Structural and thermochemical properties of sodium magnesium phosphate glasses

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Structural and thermochemical properties of sodium magnesium phosphate glasses

Refka Oueslati Omrani â , Abdeltif Kaoutar ^b , Abdelaziz El Jazouli ^b , Saida Krimi ^c , Ismail Khattech â, ^⇑ , Mohamed Jemal â , Jean-Jacques Videau ^d , Michel Couzi ê

a

Université de Tunis El Manar, Faculté des Sciences de Tunis, Chemistry Department, LR01SE10 Applied Thermodynamics Laboratory, 2092 Tunis, Tunisia

b

LCMS, URAC 17, Faculté des Sciences Ben M’Sik, UH2MC, Casablanca, Morocco

c

LPCMI, Faculté des Sciences Aïn Chok, UH2C, Casablanca, Morocco

d

ICMCB, Institut de Chimie de la matière condensée, Université de Bordeaux 1, France

e

Institut des Sciences Moléculaires, CNRS-Université de Bordeaux 1, France

a r t i c l e i n f o

Article history:

Received 6 May 2014

Received in revised form 27 January 2015 Accepted 30 January 2015

Available online 7 February 2015 Keywords:

Magnesium phosphate glasses Calorimetric dissolution Depolymerization Metaphosphate glasses Pyrophosphate groups

a b s t r a c t

Ternary phosphate based glasses with the general formula (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

(0 6 x 6 42.8 mol%), where the O/P ratio was varied from 3 to 3.75, have been prepared using a conven- tional melt quenching technique. Samples were investigated by means of density measurements, Fourier- transformed infrared (FTIR), Raman and

³¹

P solid state magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies, differential scanning calorimetry (DSC), inductively coupled plasma atomic emission spectroscopy (ICP/AES) analysis and calorimetric dissolution.

The depolymerization of metaphosphate chains are described by the decrease of Q

²

tetrahedral sites allowing the formation of pyrophosphate groups (Q

¹

) revealed by spectroscopic investigations. As a result, the increase of density and glass transition temperature when x rises. Calorimetric study shows that the dissolution phenomenon is endothermic for a lower MgO content and becomes exothermic when magnesium oxide is gradually incorporated, suggesting the disruption of phosphate chains with increas- ing O/P ratio.

Ó 2015 Published by Elsevier B.V.

1. Introduction

Phosphate Glasses have attracted a considerable technological interest due to their special properties and applications compared to silicate glasses. They have a low transition and melting temper- atures and high transmission in the UV. Amorphous materials are used as biomaterials for medical application. Many investigators are currently interested in ameliorating the ability of glasses to react with the physiological environment by forming durable and mechanically strong bonds across the biological tissue–glass inter- face. Further, phosphate glasses are suited for doping with rare earth ions allowing the manufacturing of laser ampliﬁcation [1].

They also ﬁnd enormous applications such as: glass to metal seals [2,3–9], optical waveguides [2,6,10–12], solid state laser sources [2–4,6,8,10,11,13–15], conducting materials [6–9,12,15–17], amor- phous semiconductors [13,14,17–19], biomaterials [6,13,20,21], nuclear waste immobilization matrices [6,14].

Magnesium phosphate glasses are more resistant to moisture attack and possess higher mechanical moduli than other phos- phate glasses [21]. Many investigators have classiﬁed the binary MgO–P

2

O

5

system as anomalous phosphate glasses due to the dis- continuity in their physical properties. This anomaly can be explained by the change of the coordination number between Mg and O ions [22].

The structure of phosphate glasses is based on the distribution of Q

ⁿ

tetrahedral sites (where n (n = 0–3) represents the number of bridging oxygen (BO) per PO

4

tetrahedron) in the vitreous network.

In addition, the incorporation of modiﬁer cations such as Na

⁺

, K

⁺

,

Mg

²⁺

, Ca

²⁺

. . . disrupts the glassy network, suggesting the depoly-

merization of the structure and the formation of non-bridging oxy- gen atoms (NBO) which ameliorate their chemical durability [20,23,24].

Many studies have focused on metal oxide addition such as Na

⁺

, Sr

²⁺

, Mg

²⁺

and Zn

²⁺

which are usually incorporated as oxides and considered as glass network modiﬁers. Addition of oxides improves the phosphate glass stability because the P A O A M (where M is a metal cation) bond is generally stable toward atmo- spheric hydrolysis or solution attack [25]. Among the various metal

http://dx.doi.org/10.1016/j.jallcom.2015.01.297 0925-8388/Ó 2015 Published by Elsevier B.V.

⇑ Corresponding author. Tel.: +216 98 208 884; fax: +216 71 883 424.

E-mail address: [email protected] (I. Khattech).

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m

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oxides, MgO is of interest from a biological viewpoint because Mg

²⁺

is known to play a physiological role in positively inﬂuencing bone strength when it is substituted into apatites [25].

The present work deals with the structural study of magnesium phosphate glasses pertaining the Na

2

O–MgO–P

2

O

5

ternary system according to the formula: (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

(0 6 x 6 42.8 mol%) with 3 6 O/P 6 3.75. Density measurements, molar volume evolution, DSC study, ICP analysis, FTIR, NMR of

31

P and Raman spectroscopy were investigated. The calorimetric dissolution was also applied in order to elucidate the correlation between structural changes and thermochemical properties of the glasses when magnesium oxide is gradually incorporated.

2. Experimental 2.1. Glass preparation

A series of glasses were prepared by varying the MgO content from 0 to 42.8 mol% using reagent grade compounds, NaH

2

PO

4

(Sigma Aldrich), MgO (Sigma Aldrich) with a high purity (99% purity) in the suitable proportions.

The mixture corresponding to the desired compositions was first heated in plat- inum crucible at 200 °C for 30 min. The temperature was then progressively raised from 750 °C to 900 °C, depending on glass composition, and held constant for 30 min. The batch was finally quenched to room temperature under air atmosphere in order to produce vitreous structure which is confirmed by X-ray diffraction. For the sake of stabilization, all the samples were annealed at 20 °C above their glass transition temperature for 2 h. The solids were kept in a desiccator to prevent pos- sible moisture. Their nominal and analyzed glass compositions are reported in Table 1.

2.2. ICP analysis

Phosphorus, magnesium and sodium were analyzed by inductively coupled plasma atomic emission spectroscopy (Jobin Yvon Ultra C).The nominal and analyt- ical glass compositions are reported in Table 1. Glasses will be labeled by reference to their molar content as mentioned Fig. 1.

2.3. Measurements of density

Glass density was determined on the bulk samples by the Archimedes method using diethyl orthophthalate as the immersion liquid. The relative error is ±0.03 g/

cm

³

. The molar volume was calculated from the density (V = M/ q ) and the molar weight M.

2.4. DSC investigations

The glass transition temperatures were determined on 40–50 mg of samples using DSC-ATD Netzsch 404 PC with a 10 °C/min heating rate (accuracy ±5 °C).

2.5. FTIR, Raman and NMR spectroscopies

Fourier-transformed infrared spectra (FTIR) were recorded with an FTIR spec- trometer (Perkin–Elmer) in the frequency range 400–4000 cm

¹

at room tempera- ture. The samples were prepared by grinding about 9 mg of glass powder with 300 mg spectroscopic grade dried KBr.

The Raman spectra were recorded on powder of glasses using a Labram HR 800 micro Raman model operating in the 50–4000 cm

¹

range at room temperature, equipped with an internal HeANe laser source (k = 488 nm).

31

P solid state magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra were collected at 121.456 MHZ (magnetic ﬁeld 7.1T) on a Bruker ultrashield 300 spectrometer. All the chemical shifts are expressed in ppm relative to the 85%

aqueous phosphoric acid. In general chemical shifts were independent on experi- mental parameters. The chemical shift resolution is ±0.2 ppm.

2.6. Calorimetric dissolution

The dissolution of the glasses was performed using the C80 (SETARAM) calorim- eter operating at 25 °C. This device is provided with two identical cells: the refer- ence and the measuring cell. The last one contains the solid to be dissolved which is tightly separated from the solvent by a movable cover, whereas the refer- ence cell was provided with only the solvent.

Both of the cells are surrounded by high performance thermopiles for the detec- tion of heat ﬂow resulting from the dissolution of about 20 mg solid in 4.5 ml of a 4.5% weight phosphoric acid solution. The dissolution enthalpy was determined by integrating the surface of the raw signal. The reliability of the calorimeter was ver- iﬁed by the key dissolution reaction [23,24,26–28].

3. Results and discussion 3.1. Density and molar volume

Values of density and molar volume are listed in Table 2. As can be seen, the molar volume decreases from 42 cm

³

mol

¹

for x = 0 mol% to 26 cm

³

mol

¹

for 42.8 mol%. This decrease can be explained by the higher ﬁeld strength, D F ( D F = z/r

²

; with z is the valence cation and r is the ionic radius) of Mg

²⁺

compared to that of Na

⁺

. The decrease in the molar volume is essentially due to the incorporation of magnesium oxide. Fig. 2 shows that both den- sity and molar volume are linear over the MgO concentration. As will be seen later, the variation of these properties is closely related to the structural changes of glasses when magnesium oxide is added, resulting the compactness of the vitreous network [23,24,29,30].

3.2. DSC investigations

Fig. 3 shows the dependence of glass transition temperature on MgO content. T

g

increases from 280 to 449 °C (Table 2). This behav- ior corresponds undoubtedly to some changes in the nature of bonding in the structural network. T

g

value is strictly related to the bond strength of the glass network which can be explained in terms of bond length affected by the cation ﬁeld strength (which is the charge divided by the square of the cation-oxygen distance).

When magnesium oxide is incorporated, the structure strength- ened because ionic bond Na A O are replaced by covalent bond Mg A O allowing the compactness of the vitreous network. As a result the increase of glass transition temperature.

According to Dietezel, the thermal stability of glasses can be expressed by the temperature difference between T

g

and T

c

, Table 1

Analyzed and nominal compositions for (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

(0 6 x 6 42.8 mol%) glass series.

%xMgO %xNa

2

O %xP

2

O

5

Nominal/analyzed Nominal/analyzed Nominal/analyzed

0/0 50/52 50/48

10/9.1 45/46.3 45/44.6

20/18.1 40/40.4 40/41.5

25/23 37.5/36 37.5/41

30/28.3 35/33.3 35/38.4

40/37.5 30/28.3 30/34.2

42.8/40.5 28.6/27.5 28.6/32

0,0 0,2 0,4 0,6 0,8 1,0

0,0

0,2

0,4

0,6

0,8

1,0 0,0

0,2 0,4 0,6 0,8 1,0 MgO

P

₂

O

₅

Na

₂

O

NaPO

₃

Fig. 1. Analytical ( ) and nominal ( ) glass compositions of: (50x/2)Na

2

O–

xMgO–(50x/2)P

2

O

5

(0 6 x 6 42.8 mol%) phosphate glasses.

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D T = T

c

–T

g

, where T

g

and T

c

are the glass transition and the onset crystallization temperatures. D T is reported in Table 2. Increasing D T delays the nucleation process which makes the glass more sta- ble [31].

3.3. Infrared and Raman spectroscopy

FTIR spectra of (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

(0 6 x 6 42.8 mol%) glasses with various content of magnesium oxide are presented in Fig. 4. The Infrared and Raman band assign- ments are listed in Table 3.

The characteristics features of NaPO

3

metaphosphate glass (x = 0) are: the PO

₂

asymmetric stretching vibration band ( t

as

(PO

2

)) near 1280 cm

¹

, the PO

2

symmetric stretching vibration band ( t

s

(PO

2

)) at 1150 cm

¹

, the t

as

(PO

3

) groups (chain end groups) at 1100 cm

¹

, the t

s

of PO

3

groups at 1000 cm

¹

, the t

as

of P A O A P groups at 880 cm

¹

, the t

s

of P A O A P groups at 780

and 720 cm

¹

and the deformation mode of P A O

(PO

₄³

) groups at 535 and 480 cm

¹

[4,14,18,23,24,30]. In addition, phosphate glasses are hygroscopic, so probably they contain water. Further- more, in IR spectra the band corresponding to OH groups is located near 3500 cm

¹

as mentioned Fig. 4.

With increasing MgO content, the high frequency bands corre- sponding to the PO

2

asymmetric stretching vibrations become broader and less distinct [4,14,18,23,24,30]. Fig. 4 shows the pres- ence of two bands in the 780–720 cm

¹

frequency range in magne- sium free phosphate glass (a) which are attributed to the presence of two P A O A P bridges in metaphosphate chains based on (P

2

O

6

)

²

groups [18]. For higher x content, one can note the existence of a single band at 750 cm

¹

assigned to the P A O A P linkage in pyro- phosphate group (P

₂

O

₇

)

⁴

[18,23,24]. These spectral changes can be explained by the depolymerization of the inﬁnite metaphos- phate chains as MgO oxide is introduced suggesting the formation of pyrophosphate glasses with increasing the O/P ratio from 3 to 3.75.

The Raman spectra of (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

glasses, where O/P varies, are shown in Fig. 5. The Raman spectrum of NaPO

3

(a) reveals a broad band at about 1274 cm

¹

and two sharp peaks at about 685 and 1164 cm

¹

. The bands situated at 1274 and 1164 cm

¹

are assigned to the asymmetric and symmet- ric vibrations ( m

as

(PO

2

) and m

s

(PO

2

)) of the non-bridging oxygen atoms (NBO) bonded to phosphorus atoms (O A P A O) in metaphos- phate chains (Q

²

) [10,14,23,24,28,29,32,33]. The wide band at 685 cm

¹

is assigned to the symmetric vibration of the bridging oxygen connecting two PO

4

tetrahedron ( m

s

(P A O A P)) in meta- phosphate chains [10,14,23,24,29,32,33].

For x = 10 mol%, a new band appeared at about 1020 cm

¹

attributed to the symmetric stretching vibration m

s

(PO

3

) of the Table 2

Glass composition, glass transition temperature T

g

, T

c

, DT, density q , molar volume V

m

, of (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

(0 6 x 6 42.8 mol%).phosphate glasses. (DT = T

c

–T

g

).

Composition T

g

(°C) ±5 °C T

c

(°C) ±5 °C DT (°C) Density (g/cm

³

) ±0.03 V

m

(cm

³

/mol)

50Na

2

O–50P

2

O

5

280 290 10 2.43 42.00 ± 1.30

47.5Na

2

O–10MgO–45P

2

O

5

321 553 232 2.48 38.00 ± 1.10

40Na

2

O–20MgO–40P

2

O

5

348 564 216 2.53 34.20 ± 1.00

37.5Na

2

O–25Mg–37.5P

2

O

5

364 540 176 2.55 32.40 ± 1.00

35Na

2

O–30MgO–35P

2

O

5

394 568 151 2.59 30.40 ± 1.00

30Na

2

O–40MgO–30P

2

O

5

448 524 76 2.63 27.00 ± 1.00

28.6Na

2

O–42.8MgO–28.6P

2

O

5

449 586 137 2.65 26.00 ± 1.00

Fig. 2. Density dependence ( ) and molar volume evolution ( ) of (50x/2)Na

2

O–

xMgO–(50x/2)P

2

O

5

phosphate glasses (0 6 x 6 42.8 mol%) over MgO content.

Fig. 3. T

g

dependence of (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

(0 6 x 6 42.8 mol%) glasses over MgO content.

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm

^-1

)

Transmission (a.u)

900 750 520

12803500 1000

1100-1150 720 - 780 535 - 480

(g) (f) (e)

(d) (c) (b) (a)

Fig. 4. Infrared spectra of (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

glasses: (a) 0 mol%, (b) 10 mol%, (c) 20 mol%, (d) 25 mol%, (e) 30 mol%, (f) 40 mol%, and (g) 42.8 mol%

MgO.

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NBO against the P atoms in PO

4

tetrahedron with 3 NBO in pyro- phosphate groups (Q

¹

) [23,24,29,32,33].

With increasing magnesium oxide content, the bands situated at 1274, 1164 and 685 cm

¹

decrease simultaneously as was observed by Zotov et al. for (100x)NaPO

3

–xMnO (0 6 x 6 1 6.7 mol%) glasses [32]. These changes can be attributed to the dis- ruption of P A O A P bridges and the depolymerization of phosphate chains. As a result, the formation of phosphate dimer (Q

¹

) when the O/P ratio increases. The intensity of bands situated at about 1164 and 685 cm

¹

decrease with increasing x content [23,24,32,33].

In particular, the ﬁrst one vanishes for x = 40 mol%. These changes can be attributed to the disruption of P A O A P bridges as well as the depolymerization of phosphate chains suggesting the formation of phosphate dimer (Q

¹

). When the O/P ratio increases, a new bond appears for x = 5 mol% at about 1020 cm

¹

attributed to the sym- metric stretching vibration m

s

(PO

3

) of the NBO in PO

4

tetrahedron in pyrophosphate groups (Q

¹

) [21,23,24,33].The intensity of this band increases when x rises (Fig. 5). In addition, the Raman spectra reveal the displacement of this band to 1100 cm

¹

for 30Na

2

O–

40MgO–30P

2

O

5

and 28.6Na

2

O–42.8MgO–28.6P

2

O

5

glasses. This result can be explained by the high P-NBO p character due to the low condensation of phosphate chains when MgO oxide is gradu- ally incorporated [7,23,24,33]. These changes can be also explained by the depolymerization of metaphosphate groups allowing the formation of pyrophosphate anions (P

2

O

74

) when magnesium oxide is added to the structure. As a result, the increase of the rigid- ity of the vitreous network.

Furthermore, Raman spectra of glasses with 25 and 30 mol%

show that the intensity of band attributed to Q

²

tetrahedral sites decreases and gives rise to a new band assigned to phosphate

dimer (Q

¹

). This result can be explained by the disruption of phos- phate chains generating the formation of pyrophosphate anion (P

2

O

74

).

A similar behavior have been observed with manganese and zinc phosphate glasses having the general formula (50x/

2)Na

2

O–xMO–(50x/2)P

2

O

5

(M„Mn or Zn) where x ranged from 0 to 33 mol% [23,24].

3.4.

³¹

P MAS-NMR spectroscopy

The

³¹

P MAS-NMR spectra of (50x/2)Na

2

O–xMgO–(50x/

2)P

2

O

5

glass series is shown in Fig. 6. Isotopic peaks are labeled with their respective chemical shift of Q

ⁿ

units. The remaining peaks observed on both sides of the centerbands represent the spinning sidebands that rises from the chemical shift anisotropy interaction associated with Q

ⁿ

units [10].

The major feature of NaPO

3

glass is the existence of an isotropic peak at 20.02 ppm representing the Q

²

tetrahedral sites that are the bases of metaphosphate chains [4,7,8,20,21,23,24]. The peak at +1.35 ppm is assigned to Q

¹

sites (chain end groups) [7].

Referring to literature, some authors have reported the chemi- cal shift at +1.5 ppm for chain end groups of NaPO

₃

[7]. As MgO is added to metaphosphate chains, the intensity of the peak attrib- uted to Q

¹

tetrahedral sites increases and becomes the dominant spectral feature for higher MgO level. The other isotropic peak assigned to Q

²

tetrahedral sites decreases simultaneously with Table 3

Infrared and Raman band assignments (cm

¹

) of (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

(0 6 x 6 42.8 mol%).

x m

as

(PO

2

) m

s

(PO

2

) m

as

(PO

3

) m

s

(PO

3

) m

as

(POP) m

s

(POP) (PO

43

) IR

(cm

¹

) Raman (cm

¹

)

IR (cm

¹

)

Raman (cm

¹

)

IR (cm

¹

)

Raman (cm

¹

)

IR (cm

¹

)

Raman (cm

¹

)

IR (cm

¹

)

Raman (cm

¹

)

IR (cm

¹

)

Raman (cm

¹

)

IR (cm

¹

)

Raman (cm

¹

) (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

0 1280vw 1274vw 1150vw 1164vs 1100vw – 1000vw – 880vw – 780–

720vw

685w 535–

480w

380vw

10 1280vw 1274vw 1150vw – 1100vw 1020vw – – 880vw – 750vw 685w 520vw 380vw

20 1280vw – 1150vw 1164vs – 1020w – – 900w – 750m 685w 520vw 380vw

25 – – 1150vw 1164vs – 1020m – – 900m – 750m 685vw 520m 380vw

30 – – 1150vw 1164vw – 1020w – – 900m – 750m 685w 520m 380vw

40 – – 1150vw 1164vw – 1020s – – 900m – 750s 685w 520w 380vw

42.8 – – 1150vw – – 1020vs – – 900w – 750vw 685vw 535–

480w

380vw

vs: very strong; m: medium; w: weak; vw: very weak.

(b)

(a) (c) (d) (e) (f) (g)

Intensity (a.u)

1020

Wavenumber (cm )

-1

200 400 600 800 1000 1200 1400

685 1164 1274

Fig. 5. Raman spectra of (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

glasses: (a) 0 mol%, (b) 10 mol%, (c) 20 mol% MgO, (d) 25 mol%, (e) 30 mol%, (f) 40 mol%, and (g)

42.8 mol% MgO. -100 -50 0 50 100

Q

¹

Q

²

* *

*

* *

*

(g) (f) (e) (d) (c) (b) (a)

Intesity (a.u)

31

P Chemical shift (ppm)

Fig. 6.

³¹

P MAS-NMR spectra of (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

glasses: (a)

0 mol%, (b) 10 mol%, (c) 20 mol%, (d) 25 mol%, (e) 30 mol%, (f) 40 mol%, (g)

42.8 mol% MgO,

^⁄

The spinning sidebands are marked with asterisks.

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increasing O/P ratio and vanished for 30Na

₂

O–40MgO–30P

₂

O

₅

and 28.6Na

2

O–42.8MgO–28.6P

2

O

5

glass compositions [5,11]. This result is in good agreement with the structural study of (100x)NaPO

3

–xZnO glasses (0 6 x 6 33.3 mol%) performed by Montagne et al. [7].

Fig. 6 indicates that the phosphate glasses with 40 mol% has almost only the Q

¹

tetrahedral sites assigned to pyrophosphate groups this suggests that phosphate chains are totally depolymer- ized. On the other hand, the phosphorus chemical shift depends essentially on the phosphorus-ligand bond (P A O) for phosphate compounds and the electronic density of the non-bridging oxygens (NBO) [7]. Table 4 reports the average values of the measured chemical shifts for Q

¹

and Q

²

sites. It mentioned that the Q

²

and Q

¹

chemical shift become less shielded when MgO oxide is intro- duced. This decrease can be explained by the low condensation degree of phosphate glasses when magnesium oxide is gradually incorporated resulting a higher p P-NBO character [7]. This sug- gests that Mg

²⁺

ions are only bonded to pyrophosphate groups described by Q

¹

tetrahedral sites [7].

The effect of MgO oxide can be quantiﬁed by studying the var- iation of the amount of Q

ⁿ

species over the composition. Further- more,

³¹

P MAS NMR is a suitable method to evaluate the Q species distributions in phosphate glasses deducing from their chemical shift.

It is possible to estimate the modiﬁcation resulting from the incorporation of magnesium oxide in the vitreous network by cal- culating the amount of the different modes of connexion of PO

₄

tet- rahedra, Q

²

and Q

¹

, on the base of their chemical composition, using the formula proposed by Van Wazer as shown Table 4 [32]:

CðQ

²

Þ ¼ 100 ð2 RÞ ðaÞ

where R = ([MgO] + [Na

₂

O])/[P

₂

O

₅

], and [MgO], [Na

₂

O], [P

₂

O

₅

] are the mol% of the corresponding oxides [32].

The average chain length, deﬁned as the number of P atoms in the chain can also be calculated by the expression proposed by Van Wazer [32]:

ð1=L

CH

Þ ¼ ðR 1Þ=2 ðbÞ

where R is the molar ratio deﬁned above.

The relative concentrations of the Q

ⁿ

units obtained by integrat- ing the peak surface obtained from the solid state NMR analysis of the full spectra are mentioned in Table 4. The area of each isotropic peak attributed to Q

²

and Q

¹

tetrahedral sites were calculated in order to get information about their relative fraction. Calculations are listed in Table 4. These results show that metaphosphate chains became shorter as MgO oxide is introduced over all the composi- tion range.

3.5. Calorimetric study of glasses

As mentioned previously, the calorimetric study was performed by determining the energy resulting from the dissolution of the glasses in a suitable solvent.

Phosphate glasses are soluble in mineral acids [23,24,34,35].

However, for the sake of comparison, the dissolution should be complete and give rise to no secondary phenomena. Preliminary experiments performed by recording the calorimetric proﬁle of dissolution in various solvents (such as: HNO

3

, HCl, NaOH, KOH, CH

3

COOH) showed that the 4.5% weight of phosphoric acid solu- tion is the most suitable solvent [23,24].

Complementary experiments were realized in order to deter- mine the relative error on the integrated area using a statistical cal- culation method developed by Pattengill et al. [36].

The plot of molar heat dissolution ( D

sol

H (kJ mol

¹

)) is shown in Fig. 7. We can notice that the dissolution phenomenon is endother- mic for the low MgO content glasses and becomes exothermic when x increases. It seems that the decrease in thermal signs is associated to the progressive depolymerization of metaphosphate groups (Q

²

) suggesting the formation of pyrophosphate units (Q

¹

) as magnesium oxide is gradually incorporated in the structure.

4. Conclusion

In the present work, spectroscopic investigations such as: FTIR, Raman and

³¹

PNMR revealed the formation of pyrophosphate groups with increasing MgO content. This result can be correlated to the shortening of phosphate chains indicating the depolymeriza- tion of the structure. When MgO is incorporated, phosphate glasses strengthened because covalent bond P A O A P is replaced by more ionic bond P A O A Mg suggesting the compactness of the vitreous network. A Similar behavior have been observed with manganese and zinc phosphate glasses with a general formula: (50x/

2)Na

2

O–xMO–(50x/2)P

2

O

5

(M A Mn or Zn) when x varied from 0 to 33 mol% [23,24].

Calorimetric study shows that the dissolution phenomenon is endothermic for lower MgO content and become exothermic when x is larger than 20 mol%. This result is probably related to the struc- tural modiﬁcation of metaphosphate chains when the O/P ratio increases from 0 to 3.75.

Table 4

Chemical shifts (ppm relative to 85% H

3

PO

4

) and relative concentration of the Q

ⁿ

units determined by

³¹

P MAS-NMR and distribution of different Q species (%) in (50x/2)Na

2

O–

xMgO–(50x/2)P

2

O

5

glasses from their chemical composition (Cc) (with C(Q

¹

) = 1C(Q

²

)).

Composition Q

²

(±0.2 ppm) Q

²

Fraction ±2% C (Q

²

) Q

¹

(±0.2 ppm) Q

¹

Fraction ±2% C (Q

¹

) L

CH

50Na

2

O–50P

2

O

5

20.02 0.99 1 +1.35 0.01 0 1

45Na

2

O–10MgO–45P

2

O

5

19.55 0.74 0.78 4.62 0.26 0.22 9

40Na

2

O–20MgO–40P

2

O

5

19.28 0.47 0.50 5.20 0.53 0.50 4

37.5Na

2

O–25MgO–37.5P

2

O

5

18.91 0.32 0.33 4.59 0.68 0.67 3

35Na

2

O–30MgO–35P

2

O

5

18.38 0.03 0.14 4.85 0.97 0.86 2

30Na

2

O–40MgO–30P

2

O

5

19.92 0.01 0 4.71 0.99 1 2

26.8Na

2

O–42.8MgO–26.8P

2

O

5

– 0 0 4.75 1 1 1

Fig. 7. Evolution of molar heat dissolution of (50x/2)Na

2

O–xMgO–(50x/2)P

2

O

5

(0 6 x 6 42.8 mol%) glasses over MgO content.

(6)

A similar behavior has been obtained with manganese and zinc phosphate glasses having a general formula: (50x/2)Na

2

O–xZnO–

(50x/2)P

2

O

5

, (50x/2)Na

2

O–xMnO–(50x/2)P

2

O

5

and (50x)Na

₂

O–xMnO–50P

₂

O

₅

(0 6 x 6 33 mol%) [23,24].

Acknowledgements

The authors are indebted to Morocco-Tunisian cooperation and AUF programs for ﬁnancial support and to Professor M. Fourati from Sfax University for spectroscopic recordings.

Structural and thermochemical properties of sodium magnesium phosphate glasses