Structural study of phosphorus oxynitride glasses LiNaPbPON by nuclear magnetic resonance and X-ray photoelectron spectroscopy

Structural study of phosphorus oxynitride glasses LiNaPbPON by nuclear magnetic resonance and X-ray photoelectron spectroscopy

Journal of Non-Crystalline Solids 324 (2003) 142–149 www.elsevier.com/locate/jnoncrysol Structural study of phosphorus oxynitride glasses LiNaPbPON b...

236KB Sizes 0 Downloads 44 Views

Journal of Non-Crystalline Solids 324 (2003) 142–149 www.elsevier.com/locate/jnoncrysol

Structural study of phosphorus oxynitride glasses LiNaPbPON by nuclear magnetic resonance and X-ray photoelectron spectroscopy Francisco Mu~ noz a,*, Luis Pascual a, Alicia Dur an a, Lionel Montagne b, Gerard Palavit b, Rene Berjoan c, Roger Marchand d a

b

d

Instituto de Cer amica y Vidrio (CSIC), 28500 Arganda del Rey, Madrid, Spain Laboratoire de Cristallochimie et Physicochimie du Solide, ENSCL, 59655 Villeneuve d’Ascq, France c IMP-UPR CNRS 8521, Odeillo, BP 5, 66125 Font-Romeu, France Laboratoire Verres et C eramiques, UMR CNRS 6512, Universit e de Rennes I, Rennes cedex, France Received 25 January 2002; received in revised form 8 November 2002

Abstract The structure of Li0:25 Na0:25 Pb0:25 PO33x=2 Nx ð0 < x 6 0:69Þ nitrided glasses has been studied by nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS). 31 P magic angle spinning (MAS) NMR shows that the glass network is built up of PO4 , PO3 N and PO2 N2 tetrahedral units. PO4 –PO4 , PO4 –PO3 N and PO3 N–PO2 N2 connections, characterized with double quantum (DQ) 31 P MAS NMR, are homogeneously distributed within the glass network. XPS at the N1s core level indicates that nitrogen atoms substitute for bridging (–O–) and terminal (@O) oxygen atoms, as two-coordinated –N ¼ (Nd ) or three-coordinated –N < (Nt ) species. A higher amount of Nd than Nt species is found for the highest nitrogen contents. 23 Na and 207 Pb NMR results show that the oxygen atoms coordinating modifier cations are not concerned by the nitrogen–oxygen substitution. The role played by lead in these ÔLiNaPbPONÕ glasses can be deduced from a comparison with similar ÔLiNaPONÕ glasses. The presence of Pb2þ ions clearly affects the nitridation mechanism and their network former character increases with nitridation. Ó 2003 Elsevier B.V. All rights reserved.

1. Introduction Phosphate glasses receive great interest as sealing materials [1–5]. Their particular thermal *

Corresponding author. Address: Instituto de Ceramica y Vidrio (CSIC), Campus de Cantoblanco-Universidad Aut onoma de Madrid, 28049 Madrid, Spain. Tel.: +34-917 35 5840; fax: +34-917 35 5843. E-mail address: [email protected] (F. Mu~ noz).

properties, like higher thermal expansion coefficients and lower softening points than borate and silicate glasses, make them very useful for low temperature seals. However, they have a poor chemical durability that limits their use. As reported as early as 1983 by Marchand [6], nitridation is an efficient way to improve the chemical durability of phosphate glasses, without great modification of their original thermal properties, contrary to what is observed when MO or M2 O3

0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-3093(03)00171-6

F. Mu~noz et al. / Journal of Non-Crystalline Solids 324 (2003) 142–149

oxides (M ¼ Mg, Ba, Al, Fe) are added in the glass formulation. Nitridation of phosphate glasses with ammonia gas has been mainly described for alkali metaphosphate glass compositions [7,8] as those compositions exhibit a low liquidus temperature and a very low viscosity in the melt. The nitridation process is controlled by diffusion of ammonia throughout the molten glass; consequently the viscosity of the melt determines the nitridation rate and can also affect the maximum nitrogen content reached. The higher field strength of divalent compared to monovalent cations results in stronger bonds with oxygen, and higher values of melt viscosity. This is why introduction of divalent metal oxides, such as BaO [9], in the glass formulation generally leads to a limited nitridation. PbO is an exception as it provides a low melt viscosity and thus enables incorporation of high nitrogen contents, as previously demonstrated by Pascual et al. [9,10]. Moreover, PbO improves the chemical durability of phosphate glasses while maintaining low softening points [11]. The aim of this paper is to report a structural study of lead-containing phosphorus oxynitride glasses, Li0:25 Na0:25 Pb0:25 PO33x=2 Nx ð0 < x 6 0:69Þ, this formulation being obtained, with respect to an alkali metaphosphate glass network, by replacing half of the alkali metal cations by lead. In a first study of those glasses [12], we showed that their nitrogen content increases linearly as a function of temperature for a fixed treatment time of 3 h, under NH3 flow. At a constant temperature, the nitrogen content increases as a function of t1=2 up to a maximum, then remains constant for longer times. A limited increase in glass transition temperatures and dilatometric softening points, and decrease in thermal expansion coefficients with increasing nitrogen is observed. Above all, an important increase in the chemical durability is found at high N/P ratios, making these glasses potential candidates as low temperature sealing materials [12]. In the present study, 31 P magic angle spinning (MAS) and double quantum (DQ) MAS nuclear magnetic resonance (NMR) have been used to characterize the structural units formed after nitridation, such as PO3 N and PO2 N2 tetrahedra, and

143

23

Na and 207 Pb MAS NMR to observe structural changes in the cation environments. In addition, N1s XPS spectra have been recorded to quantify the proportion of Nd and Nt nitrogen species with increasing nitrogen content.

2. Experimental procedures A base glass of composition, in mol%, 12.5Li2 O  12.5Na2 O  25PbO  50P2 O5 (labelled ÔLiNaPbPOÕ) was prepared by melting reagent grade Li2 CO3 , Na2 CO3 , Pb3 O4 and H3 PO4 (85 wt%, d ¼ 1:7 g cm3 ) in a gas furnace. Phosphorus oxynitride glasses Li0:25 Na0:25 Pb0:25 PO33x=2 Nx (labelled ÔLiNaPbPONÕ) were prepared by thermal treatment of the base glass in anhydrous ammonia atmosphere, at temperatures ranging from 600 to 750 °C, and for times up to 30 h, as previously described [12]. Nitrogen analyses were carried out in a nitrogen/oxygen LECO analyzer by the inert gas fusion method. The N/P ratio is expressed by the x value in the glass formulation Li0:25 Na0:25 Pb0:25 PO33x=2 Nx ð0 < x 6 0:69Þ. 31 P MAS NMR spectra were recorded on a Bruker ASX 100 spectrometer operating at 40.48 MHz (2.34 T). The pulse length was 1.3 ls ðp=4Þ and 120 s delay time was used. The spinning rate was 12 kHz. The 31 P spectra were fitted to Gaussian functions, in accordance with the chemical shift distribution of the amorphous state. The precision of the relative component determination was 5%. A Qn (M) notation is used: n is the number of bridging oxygen atoms in a PO4 tetrahedron, (M) refers to the cation(s) bonded to non-bridging oxygen(s). 23 Na NMR spectra were recorded on an ASX 400 spectrometer (9.4 T), at a Larmor frequency of 105.8 MHz, using 2 ls pulse length and 5 s repetition delay. The chemical shift references were 85% H3 PO4 for 31 P, and 1 M aqueous NaCl solution for 23 Na. All NMR vertical scales in this paper are signal amplitude, normalized to the highest peak. 31 P DQ MAS NMR measurements were performed at 2.34 T, using a previously described procedure [13,14]. A 4 mm MAS probe was used at a MAS frequency of 12.5 kHz. The back to back (BaBa) sequence was applied, using time

144

F. Mu~noz et al. / Journal of Non-Crystalline Solids 324 (2003) 142–149

proportional phase increment (TPPI) [13–15]. A 3 ls pulse was used ðp=2Þ, and a 5 s recycling delay. 64 t1 increments of 80 ls were used, with 32 accumulations per slice. The contour plots are drawn with linear scale from 5% or 10%, to 70% or 80%, each 5–7%. 207 Pb NMR spectra were obtained on a Bruker ASX 400 spectrometer. The large electronic shield around the lead nuclei results in a large chemical shift anisotropy (CSA). Hence, the free induction decays have to be recorded from echoes measured at three frequency offsets. Then, three echoes are summed to obtain the whole spectrum. This method, called variable offsets cumulative spectrum (VOCS) was developed recently for nuclei with large CSA [16]. The 207 Pb frequency is 83.69 MHz at 9.4 T. Static echoes were obtained with a ½p=2–s–p pulse sequence, with p=2 ¼ 3 ls and s ¼ 30 ls. The delay time was 30 s, sufficient to avoid signal saturation, and the number of scans was 400. A 0.5 M Pb(NO3 )2 solution was used as a secondary reference for 207 Pb (d ¼ 2941 ppm vs. Pb(CH4 )4 at 0 ppm). XPS measurements were performed on an electron spectrometer for multi-technique surface analysis system. This system is equipped with a double stage cylindrical mirror electron energy analyzer. The photon source was a Cameca SCX 700 dual anode X-ray source. A non-monochromatized AlKa X-ray source (hm ¼ 1486:6 eV) was used as the excitation source in all cases. A spectrometer energy calibration was made by using the Au 4f7=2 and Cu 2p3=2 photoelectron lines. Due to surface electrostatic charges detected for some samples and resulting in variable retarding effects, all the energy scales corresponding to the XPS spectra reported in this paper were normalized from the energy position of the C1s photoelectron line of atmospheric carbon (CH2 )n : 285 eV. The photoemission measurements were carried out on a series of samples before and after removing the surface carbon contamination by ion sputtering. The sputtering was performed for 10 min in the ultra high vacuum (UHV) analysis chamber with a 0.6 keV Arþ ion beam (30 mA cm2 ). The elemental composition of the samples was evaluated using the integrated areas of the core-level peaks O1s , C1s , P2p , Pb4f , Na1s and N1s . The Li atomic

concentration could not be measured due to the very low value of the ionization cross-section for this element.

3. Results 3.1.

31

P MAS and Double Quantum MAS NMR

31

P MAS NMR spectra of Li0:25 Na0:25 Pb0:25 PO33x=2 Nx glasses ð0 6 x 6 0:69Þ are given in Fig. 1. The base glass spectrum ðx ¼ 0Þ shows a main resonance at )23 ppm, attributed to Q2 -type sites, a shift very close to that obtained by Sato et al. [17] in alkali metaphosphate glasses. The spectra show in addition a weak resonance at )8.5 ppm, that is assigned to a small amount of Q1 -type sites in accordance with [18], thus indicating a small deviation of the base glass composition from the metaphosphate one. The Q2 chemical shift value is in accordance with an average distribution of the cations around the Q2 sites. Since the Q2 (Na) chemical shift is )19 ppm, Q2 (Li) )23 ppm, and Q2 (Pb) )24.5 ppm, we get a Q2 (Na,Li,Pb) average value of )22.8 ppm for a Li0:25 - Na0:25 Pb0:25 PO3 glass composition, in agreement with the experimental value of )23 ppm. Nitridation of glasses up to x ¼ 0:27 results in an increase in intensity of the )8.5 ppm resonance (Fig. 1), that is now attributed to the formation of PO3 N sites, according to previous results obtained

Fig. 1. 31 P MAS NMR spectra of Li0:25 Na0:25 Pb0:25 PO33x=2 Nx glasses ð0 6 x 6 0:69Þ.

F. Mu~noz et al. / Journal of Non-Crystalline Solids 324 (2003) 142–149

on nitrided 25Li2 O  25Na2 O  50P2 O5 [19] and 50Na2 O–50P2 O5 [20] glass compositions. For those glasses, the PO3 N resonance was located at )10 ppm. The )8.5 ppm value found in our case can be explained by a shielding effect of Pb2þ . The 31 P spectra corresponding to x ¼ 0:5 and 0.69 show a broad resonance centered on )5 ppm, and a smaller one at ca. )20 ppm. The latter is attributed to Q2 sites, the deshielded chemical shift indicating that they are involved in shorter chains. Although the presence of PO2 N2 sites could be expected, there is no distinct feature for them, even on MAS NMR spectra of nitrogen-rich glasses. The 31 P DQ NMR spectrum of the Li0:25 Na0:25 Pb0:25 PO2:25 N0:5 glass composition is shown in Fig. 2(a). Q2 sites at )21 ppm (measured in the MAS dimension) are located on the diagonal, what means that they are connected to each other. The two off-diagonal resonances at )22 and )8 ppm can be attributed to Q2 sites connected with PO3 N units, and to PO3 N units connected with Q2 sites, respectively. Although less visible, we can also detect two off-diagonal resonances at )8 and )5 ppm. They are attributed to PO3 N– PO2 N2 correlated units, thus indicating the presence of PO2 N2 units. The 31 P DQ NMR spectrum of the Li0:25 Na0:25 Pb0:25 PO1:97 N0:69 glass composition (Fig. 2(b)) shows that some Q2 –Q2 connections still remain at this high nitrogen content (ondiagonal peak at )21 ppm in the MAS dimension). There is also evidence for Q2 –PO3 N connections (off-diagonal peaks at )21 and )7.5 ppm), and for PO3 N–PO2 N2 connections (off-diagonal peaks at )7.5 and )3 ppm). Although not well resolved in the DQ spectrum, a weak resonance is also detected on the isotropic projection (Fig. 2(b)), with a chemical shift of +4 ppm in the MAS dimension. It is attributed to the presence of Q1 sites of pyrophosphate groups [21]. Although PO3 N and PO2 N2 units are not resolved on 31 P MAS NMR spectra, their presence is therefore asserted from DQ spectra. DQ filtered spectra are not quantitative, but we have the possibility to decompose the 31 P MAS NMR spectra by using chemical shift values of the different sites constrained in ranges measured on the

145

Fig. 2. (a) 31 P double quantum-MAS NMR spectrum of the Li0:25 Na0:25 Pb0:25 PO2:25 N0:5 glass composition. (b) 31 P double quantum-MAS NMR spectrum of the Li0:25 Na0:25 Pb0:25 PO1:97 N0:69 glass composition.

DQ spectra: Q2 : )20 to )23 ppm, PO3 N: )7.5 to )9 ppm, PO2 N2 : )3 to )5 ppm, Q1 (pyrophosphates): 4 ppm. Fig. 3 displays the relative

146

F. Mu~noz et al. / Journal of Non-Crystalline Solids 324 (2003) 142–149

3.3.

100

Pb MAS NMR

error

90

20

The 207 Pb NMR static spectra (Fig. 4) show a broadening and a shift towards less negative chemical shift values with increasing nitrogen content. The bands are centered on )2500 ppm, which corresponds to typical chemical shifts of lead cations with high coordination number, from 7 to 12, and to Pb–O bonds with dominating ionic character [22]. Let us note that the spectra of Fig. 4 are similar to those of ð1  xÞPbO–xP2 O5 glasses [23].

10

3.4. XPS results

PO4 (Q2) PO3 N PO 2 N2 Pyrophosphate (Q1)

80 70

% groups

207

60 50 40 30

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x Fig. 3. Relative proportions of P(O,N)4 groups in Li0:25 Na0:25 Pb0:25 PO33x=2 Nx glasses versus nitrogen content, x, obtained from 31 P MAS NMR spectra decomposition. Lines are drawn as a guide to the eye.

proportions of P(O,N)4 groups with increasing nitrogen content. PO2 N2 tetrahedra are clearly present only from x ¼ 0:27 and their proportion never reaches that of PO3 N tetrahedra, even at the highest nitrogen content. 3.2.

23

Na MAS NMR

In Fig. 5, two representative N1s XPS spectra of nitrided glasses with x ¼ 0:11 and 0.69 are shown. The spectra were deconvoluted into two Gaussian curves with a Lorentzian contribution to the lineshape of the components centered on 397.5 and 399.4 eV, assigned to –N ¼ (Nd ) and –N < (Nt ) nitrogen species, respectively, on the basis of the P3 N5 Hx spectrum [24] and other nitrided metaphosphate glasses [25–27]. No NHx species were detected after ion beam sputtering of the samples. Table 1 shows the N/P ratios ð¼ xÞ obtained from a LECO elemental analysis, those measured by XPS and the O/P ratios obtained from O1s XPS spectra. The results confirm a good agreement between nitrogen contents measured by different methods and corroborate the general glass formulation.

23

Na MAS NMR spectra (not shown) of Li0:25 Na0:25 Pb0:25 PO33x=2 Nx glasses ð0 6 x 6 0:69Þ have an asymmetric lineshape, due to the distribution of chemical shifts and of quadrupolar constants. The chemical shift at the largest amplitude of the non-nitrided glass spectrum is )9 ppm, which is smaller than that observed for NaPO3 glass, )9.9 ppm [17], and for Li0:5 Na0:5 PO3 glass, )12.9 ppm [17]. The presence of Pb2þ ions in the glass composition affects the non-bridging oxygen charge in the phosphate groups, thus modifying the shielding on sodium ions. Whereas 23 Na NMR spectra are sensitive to modifications of the glass network condensation [18] and of the cationic bonding to non-bridging oxygens [17], there is no significant effect of the nitridation on our 23 Na NMR spectra.

Fig. 4. 207 Pb NMR static VOCS-echoes Li0:25 Na0:25 Pb0:25 PO33x=2 Nx glasses.

spectra

of

F. Mu~noz et al. / Journal of Non-Crystalline Solids 324 (2003) 142–149

147

0 .4 0

N/P=0.69

0 .3 5

N d /P N t /P

0. 3 0

(%)

0 .2 5 0 .2 0 0. 1 5

N/P=0.11

error

0 .1 0 0 .0 5 0. 0 0 0 .0

0 .1

0 .2

0.3

0. 4

0. 5

0. 6

0. 7

x

401

399

397

Fig. 6. Nt /P and Nd /P ratios versus x in Li0:25 Na0:25 Pb0:25 PO33x=2 Nx glasses, measured from N1s XPS spectra deconvolution. Lines are drawn as a guide to the eye.

395

Binding Energy (eV) Fig. 5. N1s XPS spectra of Li0:25 Na0:25 Pb0:25 PO2:84 N0:11 and Li0:25 Na0:25 Pb0:25 PO1:97 N0:69 glasses. The continue lines are the experimental spectra, the dotted lines are the calculated components form spectra decomposition.

Table 1 N/P ratios (x) in Li0:25 Na0:25 Pb0:25 PO33x=2 Nx glasses measured with LECO analyzer and by XPS N/P LECO

N/P (0.02) XPS

O/P (0.02) XPS

Nd /P

Nt /P

0.11 0.19 0.27 0.38 0.52 0.69

0.11 0.16 0.28 0.34 0.54 0.67

2.99 2.73 2.66 2.54 2.21 2.03

0.04 0.06 0.14 0.16 0.29 0.36

0.07 0.10 0.13 0.17 0.25 0.31

The variation of the Nd /P and Nt /P ratios (Table 1) as a function of x is drawn in Fig. 6. Like in nitrided alkali metaphosphate glasses [19,25], both kinds of nitrogen simultaneously appear from beginning nitridation. At the highest nitrogen content, the proportion of Nd species is higher than Nt , thus indicating a higher stability with increasing nitrogen content.

4. Discussion As shown in Figs. 2 and 3, PO4 , PO3 N and PO2 N2 groups are the three kinds of phosphate tetrahedral units detected in LiNaPbPON glasses by 31 P MAS and DQ MAS NMR spectra. In contrast with PO4 and PO2 N2 , the proportion of PO3 N tetrahedra remains approximately constant from x ¼ 0:27; a similar result was previously observed in ÔLiNaPONÕ [19] and ÔNaPONÕ [20] glasses. However, in the ÔLiNaPbPONÕ glasses, some distinct features can be underlined: (i) a fast initial increase of PO3 N units with increasing x, (ii) a proportion of PO2 N2 units near zero at low x values, while the PO3 N concentration is close to its maximum, (iii) a maximum of 38% of PO3 N units, higher than that observed for nitrided alkali metaphosphate glasses (33%) [19]. In the 31 P DQ NMR spectra of both Li0:25 Na0:25 Pb0:25 PO2:25 N0:5 and Li0:25 Na0:25 Pb0:25 PO1:97 N0:69 glass compositions (Fig. 2), PO4 –PO4 , PO4 –PO3 N and PO3 N–PO2 N2 connections are observed. However, there is no evidence for PO3 N–PO3 N or

148

F. Mu~noz et al. / Journal of Non-Crystalline Solids 324 (2003) 142–149

PO2 N2 –PO2 N2 connections (on-diagonal signal). Hence, DQ MAS NMR enables to conclude that there is an homogeneous distribution of oxynitride (micro)domains within the glass network. According to the nitridation model proposed by Le Sauze et al. [19], the constant number of PO3 N units from x  0:27 implies that PO4 and PO3 N tetrahedra are transformed at the same time into PO3 N and PO2 N2 , respectively. The structure of the oxide base glass consists of –PO4 –PO4 – chains interconnected through the modifier ions, which are coordinated by several oxygen atoms belonging to different PO4 tetrahedra and chains. By comparing the formation of nitrided P(O,N)4 tetrahedra in our ÔLiNaPbPONÕ and in the ÔunleadedÕ ÔLiNaPONÕ glasses previously studied by Le Sauze et al. [19], it clearly appears that the presence of lead affects the nitridation mechanism. Lead atoms, which replace alkali metal cations with respect to a MPO3 metaphosphate composition, have not only a higher ionic radius but also a divalent character. That means that not only numerous PO4 tetrahedra are interacting with lead, but also that each coordinating oxygen atom has a higher partial charge than those surrounding lithium or sodium atoms. A direct consequence should be a weakening of the adjacent P–O bonds in each tetrahedron, in other words a less covalent character of the adjacent P–O bonds. We assume that those bonds are more easily transformed into P–N bonds. Due to their high concentration in the starting glass composition, Pb2þ ions would therefore explain, on one hand, the homogeneous nitridation told before and, on the other hand, the rapid increase of the PO3 N proportion at beginning nitridation, as the amount of PO4 tetrahedra not affected by their presence is relatively small. Let us note that the absence of PO3 N– PO3 N connections implies that two consecutive non-bridging oxygen atoms of a chain cannot belong to the coordination sphere of the same lead atom. So, we think that, at first, PO4 tetrahedra around lead atoms are preferentially nitrided and transformed into PO3 N. When all of these PO4 are converted into PO3 N units, the PO4 tetrahedra whose oxygen atoms are not coordinated with

lead, as well as the previously PO3 N formed, are now substituted, forming new PO3 N and PO2 N2 units. Further studies are in progress on ÔLiNaPbPONÕ glasses with different lead contents in order to make totally clear this influence of lead on the structure and nitridation of phosphate glasses. The N1s XPS spectra and the variation of the Nd and Nt nitrogen species versus total nitrogen content (Fig. 6) also enable a similar comparison between both ÔleadedÕ and ÔunleadedÕ nitrided systems: the Nd proportion is greater in ÔLiNaPbPONÕ than in ÔLiNaPONÕ glasses [19]. This is possibly due to a steric effect resulting from the large size of lead atoms which facilitates the formation of di-coordinated –N@ rather than tricoordinated –N< species. The broadening and the shift observed in 207 Pb NMR spectra when the nitrogen content increases (Fig. 4) indicate that nitrogen affects the lead environment. This could be interpreted by: (i) a lower coordination number of lead atoms, (ii) a more pronounced covalent character of Pb– O bonds, (iii) a possible presence of nitrogen in the first coordination sphere of lead. Nitrogen is already known to induce a decrease in the shielding coefficient, resulting in higher chemical shift values. For instance, the 27 Al chemical shift observed for Al(O,N)4 tetrahedra is 75 ppm for AlO3 N and 95 ppm for AlON3 tetrahedra [28]. However, no effect of the presence of nitrogen was detected in 27 Al MAS NMR spectra of ÔAlPONÕ glasses previously studied [29], and it was concluded that there is no nitrogen in the first coordination sphere of Al atoms. Such a conclusion is also in agreement with results obtained for ÔSiAlONÕ ceramics, in which only formation of Si–N bonds was reported, as no Al–N bonds were detected [28]. Thus, the broadening of the static lead NMR spectra could indicate a modification of the oxygen coordination sphere around lead atoms as a consequence of the nitrogen/oxygen substitution into the glass network. As more nitrogen is introduced within the phosphate network, substituting

F. Mu~noz et al. / Journal of Non-Crystalline Solids 324 (2003) 142–149

for oxygen, less oxygen atoms are available for bonding with lead, which results in a smaller coordination number of lead atoms and in more covalent Pb–O bonds. Hence, a greater network former character can be attributed to lead atoms as the nitrogen content increases. Finally, the fact that no change is observed in the 23 Na NMR spectra confirms that the nonbridging oxygen atoms coordinating modifier cations are not replaced by nitrogen.

5. Conclusions The NMR and XPS structural study of oxynitride compositions Li0:25 Na0:25 Pb0:25 PO33x=2 Nx ð0 < x 6 0:69Þ shows that they are composed of PO4 , PO3 N and PO2 N2 tetrahedral units with twoand three-coordinated nitrogen atoms. The distribution of P(O,N)4 tetrahedra is homogeneous inside the glass network, i.e. without separated microdomains. The experimental results agree with a model in which oxygen atoms belonging to PO4 units around lead atoms are substituted first, inducing a rapid increase in PO3 N units, after that substitution of the other PO4 tetrahedra takes place. The presence of lead prevents formation of PO3 N–PO3 N and PO2 N2 –PO2 N2 connections.The oxygen coordination number around lead atoms decreases as the nitrogen proportion increases, thus forming more covalent Pb–O bonds and leading consequently to a greater network former character of Pb2þ . Nitrogen does not substitute for non-bridging oxygen atoms coordinating modifier cations.

Acknowledgements This work was financed by CICYT through the project MAT (2000-0952-C02-01) and by an Integrated Action CSIC-CNRS 2001. L.M. and G.P. acknowledge B. Revel, from the Ôcentre commun de mesures RMN de lÕUSTLÕ for recording the NMR spectra.

149

References [1] N.H. Ray, C.J. Lewis, J.N.C. Laycock, W.D. Robinson, Glass Technol. 14 (2) (1973) 50. [2] N.H. Ray, J.N.C. Laycock, W.D. Robinson, Glass Technol. 14 (2) (1973) 5. [3] N.H. Ray, R.J. Plaisted, W.D. Robinson, Glass Technol. 17 (2) (1976) 66. [4] Y.B. Peng, D.E. Day, Glass Technol. 32 (5) (1991) 166. [5] Y.B. Peng, D.E. Day, Glass Technol. 32 (6) (1991) 200. [6] R. Marchand, J. Non-Cryst. Solids 56 (1983) 173. [7] M.R. Reidmeyer, M. Rajaram, D.E. Day, J. Non-Cryst. Solids 85 (1986) 186. [8] R.W. Larson, D.E. Day, J. Non-Cryst. Solids 88 (1986) 97. [9] L. Pascual, A. Duran, Mater. Res. Bull. 31 (1) (1996) 77. [10] L. Pascual, A. Duran, Proc. XVII Int. Congr. Glass Beijin (China), vol. 5, 1995, p. 157. [11] Y. He, D.E. Day, Glass Technol. 33 (6) (1992) 214. [12] F. Mu~ noz, L. Pascual, A. Duran, R. Marchand, Phys. Chem. Glasses 43C (2002) 113. [13] M. Feike, R. Graf, I. Schnell, C. J€ager, H.W. Speiss, J. Am. Chem. Soc. 118 (1996) 9631. [14] K.K. Olsen, J.W. Zwanziger, P. Hartmann, C. J€ager, J. Non-Cryst. Solids 222 (1997) 199. [15] R. Ernst, G. Bodenhausen, A. Wokaun, in: Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Clarendon, Oxford, 1987. [16] D. Massiot, I. Farnan, N. Gautier, D. Trumeau, A. Trokiner, J.P. Coutures, Solid State NMR 4 (1995) 241. [17] R.K. Sato, R.J. Kirkpatrick, R.K. Brow, J. Non-Cryst. Solids 143 (1992) 257. [18] R.K. Brow, R.J. Kirkpatrick, G.L. Turner, J. Non-Cryst. Solids 116 (1990) 39. [19] A. Le Sauze, L. Montagne, G. Palavit, F. Fayon, R. Marchand, J. Non-Cryst. Solids 263&264 (2000) 139. [20] B.C. Bunker, D.R. Tallant, C.A. Balfe, R.J. Kirkpatrick, G.L. Turner, M.R. Reidmeyer, J. Am. Ceram. Soc. 70 (9) (1987) 675. [21] S. Prabhakar, K.J. Rao, C.N.R. Rao, Chem. Phys. Lett. 139 (1) (1987) 96. [22] F. Fayon, I. Farnan, C. Bessada, D. Massiot, J.P. Coutures, J. Am. Ceram. Soc. 119 (1997) 6837. [23] F. Fayon, C. Bessada, J.P. Coutures, D. Massiot, Inorg. Chem. 38 (1999) 5212. [24] S. Veprek, S. Iqbal, J. Brunner, M. Scharli, Philos. Mag. 43 (3) (1981) 527. [25] R. Marchand, D. Agliz, L. Boukbir, A. Quemerais, J. NonCryst. Solids 103 (1988) 35. [26] R.K. Brow, M.R. Reidmeyer, D.E. Day, J. Non-Cryst. Solids 99 (1988) 178. [27] B. Wang, B.S. Kwak, B.C. Sales, J.B. Bates, J. Non-Cryst. Solids 183 (1995) 297. [28] M.E. Smith, J. Phys. Chem. 96 (1992) 1444. [29] A. Le Sauze, PhD thesis, University of Rennes 1, France, 1998.