Redox Phenomenon

The 300K Mössbauer spectra of some Na-Al-Fe phosphate glasses are illustrated in Fig. 3. As those obtained at 77K, they exhibit a central prevailing quadrupolar doublet with broadened lines and a small line at higher velocities. The first part has to be well described by means of at least 4 quadrupolar doublets which can be unambiguously attributed to high spin state Fe

ions in octahedral sites. Nevertheless, it is important to notice that a quadrupolar splitting distribution can be also successfully used, that is consistent with a disordered local structure as expected in a glassy system.

The second part of the Mössbauer spectra (Fig. 2) has to be associated to a minor quadrupolar component, well described by two doublets, unambiguously ascribed to the presence of Fe

ions. The refined values of mean Mössbauer hyperfine parameters, as isomer shift (), quadrupole splitting (E

), and fraction of Fe

are listed in Table 2.

Table 2. Mössbauer characteristics of the glass series (40-y)Na

O-5Al

O

-yFe

O

-55P

O

Sample [Fe

]/[Fe

+Fe

]; % Molar Stage 1 Chi Sqr.

(N=15, df=1)

p

Stage 2 Chi Sqr.

(N=15, df=1)

G

6 0.43 1.07 0.64 2.51

G

7 0.42 1.00 0.73 2.55

G

8 0.41 0.97 0.63 2.61

G

7 0.40 0.93 0.88 2.66

The values of quadrupolar splitting suggest Fe

and Fe

ions with distorted octahedral coordination [24, 25]. This description allows thus to determine the iron redox ratio Fe

/(Fe

+Fe

) from the corresponding spectral area of each component, assuming the same value of f-Lamb Mössbauer factor. It can be concluded that the increasing Fe

O

content favours a reduction of Fe

into Fe

ions during melting in air. It seems thus that the significant factor affecting the redox state of the iron in sodium-aluminium-iron phosphate glasses could be the presence of ammonium phosphate (NH

)

HPO

in a batch composition as a source of P

O

. As a matter of fact, reducing conditions were created during the melting process which favours the reduction of Fe

into Fe

[24]. However this reduction does not exceed 10%. Some authors have suggested that the Fe

/(Fe

+Fe

) ratio is more related to the melting temperature than to the glass composition of [ 8, 16]. The molar fractions of Fe

and Fe

in the title of series of glasses were also determined from the potentiometric technique. As it can be seen in Fig 4, the obtained values are slightly higher than those determined from the Mössbauer study. However all the values lie in the same range of about [Fe

]/[Fe

]+[Fe

] = 6-10%.

MATERIAUX 2010 IOP Publishing

IOP Conf. Series: Materials Science and Engineering 27 (2012) 012003 doi:10.1088/1757-899X/27/1/012003

4

Figure 2. IR spectra of the series of (40-y)Na

O -yFe

O

-5Al

O

-55P

O

glasses, y= 5, 10, 15 and 20

Figure 3. Room temperature Mossbauer spectra of (40-y)Na

O -yFe

O

-5Al

O

- 55P

O

glasses

Figure 4. Variation of relative % of Fe

versus initial molar fraction in the glass series (40-y)Na

O-5Al

O

-yFe

O

-55P

O

3.3. Density and Molar Volumes

The specific mass (Density) of vitrified phosphates is increasing with increasing molar fraction along the series (40-y)Na

O-yFe

O

-5Al

O

-55P

O

. As it is shown in Fig. 5, from density measurements, it was possible to deduce the value of the molar volume and oxygen radius, calculated from the approximate hypothesis of close packing of oxygen anions O

, having r(O

)cal recapitulated for each composition in Table 3.

MATERIAUX 2010 IOP Publishing

IOP Conf. Series: Materials Science and Engineering 27 (2012) 012003 doi:10.1088/1757-899X/27/1/012003

Table 3. Variation of the density and related molar volume in (40-y)Na

O-yFe

O

-5Al

O

-55P

O

. system (

M = molar mass;  = density; N

= Avogadro number; N

= number of oxygen atoms in the molar formula)

Figure 5. Variation of the specific mass (density) versus mol% in (40-y)Na

O -yFe

O

-5Al

O

-55P

O

system

3.4. Analysis of the Chemical Durability of the Glass Series (40-y)Na

O-yFe

O

-5Al

O

55P

O

The chemical durability of the series of glasses (40-y)Na

O-yFe

O

-5Al

O

-55P

O

was approached using the measurement of the dissolution (D

) rate which was defined as the weight loss of the glass expressed in terms of g.cm

.mn

. The values of D

reported in Table 1, show a dissolution decrease versus the Fe

O

content of our samples after their immersion in 100ml of distilled water, heated at 90°C for 20 consecutive days (Fig 6). There was no detectable difference in the visual appearance of the glass samples G

55

, G

55

, and G

55

immersed in water at 90°C during the attack’s period.

However the glass sample G

55

was weakly corroded. The glass containing 20 mol % Fe

O

had a (D

) 30 times less than the D

for window glass and ~ 100 times less than the D

for BABAL glass of composition 27BaO

-45B

O

-18Al

O

-10Fe

O

which has been studied as alternative materials for nuclear waste immobilization [9, 18].

MATERIAUX 2010 IOP Publishing

IOP Conf. Series: Materials Science and Engineering 27 (2012) 012003 doi:10.1088/1757-899X/27/1/012003

6

Figure 6. Dissolution rates (D

) of the glass series (40-y)Na

O-yFe

O

- 5Al

O

-55P

O

versus: a)Fe

O

(mol%); b) ratio [O/P].

4. Correlation between the structure and the high durability of Iron Phosphate Glasses

The IR techniques have confirmed the structural evolution of the glass network along series (40-y) Na

O-yFe

O

-5Al

O

-55P

O

, from a mixture of pyrophosphate and metaphosphate 3 < [O/P] < 3,5 towards the pyrophosphate line with [O/P] = 3,5 (Fig. 1). The determination of the molar volume (Table 3) tends to confirm the increase of the covalent character of the chemical bond with increasing Fe

O

content

The increase of this covalent character may explain the chemical strength of glass network which becomes more resistant to the chemical attack in an aggressive aqueous medium at 90°C. The presence of a small content of Al

O

in the glass seems to play a role as a network modifier when Fe

O

content increases (y≥15) [10]. The chemical resistance of Aluminium-iron phosphate glasses (regarding aqueous attackat 90°C) is attributed to the increasing number of Fe-O-P bonds in the glass [19-24]. Such bonds are expected to be more water resistant than the P-O-P ones, which constitute the majority of bonds in the low iron concentrated glasses.

5. Conclusion

The structural and proprieties physic-chemical of sodium-aluminium-iron-phosphate glasses have been studies by using several techniques. The structural approach of these glasses by I.R spectroscopy show a radical change of structure when the Fe

O

content increases. This change lead to the formation of short units groups of pyrophosphates. Results of spectroscopy Mössbauer indicate the proportions of Fe

and Fe

in the glass. The parameters hyperfins show that the last occupied the merely octahedral sites more or less distorted. The structure glass can be considered as pyrophosphate units connected with ferric and ferrous iron in octahedral or distorted octahedral coordination. The improved chemical durability is attributed to the replacement of the easily hydrated Na-O-P and P-O-P bonds by corrosion resistant Fe-O-P bands. The O/P ratio is also an important factor to aqueous chemical durability. These glasses have a dissolution rate (D

) 30 times less than the D

for window glass and ~ 100 times less than the D

for BABAL glass which has been considered as alternative materials for the immobilization of nuclear waste substance.

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MATERIAUX 2010 IOP Publishing

IOP Conf. Series: Materials Science and Engineering 27 (2012) 012003 doi:10.1088/1757-899X/27/1/012003

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IOP Conf. Series: Materials Science and Engineering 27 (2012) 012003 doi:10.1088/1757-899X/27/1/012003

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