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Glass-forming ability and structure of ZnO–MoO3–P2O5 glasses
Journal of Non-Crystalline Solids 356 (2010) 2509–2516
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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Glass-forming ability and structure of ZnO–MoO3–P2O5 glasses Jiří Šubčík a, Ladislav Koudelka a,⁎, Petr Mošner a, Lionel Montagne b, Gregory Tricot b, Laurent Delevoye b, Ivan Gregora c a b c
Department of General and Inorganic Chemistry, University of Pardubice, Faculty of Chemical Technology, 53210 Pardubice, Czech Republic Unité de Catalyse et Chimie du Solide UMR CNRS 8181, University of Science and Technologies of Lille, National Chemistry School of Lille, 59655 Villeneuve d'Ascq, France Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Prague, Czech Republic
a r t i c l e
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Article history: Received 15 October 2009 Received in revised form 10 February 2010 Available online 2 June 2010 Keywords: Phosphate glasses; Structure; MAS NMR spectra; Raman spectra
a b s t r a c t Glasses in the ternary system ZnO–MoO3–P2O5 were studied. The determined glass-forming region is very large and covers the concentration range from pure P2O5 up to the glass compositions with only 20 mol% P2O5 and 70 mol% MoO3. Structure of these glasses was analyzed in four compositional series containing 50 mol% of P2O5 in the series of (50 − t)ZnO–tMoO3–50P2O5, 10 mol% of ZnO in the series of 10ZnO–yMoO3– (90 − y)P2O5 and 20 mol% MoO3 in the series of zZnO–20MoO3–(80 − z)P2O5. The fourth series contained equimolar ratio of ZnO/P2O5 and its composition was (100 − x)[0.5ZnO–0.5P2O5]–xMoO3. Basic structural units in this ternary system were identified in specific compositional regions using Raman and 31P MAS NMR spectroscopy. By the deconvolution of NMR spectra compositional dependences of Qn units were obtained. The presence of isolated MoO6 octahedra was detected in the glass with 70 mol% P2O5. In the MoO3-rich and ZnO-rich region clustering of MoO6 octahedra was observed, but the formation of Mo–O–Mo bonds was found already in the P2O5-rich glasses containing 60 mol% P2O5. Molybdate structural units are preferentially bonded to non-bridging oxygen atoms of PO4 units and P–O–Mo bonds do not tend to replace P–O–P bonds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Phosphate glasses with additions of tungsten oxide or molybdenum oxide offer prospective applications for electro-optical applications due to their electrochromic properties and high ionic conductivity [1,2], and for the formulation of low Tg glasses for sealing applications [3]. Moreover, the knowledge of molybdenum phosphate interactions in glasses is useful for the management of radioactive wastes. This includes the formulation of molybdenum phosphate glasses [4], and also the understanding of phase separation that occurs in borosilicate glasses containing phosphate and molybdenum wastes [5]. Molybdenum oxide was found to be able to form broad glass-forming regions in several MoO3-containing phosphate glasses. Silver- and lithium-containing molybdato-phosphate glasses were investigated for their prospective applications as ionic conductors [6–10]. Several studies were devoted also to various sodium-containing molybdato-phosphate glasses [11–15]. Glass-forming region in the ternary Na2O–MoO3–P2O5 system was determined by Bih et al. [11] who studied also electrical properties of these glasses. A large glass-forming region includes glasses containing more than 20 mol% P2O5. Bih et al. [12] investigated physicochemical properties of NaPO3–MoO3 glasses and Santagneli et al. [14] studied their structure using NMR spectroscopy of 31P, 23Na and 95Mo
⁎ Corresponding author. Tel.: +420 466037166; fax: +420 466037068. E-mail address: [email protected] (L. Koudelka). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.02.013
nuclei combined with Raman and FTIR spectroscopy. Santagneli et al. [14] prepared homogeneous glasses of (100−x)NaPO3–xMoO3 system in the range of x=0–70 and revealed increasing network connectivity within the compositional range 0≤x≤45 associated with an increase in the glass transition temperature and the formation of isolated phosphate species and molybdenum oxide clusters within the range of 45≤x≤70 associated with decreasing thermal and crystallization stability. Glass-forming region in the Li2O–MoO3–P2O5 system, as determined by Chowdari et al. [15], is somewhat larger than in Na2O-based glasses [11], reaching a little over 80 mol% MoO3. Zinc phosphate glasses xZnO–(100−x)P2O5 were investigated in several papers [16–21]. Glass-forming region in this system is relatively broad depending on the cooling rate of the melt. By slow cooling of the melt xZnO–(100−x)P2O5 glasses can be obtained with x=0–60 mol% ZnO [16], whereas fast quenching enables obtaining glasses with x=60– 80 [17]. Structure of zinc ultraphosphate glasses was studied by Raman and infrared spectroscopy by K. Meyer [18] and by 31P MAS NMR spectroscopy by Mercier et al. [19]. Structure of zinc polyphosphate glasses was studied by Walter et al. [20] using diffraction methods and 31P NMR. All the results showed that in the xZnO–(100−x)P2O5 glasses in ultraphosphate region Q3 structural units dominate and their number decreases with increasing ZnO content, whereas the number of Q2 units increases and reaches a maximum just at x=50 mol%. In the polyphosphate region with increasing x the number of Q2 units decreases, while the number of Q1 units increases. U. Hoppe [21] studied also the coordination of Zn atoms, using neutron and X-ray diffraction, and came
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to the conclusion that in glasses with the molar ratio n(ZnO)/n(P2O5) from 0.5 to 1.0, a change from ZnO6 coordination to ZnO4 causes a decrease in the packing densities. In this work glass formation was investigated in the whole ZnO– P2O5–MoO3 ternary system and the glass-forming region was determined under slow cooling of the melt in air. Structural studies were carried out using Raman and 31P MAS NMR spectroscopy throughout the whole glass-forming region in order to propose a complete view on structural changes in this ternary system. 2. Experimental Glasses of the ternary system ZnO–MoO3–P2O5 were prepared from reagent grade ZnO, H3PO4 and MoO3 in batches of 20 g. In the first stage, the reaction mixture was heated slowly in a platinum crucible up to 600 °C with the final calcination at the maximum temperature for 2 h to remove the water. After the calcination the reaction mixture was heated slowly up to 910–1260 °C, according to the composition. After 20 min heating at the maximum temperature, the melt was cooled by pouring into a graphite mould. The amorphous character of the obtained glasses was checked by X-ray diffraction. The obtained glasses were separately annealed for 15 min at a temperature close to their Tg and then slowly cooled to room temperature. Glass composition was checked by weighing and also by chemical analysis. The changes in the MoO3 content compared to target value were lower than 0.5 mol%. The glass density, ρ, was determined at 25 °C by the Archimedes method using toluene as the immersion liquid. The molar volume VM was calculated as VM = M̄ / ρ, where M̄ is the average molar weight of the glass composition. Glass transition temperature, Tg, and crystallization temperature, Tc, of the glasses were obtained from DTA curves (Netzsch DTA 404 PC) at the heating rate of 10 K min− 1. Crystallization temperature was taken as the onset of the crystallization peak. 31 P MAS NMR spectra were measured using a 9.4 T NMR spectrometer (Larmor frequency 162.3) with a 4 mm probe. The spinning speed was 12.5 kHz. The pulse length was 1.2 μs (π/4), and the recycle delay was 60 s which was sufficient to enable relaxation at this field strength. The Larmor frequency was 162.3 MHz for 31P nuclei. The chemical shift of 31P nuclei is given relative to 85% H3PO4 at 0 ppm. Raman spectra were measured on bulk samples at room temperature. The spectra were recorded in back-scattering geometry under excitation with Ar-laser radiation (514.5 nm) at a power of 5 mW. The spectral slit width was 1.5 cm− 1 and the total integration time was 100 s. 3. Results We have prepared and characterized 30 samples of the ZnO–MoO3– P2O5 ternary system by slow cooling of the corresponding melt in air to room temperature and investigated their amorphous character by X-ray diffraction. The experiments enable to present the glass-forming region on the composition diagram shown in Fig. 1. Open circles denote glassy samples while half-open circles denote glass-crystalline samples and closed circles denote crystalline samples. Glass-forming region in the MoO3–P2O5 glasses was taken from Bridge and Patel [22], who reported glass formation in the range of 0–83 mol% MoO3 and in ZnO–P2O5 system from Koudelka and Mošner [16]. As can be seen, in the ternary system glasses can be obtained even with the P2O5 content as low as 15 mol%. The glass samples containing MoO3 were of blue color showing on the presence of Mo+ V species in the glasses which was confirmed by EPR spectroscopy. The EPR studies also showed that the ratio of Mo+ V/ Mototal is lower than 0.03. Table 1 contains basic characterization data of the prepared glasses — density, molar volume, glass transition temperature, crystallization temperature and their difference Tc −Tg which we take as a measure of the thermal stability of glasses. For structural studies we have chosen four compositional series to facilitate the evaluation of structural changes in the glasses with
Fig. 1. Glass formation region of the ternary system ZnO–P2O5–MoO3.
composition. The first series (A) has equimolar content of ZnO/P2O5 in the series (100 − x)[0.5ZnO–0.5P2O5]–xMoO3. The other three series have a constant content of one of the components, (B) 10 mol% of ZnO in the series of 10ZnO–yMoO3–(90 − y)P2O5, (C) 20 mol% MoO3 in the series of zZnO–20MoO3–(80 − z)P2O5 and (D) 50 mol% of P2O5 in the series of (50 − t)ZnO–tMoO3–50P2O5. All the studied compositional series are marked in the ternary diagram (see Fig. 1). In the (A) series (100−x)[0.5ZnO–0.5P2O5]–xMoO3 (0b xb 70) we were able to prepare glasses in the compositional range of 0–70 mol% MoO3. 31P MAS NMR spectra of the (A) series are shown in Fig. 2. The NMR spectrum of zinc metaphosphate glass (x=0) is characterized by the dominant signal of Q2 units with the chemical shift δ=−31 ppm. A small contribution of Q1 resonance at −5 ppm indicates a small deviation from the metaphosphate composition [23]. When MoO3 is added, new signals appear on the low field side of the spectra (towards positive Table 1 Compositions, density, ρ, molar volume, VM, glass transition temperature, Tg, and crystallization temperature, Tc, of ZnO–MoO3–P2O5 glasses. Samples
ZnO MoO3 P2O5 ρ±0.02 VM ±0.5 Tg ±2 °C Tc ±5 °C Tc −Tg [g cm−3] [cm3 mol− 1] [°C] [°C] [°C] mol%
Series A 50 45 40 35 30 25 20 15 Series B 10 10 10 10 10 10 Series C 10 20 30 40 50 Series D 50 40 30 20 10 –
– 10 20 30 40 50 60 70 20 30 40 50 60 70 20 20 20 20 20 – 10 20 30 40 50
50 45 40 35 30 25 20 15 70 60 50 40 30 20 70 60 50 40 30 50 50 50 50 50 50
2.90 3.08 3.20 3.34 3.44 3.54 3.65 3.78 2.72 2.95 3.03 3.11 3.31 3.56 2.72 2.74 3.00 3.20 3.72 2.90 2.96 3.00 3.02 3.03 3.03
38.6 37.3 36.9 36.4 36.2 36.1 35.9 35.5 50.1 46.2 45.1 44.0 41.4 38.5 50.1 47.5 41.5 36.9 30.1 38.6 39.9 41.5 43.2 45.1 47.2
460 452 448 443 430 413 388 368 – 482 538 487 415 391 – 386 492 448 449 460 440 492 514 536 530
571 – 663 589 537 523 498 425 – – – – – – – – – 663 553 571 – – – – –
111 – 215 146 107 110 110 57 – – – – – – – – – 215 104 111 – – – – –
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Fig. 3. Concentrations of Qn species derived from the deconvolutions of the 31P MAS NMR spectra of the (100 − x)[0.5ZnO–0.5P2O5]–xMoO3 glasses. The lines connecting the points are just guides for the eye. Dashed lines depict the theoretical model explained in the text.
31
Fig. 2. 31P MAS NMR spectra of the (100 − x)[0.5ZnO–0.5P2O5]–xMoO3 glasses.
chemical shifts), indicating a decrease of the phosphate polymerization. The resonances centered on −17 to −20 ppm and on −7 to −10 ppm are assigned to Q1 and Q0 sites, respectively. By fitting the spectra we obtained the evolution of the relative number of structural units in these glasses with composition showed in Fig. 3. Raman spectra of the (A) glass series are shown in Fig. 4. The spectrum of zinc metaphosphate glass is characterized by the dominant band at 1207 cm− 1 (symmetrical stretching vibration of non-bridging oxygen atoms in Q2 units [24]), medium band at 701 cm− 1 (symmetrical stretching vibration of bridging oxygen atoms between Q2 units [24]) and the weak band at 1253 cm− 1 (asymmetrical stretching vibration of non-bridging oxygen atoms in Q2 units [24]). Even a small amount of MoO3, as low as 2.5 mol%, induces an appearance of the bands at 966 and 891 cm− 1, caused by the presence of Mo–O bonds in the glass structure. These bands are assigned to terminal stretching vibration in MoO6 octahedra [7]. We assume that the band at 976 cm− 1 should be ascribed to the symmetric stretching vibration of MoO6 tetrahedra, rather than to the vibrations of Mo–O or Mo O bonds only [7,14]. Characteristic spectral features of phosphate units decrease in intensity with increasing MoO3 content due to a much higher Raman scattering efficiency for Mo–O vibrations than that for P–O vibrations [25]. A high Raman scattering efficiency of the vibrations of molybdenum species is reflected by a rapid increase in the intensity of bands at ∼968 and ∼896 cm− 1 within the concentration region of 0–40 mol% MoO3. The band at ∼ 968 cm− 1 is dominant in the spectra within the concentration region of 10–30 mol% MoO3, but in the spectra of glasses with x ≥ 50 mol% MoO3 the band at 891 cm− 1 broadens and shifts its maximum to lower wavenumbers with increasing MoO3 content. This is due to the progressive formation of interconnected molybdate species with increasing number of interconnected molybdate species in MoO3rich glasses. Characteristic features of phosphate structural units (at ∼705 and 1202 cm− 1) disappear in the spectra of glasses with x ≥ 30 mol% MoO3. The small band at ∼395 cm− 1 has the highest intensity in the glasses with 20–50 mol% MoO3.
P MAS NMR spectra of the (B) series 10ZnO–yMoO3–(90 − y)P2O5 (20 b y b 70) are shown in Fig. 5. The y = 20 spectrum shows resonances at −15, −30 and −45 ppm, which are characteristic of Q1, Q2 and Q3 sites, respectively [26]. NMR spectra show on the steady transformation of structural units in the direction Q3 → Q2 → Q1 → Q0 with increasing MoO3 content. In the spectrum of the glasses containing more than 50 mol% P2O5 we can see two resonances at −12 ppm and −1 ppm which are not apparent in other spectra of this glass series. Their signals are included in the distributions of Qn units as a part of the number of Q1 and Q0 units, respectively. The evolution of the relative number of structural units with composition is shown in Fig. 6. Q2 units dominate the NMR spectra within the region of 20–40 mol% MoO3, but with a
Fig. 4. Raman spectra of the (100 − x)[0.5ZnO–0.5P2O5]–xMoO3 glasses.
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Fig. 7. Raman spectra of the 10ZnO–yMoO3–(90 − y)P2O5 glasses. Fig. 5. 31P MAS NMR spectra of the 10ZnO–yMoO3–(90 − y)P2O5 glasses.
further increase of MoO3 content in this glass series, their relative quantity falls rapidly down leading to a temporary maximum in the number of Q1 units which are then replaced by isolated Q0 units. Raman spectra of the (B) series are shown in Fig. 7. In the glass with 20 mol% MoO3 there is a dominant band at 975 cm− 1 accompanied by 2 small bands at 936 and 1007 cm− 1. There are also 2 small bands at 689 and 1189 cm− 1 ascribed to the vibrations of phosphate units νs(P–O–P) and νs(PO2), respectively [24]. A weak band at 403 cm− 1 shifts a little to lower wavenumbers with increasing MoO3 content, and its relative intensity is maximal in the spectra of glasses with 30–50 mol% MoO3. With increasing MoO3 content a new band at 884 cm− 1 appears in the Raman spectra, which broadens and shifts its maximum to lower wavenumbers. Thermal stability of glasses of this series is very high, they do not crystallize on heating and the compositional dependence of the glass transition temperature reaches a maximum at the glass with 50 mol% P2O5 (see Table 1).
Fig. 6. Concentrations of Qn species derived from the deconvolutions of the 31P MAS NMR spectra of the 10ZnO–yMoO3–(90 − y)P2O5 glasses. The lines connecting the points are just guides for the eye.
31 P MAS NMR spectra of the (C) glass series zZnO–20MoO3–(80 − z) P2O5 (20 b z b 50) are shown in Fig. 8. The z = 20 spectrum, shown previously in series B, reveals resonances at −15, −30 and −45 ppm, which are characteristic of Q1, Q2 and Q3 sites, respectively [26]. The spectra show relatively small changes within the concentration region of 10–30 mol% ZnO, where the signal of Q2 units at −31 ppm dominates the spectra. A further increase in ZnO content to 40 and 50 mol% results in an appearance of signals of Q1 and Q0 units, and a rapid decrease in the number of Q2 units. The changes in the relative number of Qn units in this glass series are shown in Fig. 9.
Fig. 8. 31P MAS NMR spectra of the zZnO–20MoO3–(80 − z)P2O5 glasses.
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Fig. 9. Concentrations of Qn species derived from the deconvolutions of the 31P MAS NMR spectra of the zZnO–20MoO3–(80 − z)P2O5 glasses. The lines connecting the points are just guides for the eye.
Raman spectra of the (C) glass series are shown in Fig. 10. Only small changes in the spectra can be observed within the composition region 10–30 mol% ZnO, but drastic changes can be seen in the spectra when ZnO content increases from 30 to 50 mol% and P2O5 content decreases from 50 to 30 mol%. The most pronounced change is a dramatic increase in the intensity of the broad Raman band at 889 cm− 1. The Raman band characteristic of vibrations of Q2 units at 1191 cm− 1, νs(PO2), reaches its maximum at the glass with 30 mol% ZnO. Thermal stability of glasses of this series is very high, they do not crystallize on heating and the compositional dependence of the glass transition temperature reaches a maximum at the glass with 50 mol% P2O5 (see Table 1). 31 P MAS NMR spectra of the (D) series (50−t)ZnO–tMoO3–50P2O5 are shown in Fig. 11. When zinc oxide is replaced by MoO3 in zinc metaphosphate (t=0), new signals at −2 ppm (Q0 units), −17 ppm (Q1 units) and −49 ppm (Q3 units) appear in the spectra. A further resonance appears on the t=50 spectrum, which is tentatively assigned to Q0 with a different quantity of Mo second neighbours than the −2 ppm resonance. We can see a steady decrease of Q2 units and a slow increase of Q1 and Q0 units with increasing MoO3 content (see Fig. 12). Raman spectra of the series (D) are shown in Fig. 13. The spectrum of the parent zinc metaphosphate glass was commented above. With increasing MoO3 content new vibrational bands appear within the region of 800–1000 cm− 1. A dominant band at 970 cm− 1 shifts up to 988 cm− 1 in the spectrum of the glass with 50 mol% MoO3. This band is accompanied by a weak band at 932–938 cm− 1. A middle-strength band at 880–888 cm− 1 increases its relative intensity with increasing MoO3 content. In the Raman spectra of Mo-containing glasses a weak band appears at 399–402 cm− 1, the intensity of which increases also with increasing MoO3 content. In this series thermal stability of glasses with MoO3 is high, as they do not crystallize on heating. Their glass transition temperature values generally tend to increase nonmonotonously with MoO3 content.
Fig. 10. Raman spectra of the zZnO–20MoO3–(80 − z)P2O5 glasses.
broad range. Nevertheless, the resolution of 31P NMR spectra of glasses is limited by the large width of the resonances, due to the distribution of structural parameters like bond length and bond angles. Improved structural information on phosphate glass network structure was obtained by the use of 2D NMR pulse sequences that enable to observe
4. Discussion The data given in Table 1 cover also density and molar volume of the studied glasses. For the observed compositional changes in the density, molar weight of replaced oxides plays the major role. For the changes in the molar volume the changes in the number of atoms affect decisively the observed compositional trends. Previous 31P MAS NMR studies of binary and ternary phosphate glasses [23,26] have shown that it is possible to get quantitative information on structural units formed by phosphorus atoms, because the chemical shift of the different Qn units formed by PO4 tetrahedra are observed over a
Fig. 11.
31
P MAS NMR spectra of the (50 − t)ZnO–tMoO3–50P2O5 glasses.
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Fig. 12. Concentrations of Qn species derived from the deconvolutions of the 31P MAS NMR spectra of the (50 − t)ZnO–tMoO3–50P2O5 glasses. The lines connecting the points are just guides for the eye. 31
P–31P homonuclear and 31P–X (X= 27Al, 11B, and 23Na) heteronuclear correlations. These methods enable to evaluate the spatial proximity of different sites, or their direct connection using through bond correlation detection [27]. For the present study we did not use correlation methods since the resolution of spectra was too low due to overlapping of resonances. We decided to decompose the spectra using the minimum number of individual component that enabled to reproduce the spectrum lineshape. Gaussian lineshape was used, in accordance with the expected distribution of chemical shift in the glasses. The envelope curves of the bands obtained by the decomposition of the 31P MAS NMR spectra coincide with the measured spectra. The individual components were assigned to Qn sites (where n is the number of bridging oxygen of a given PO4 site), although one must notice that they include sites with variable connectivity (i.e. with a variable nature of second neighbours). After the decomposition, the chemical shift and width of the different Qn sites show systematic variation with the glass compositions, which is due to the evolution of bonding on the PO4 sites (covalency, nature of second neighbours, bond length and angles). In the present study, we did not manage to interpret these variations, we limited the discussion to the
Fig. 13. Raman spectra of the (50 − t)ZnO–tMoO3–50P2O5 glasses.
evolution of the proportion of the Qn sites with the composition of the four glass series. A relative fraction of Qn units obtained by the deconvolution of the spectra was estimated with the error of ∼5%. The coordination of molybdenum in the molybdato-phosphate glasses is still a matter of discussion. Chowdari et al. [7] in their studies of the Li2O–P2O5–MoO3 glasses came to the conclusion that in the 50P2O5–50MoO3 glass molybdenum forms MoO6 octahedra characterized in the Raman spectra by the stretching vibration at ∼983 cm− 1. With increasing additions of Li2O the MoO6 octahedra are converted partly to MoO4 tetrahedra characterized by the vibrational band at 945 cm− 1. Santagneli et al. [11] obtained 95Mo NMR spectra for the investigation of molybdenum coordination in (100 − x)NaPO3–xMoO3 glasses and on the basis of chemical shift trends they came to the conclusion that both MoO6 and MoO4 units are present in these glasses. On the other side, Poirier et al. [28] studied (100 − x)NaPO3–xMoO3 glasses by X-ray absorption spectroscopy and from the course of XANES spectra they concluded that Mo atoms form only MoO6 octahedra in these glasses. In the structure of all crystalline compounds described in the P2O5–MoO3 system, i.e. MoO2(PO3)2 [29], (MoO2)2P2O7 [30], MoOPO4 [31] and Mo2O2P4O13 [32] molybdenum atoms form MoO6 octahedra regardless of the molybdenum valence. Glasses of the series 10ZnO–yMoO3–(90−y)P2O5, lying close to the P2O5–MoO3 tie-line, reveal in their Raman spectra a characteristic strong vibrational band at 975–992 cm− 1. We assume that these glasses have similar coordination of Mo atoms as the crystalline compounds with a similar composition and thus we ascribed this strong band to the vibrations of MoO6 octahedra. The observed values are also close to those observed by Morgan and Magruder [33] in the Raman spectra of MoO2 (PO3)2 (975 cm− 1) and (MoO2)2P2O7 (984 cm− 1) glasses containing MoO6 octahedra. As the dominant peak vibration of molybdate units in the Raman spectra of ZnO–MoO3–P2O5 glasses lies close to these values, we suppose that MoO6 units are formed in these glasses throughout the whole glass-forming region. Raman spectra of all the studied glasses containing molybdenum and phosphorus contain also a weak band at 386–403 cm− 1. Its relative strength is the highest at the glasses with a medium content of MoO3 and P2O5 and therefore we ascribe it to the vibration of Mo–O–P bonds in the ZnO–MoO3–P2O5 glasses. MoO6 octahedra reveal a strong tendency towards clustering and only in the glass 10ZnO–70P2O5–20MoO3 we assume the presence of isolated MoO6 octahedra. This conclusion was made on the basis of the absence of Raman band at 884 cm− 1 (Fig. 7), ascribed to the vibrations of Mo–O–Mo bonds [6,14,25]. We suppose that the structure of this glass is built up of phosphate chains interconnected by bridging oxygens or by –O–Mo–Obridges of MoO6 octahedra as well as in Mo2O2P4O13 [32]. The structure of this crystalline compound is indeed formed by MoO6 octahedra linked via PO4 units and has a similar ratio of Mo/P as the P2O5-rich glass. Structural data obtained in our (A) glass series (100−x)[0.5ZnO– 0.5P2O5]–xMoO3 can be compared with those obtained by Santagneli et al. [14] on similar NaPO3–MoO3 glasses. Some common features can be found with our (A) glass series: In the low to medium content of MoO3 (x=0–30 mol% MoO3), the evolution of the relative proportion of Q2 units (Fig. 3) decreases with MoO3 addition in a way similar to [14]. In sodium metaphosphate glasses, the local structure around Mo6+ ions was described as MoO4 tetrahedra in the form of Mo(=O)2(O1/2)2, each species forming two Mo–O–P links to phosphorus. The similarity of our Raman data with those of sodium phosphate glasses enables us to assume similar bonding relations in the studied zinc molybdato-phosphate glasses. In the low to medium content of MoO3 (x=0–30 mol% MoO3) the evolution of Qn units in (A) series, corresponds with the model of Santagneli [14], which supposes that the majority of the molybdenum species form MoO4 tetrahedra. The same distribution will be valid also for MoO6 octahedra forming two Mo–O–P bonds. This model results in the Qn distribution shown in Fig. 3 by the dashed line. In the region of high MoO3 content the number of Qn species deviates from the model of Santagnelli [14] in such a way that the number of Q1 units is higher and Q0 lower than predicted by the model. This is due to the higher clustering of MoO6
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octahedra in ZnO-based glasses as confirmed also by the Raman spectra (Fig. 4). Glass transition temperature in the (A) glass series (100−x)[0.5ZnO– 0.5P2O5]–xMoO3 decreases with increasing MoO3 content (see Table 1). This result is in good agreement with the strength of chemical bonds in diatomic molecules of P–O, and Mo–O [34] determined by spectroscopic methods. When we are comparing their strength we can find a higher value for the P–O bonds (599.1 kJ/mol), whereas a lower value was reported for the Mo–O bonds (560.2 kJ/mol). Thus the replacement of P–O bonds by Mo–O bonds results in a decrease of bonding forces in the structural network of this glass series. Comparing the maxima of Q1 units in Qn units evolution of (A) series of our Zn-glasses (see Fig. 3) and in Na-glasses [14] we observe that there is a preferential formation of Q1 units in Na-glasses (maximal portion is 80% of all Qn units). In Zn-glasses, the Q1 units (maximal portion is 60%) tend to be transformed to Q0 units, even if there is still a significant number of Q2 units that could be transformed. It follows that Q2 units possessing stronger covalent bonds with zinc atoms are more stable than those charge-compensated by Na+ ions. Moreover, the much stronger Raman band characteristic of Mo–O–Mo vibrations in Zn-glasses (at 849 cm− 1) compared to that in Na-glasses (at 840 cm− 1) shows a higher degree of clustering of MoO6 octahedra in Zn-glasses. In the P2O5-rich glasses in the series (B) 10ZnO–yMoO3–(90 − y) P2O5 and (C) zZnO–20MoO3–(80 − z)P2O5 (20 b z b 50) metaphosphate (Q2) units dominate the structure of glasses with a high P2O5 content, but some Q3 and Q1 units appear in the structure as well (Figs. 6 and 9). It is known that ultraphosphate glasses prepared in air accommodate certain amount of water in the form of P–OH bonds [23]. The two resonances at − 12 ppm and −1 ppm, which appear only at the spectra of glasses containing more than 50 mol% P2O5 (see Figs. 5 and 6), are probably due to the presence of phosphate units containing various amounts of terminal P–OH bonds. Consequently, the amount of phosphate units with two and three bridging oxygens (Q2 and Q3) obtained from fitting the spectra is lowered as showed by Brow [35]. With a decrease of P2O5 content down to 60 mol% the amount of water in the glass lowers and the oxygen atoms, which were used up for the formation of P–OH bonds can now form P–O–P bridges. That is why the number of Q2 units decreases only very slowly with a decrease of P2O5 content within the range of 70–50 mol% P2O5 in the glasses of the (B) and (C) series. Thermal stability of glasses of these series is high, they do not crystallize on heating and their reaches a maximum at the glass with 50 mol% P2O5. In the MoO3-rich glasses in the (B) series, phosphate chains are interrupted by MoO6 units and thus Q2 and Q3 units are gradually transformed to Q1 and further to isolated Q0 units. Thus in the glass with 70 mol% MoO3 there are nearly no linkages between individual phosphate groups and mostly isolated phosphate units in the form of P(OMo)4 can be found in the glass structure (Fig. 6). This is similar to the structure found in sodium phosphomolybdates [14]. Simultaneously, with increasing MoO3 content a band at 884 cm− 1 appears in the Raman spectra, which shows on the formation of Mo–O–Mo bridges between MoO6 octahedra [6,14,25]. This band broadens and shifts to lower wavenumbers in glasses with more than 50 mol% MoO3, which can be caused by the formation of three-dimensional clusters composed of several MoO6 octahedra interconnected by common edges. In the ZnO-rich part of the ternary diagram in the (C) series zZnO– 20MoO3–(80 −z)P2O5 with the constant MoO3 content within the concentration range of 30–50 mol% ZnO the number of Q2 units steeply decreases as phosphate chains are rapidly interrupted (Fig. 9). At the same time a large increase in the number of Mo–O–Mo bonds takes place as manifested by a significant increase in the intensity of the Raman band at 885–889 cm− 1 (Fig. 10). We suppose that the main reason for the observed steep decrease in the number of Q2 units is a decreasing P2O5 content and a consequent decrease in the number of oxygen atoms available for the formation of Mo–O–P bonds.
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The (D) series (50 − t)ZnO–tMoO3–50P2O5 with the constant P2O5 content is very useful for comparison of the differences in the role of ZnO and MoO3 in the glass structure. From the evolution of Qn phosphate units (Fig. 12) with the replacement of ZnO by MoO3 we can see that the number of Q1 units increases at the expense of Q2 units. We assume that Mo–O–P bonds do not tend to replace P–O–P bonds and MoO6 units are preferentially bonded to non-bridging oxygen atoms on PO4. This conclusion is in good agreement with the lower strength of Mo–O bonds (560.2 kJ/mol) than P–O bonds (599.1 kJ/mol) [34] as discussed above. Nevertheless, increasing number of oxygen atoms in the glass structure (1 O atom of ZnO is replaced by 3 atoms of MoO3) results in the transformation of Q2 units into Q1 units. The presence of small signals characteristic of Q0 and Q3 units is probably due to a partial disproportíonation of Q2 and Q1 units [23]. A relatively high number of oxygen atoms in the glasses with 50 mol% P2O5 results also in a lower degree of clustering of MoO6 units as revealed by a relatively small increase in the intensity of the vibrational band at 880–888 cm− 1 (Fig. 13) ascribed to the vibration of Mo–O–Mo bonds. 5. Conclusions A very large glass-forming region was found in the ternary system ZnO–MoO3–P2O5, which made it possible to analyze structural features in this system by combination of 31P MAS NMR spectroscopy and Raman spectroscopy. In the P2O5-rich region the phosphate chains are interconnected to form three-dimensional network in glasses with more than 50 mol% of P2O5. However these glasses tend to accommodate certain amount of water in form of terminal P–OH bonds which lowers their degree of cross-linking. Each species of MoO3 and ZnO make two Mo–O–P or Zn– O–P bonds with the phosphate units and thereby interrupt phosphate chains. Nevertheless, Mo–O–Mo bonds were found to be already present in glasses with 60 mol% P2O5. In the ZnO-rich region the phosphate network is highly depolymerized by a high number of Zn–O–P covalent bonds. Molybdate polyhedra are mutually interconnected in this region by Mo–O–Mo bonds even in the glasses with a small amount of MoO3. In the MoO3-rich region the glass structure is characterized by the formation of three-dimensional clusters of several MoO6 octahedra interconnected by corners and edges. Remaining phosphate units are isolated and form P(OMo)4 units and thus the number of P–O–Mo bonds created by one MoO3 species is lowered in this concentration region under one bond for a species. Acknowledgements The Czech authors are grateful for the financial support from the research project no. P106/10/0283 of the Grant Agency of Czech Republic, from the research project no. 0021627501 of the Ministry of Education of Czech Republic and the research project KAN301370701 of the Academy of Sciences of the Czech Republic. LM thanks the Feder, the USTL, the Région Nord Pas de Calais and the CNRS for funding of NMR spectrometers. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
I.R. Zakis, A.R. Lusis, Y.L. Lagzdons, J. Non-Cryst. Solids 47 (1982) 267. F. Studer, A. Lebail, B. Raveau, J. Solid State Chem. 63 (1986) 414. B.G. Aitken, US Patent 6432851. R.S. De Oliveira, J.A.C. De Paira, M.A.B. De Araujo, A.S.B. Sombra, Il Nuovo Cimento D 20 (1998) 209. O. Pinet, J.L. Dussossoy, C. David, C. Fillet, J. Nucl. Mater. 377 (2008) 307. B.V.R. Chowdari, R. Gopalakrishnan, S.H. Tang, M.H. Kuok, Solid State Ionics 28–30 (1988) 704. B.V.R. Chowdari, K.L. Tan, W.T. Chia, Mater. Res. Soc. Symp. Proc. 293 (1993). L. Bih, M. El Omari, J.M. Réau, M. Haddad, D. Boudlich, A. Yacoubi, A. Nadiri, Solid State Ionics 132 (2000) 71. L. Bih, A. Nadiri, M. El Omari, A. Yacoubi, M. Haddad, Phys. Chem. Glasses 43 (2002) 153. L. Bih, A. Nadiri, J. Aride, J. Therm. Anal. Calorim. 68 (2002) 965.
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[11] L. Bih, M. El Omari, J.M. Réau, A. Nadiri, A. Jacoubi, M. Haddad, Mater. Lett. 50 (2001) 308. [12] L. Bih, A. Nadiri, Y. El Amraoui, J. Phys. IV France 123 (2005) 165. [13] L. Abbas, L. Bih, A. Nadiri, Y. El Amraoui, D. Mezzane, B. Elouadi, J. Mol. Struct. 876 (2008) 194. [14] S.H. Santagneli, C.C. de Araujo, W. Strojek, H. Eckert, G. Poirier, S.J.L. Ribeiro, Y. Messadeq, J. Phys. Chem. B 111 (2007) 10109. [15] B.V.R. Chowdari, K.L. Tan, W.T. Chia, R. Gopalakrishnan, J. Non-Cryst. Solids 128 (1991) 18. [16] L. Koudelka, P. Mošner, Mater. Lett. 42 (2000) 194. [17] B. Tischendorf, J.U. Otaigbe, J.W. Wiench, M. Pruski, B.C. Sales, J. Non-Cryst. Solids 282 (2001) 147. [18] K. Meyer, J. Non-Cryst. Solids 209 (1997) 227. [19] C. Mercier, L. Montagne, H. Sfihi, G. Palavit, J.C. Boivin, A.P. Legrand, J. Non-Cryst. Solids 224 (1998) 163. [20] G. Walter, U. Hoppe, J. Vogel, G. Carl, P. Hartmann, J. Non-Cryst. Solids 333 (2004) 252. [21] U. Hoppe, G. Walter, G. Carl, J. Neuefeind, A.C. Hannon, J. Non-Cryst. Solids 351 (2005) 1020–1031. [22] B. Bridge, D. Patel, J. Non-Cryst. Solids 91 (1987) 27.
[23] R. Brow, J. Non-Cryst. Solids 263–264 (2000) 1. [24] Ya.S. Bobovich, Opt. Spektroskop. 13 (1962) 459. [25] J. Šubčík, L. Koudelka, P. Mošner, L. Montagne, B. Revel, I. Gregora, J. Non-Cryst. Solids 355 (2009) 970. [26] P. Mustarelli, Phosphorus Res. Bull. 10 (1999) 25. [27] G. Tricot, L. Delevoye, L. Montagne, G. Palavit, Recent advances in solid state NMR of polymeric phosphate materials, in: R. de Jaeger, M. Gleria (Eds.), Inorganic Polymers, Nova Science Publishers, Hauppauge, 2007, p. 663. [28] G. Poirier, F.S. Ottoboni, F.S. Cassanjes, A. Remonte, Y. Messaddeq, S.J.L. Ribeiro, J. Phys. Chem. B112 (2008) 4481. [29] P. Kierkegaard, Arkiv Kemi 18 (1961) 521. [30] P. Kierkegaard, Arkiv Kemi 19 (1961) 1. [31] G.T. Stranford, R.A. Condrate Sr., J. Sol. State Chem. 52 (1984) 248. [32] G. Costentin, A. Leclaire, M.M. Borel, A. Grandin, B. Raveau, Z. Kristallogr. 201 (1992) 53. [33] S.H. Morgan, R.H. Magruder III, J. Am. Ceram. Soc. 73 (3) (1990) 753. [34] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 2001, pp. 9–51. [35] R.K. Brow, R.J. Kirkpatrick, G.L. Turner, J. Non-Cryst. Solids 116 (1990) 39.
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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Glass-forming ability and structure of ZnO–MoO3–P2O5 glasses Jiří Šubčík a, Ladislav Koudelka a,⁎, Petr Mošner a, Lionel Montagne b, Gregory Tricot b, Laurent Delevoye b, Ivan Gregora c a b c
Department of General and Inorganic Chemistry, University of Pardubice, Faculty of Chemical Technology, 53210 Pardubice, Czech Republic Unité de Catalyse et Chimie du Solide UMR CNRS 8181, University of Science and Technologies of Lille, National Chemistry School of Lille, 59655 Villeneuve d'Ascq, France Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Prague, Czech Republic
a r t i c l e
i n f o
Article history: Received 15 October 2009 Received in revised form 10 February 2010 Available online 2 June 2010 Keywords: Phosphate glasses; Structure; MAS NMR spectra; Raman spectra
a b s t r a c t Glasses in the ternary system ZnO–MoO3–P2O5 were studied. The determined glass-forming region is very large and covers the concentration range from pure P2O5 up to the glass compositions with only 20 mol% P2O5 and 70 mol% MoO3. Structure of these glasses was analyzed in four compositional series containing 50 mol% of P2O5 in the series of (50 − t)ZnO–tMoO3–50P2O5, 10 mol% of ZnO in the series of 10ZnO–yMoO3– (90 − y)P2O5 and 20 mol% MoO3 in the series of zZnO–20MoO3–(80 − z)P2O5. The fourth series contained equimolar ratio of ZnO/P2O5 and its composition was (100 − x)[0.5ZnO–0.5P2O5]–xMoO3. Basic structural units in this ternary system were identified in specific compositional regions using Raman and 31P MAS NMR spectroscopy. By the deconvolution of NMR spectra compositional dependences of Qn units were obtained. The presence of isolated MoO6 octahedra was detected in the glass with 70 mol% P2O5. In the MoO3-rich and ZnO-rich region clustering of MoO6 octahedra was observed, but the formation of Mo–O–Mo bonds was found already in the P2O5-rich glasses containing 60 mol% P2O5. Molybdate structural units are preferentially bonded to non-bridging oxygen atoms of PO4 units and P–O–Mo bonds do not tend to replace P–O–P bonds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Phosphate glasses with additions of tungsten oxide or molybdenum oxide offer prospective applications for electro-optical applications due to their electrochromic properties and high ionic conductivity [1,2], and for the formulation of low Tg glasses for sealing applications [3]. Moreover, the knowledge of molybdenum phosphate interactions in glasses is useful for the management of radioactive wastes. This includes the formulation of molybdenum phosphate glasses [4], and also the understanding of phase separation that occurs in borosilicate glasses containing phosphate and molybdenum wastes [5]. Molybdenum oxide was found to be able to form broad glass-forming regions in several MoO3-containing phosphate glasses. Silver- and lithium-containing molybdato-phosphate glasses were investigated for their prospective applications as ionic conductors [6–10]. Several studies were devoted also to various sodium-containing molybdato-phosphate glasses [11–15]. Glass-forming region in the ternary Na2O–MoO3–P2O5 system was determined by Bih et al. [11] who studied also electrical properties of these glasses. A large glass-forming region includes glasses containing more than 20 mol% P2O5. Bih et al. [12] investigated physicochemical properties of NaPO3–MoO3 glasses and Santagneli et al. [14] studied their structure using NMR spectroscopy of 31P, 23Na and 95Mo
⁎ Corresponding author. Tel.: +420 466037166; fax: +420 466037068. E-mail address: [email protected] (L. Koudelka). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.02.013
nuclei combined with Raman and FTIR spectroscopy. Santagneli et al. [14] prepared homogeneous glasses of (100−x)NaPO3–xMoO3 system in the range of x=0–70 and revealed increasing network connectivity within the compositional range 0≤x≤45 associated with an increase in the glass transition temperature and the formation of isolated phosphate species and molybdenum oxide clusters within the range of 45≤x≤70 associated with decreasing thermal and crystallization stability. Glass-forming region in the Li2O–MoO3–P2O5 system, as determined by Chowdari et al. [15], is somewhat larger than in Na2O-based glasses [11], reaching a little over 80 mol% MoO3. Zinc phosphate glasses xZnO–(100−x)P2O5 were investigated in several papers [16–21]. Glass-forming region in this system is relatively broad depending on the cooling rate of the melt. By slow cooling of the melt xZnO–(100−x)P2O5 glasses can be obtained with x=0–60 mol% ZnO [16], whereas fast quenching enables obtaining glasses with x=60– 80 [17]. Structure of zinc ultraphosphate glasses was studied by Raman and infrared spectroscopy by K. Meyer [18] and by 31P MAS NMR spectroscopy by Mercier et al. [19]. Structure of zinc polyphosphate glasses was studied by Walter et al. [20] using diffraction methods and 31P NMR. All the results showed that in the xZnO–(100−x)P2O5 glasses in ultraphosphate region Q3 structural units dominate and their number decreases with increasing ZnO content, whereas the number of Q2 units increases and reaches a maximum just at x=50 mol%. In the polyphosphate region with increasing x the number of Q2 units decreases, while the number of Q1 units increases. U. Hoppe [21] studied also the coordination of Zn atoms, using neutron and X-ray diffraction, and came
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to the conclusion that in glasses with the molar ratio n(ZnO)/n(P2O5) from 0.5 to 1.0, a change from ZnO6 coordination to ZnO4 causes a decrease in the packing densities. In this work glass formation was investigated in the whole ZnO– P2O5–MoO3 ternary system and the glass-forming region was determined under slow cooling of the melt in air. Structural studies were carried out using Raman and 31P MAS NMR spectroscopy throughout the whole glass-forming region in order to propose a complete view on structural changes in this ternary system. 2. Experimental Glasses of the ternary system ZnO–MoO3–P2O5 were prepared from reagent grade ZnO, H3PO4 and MoO3 in batches of 20 g. In the first stage, the reaction mixture was heated slowly in a platinum crucible up to 600 °C with the final calcination at the maximum temperature for 2 h to remove the water. After the calcination the reaction mixture was heated slowly up to 910–1260 °C, according to the composition. After 20 min heating at the maximum temperature, the melt was cooled by pouring into a graphite mould. The amorphous character of the obtained glasses was checked by X-ray diffraction. The obtained glasses were separately annealed for 15 min at a temperature close to their Tg and then slowly cooled to room temperature. Glass composition was checked by weighing and also by chemical analysis. The changes in the MoO3 content compared to target value were lower than 0.5 mol%. The glass density, ρ, was determined at 25 °C by the Archimedes method using toluene as the immersion liquid. The molar volume VM was calculated as VM = M̄ / ρ, where M̄ is the average molar weight of the glass composition. Glass transition temperature, Tg, and crystallization temperature, Tc, of the glasses were obtained from DTA curves (Netzsch DTA 404 PC) at the heating rate of 10 K min− 1. Crystallization temperature was taken as the onset of the crystallization peak. 31 P MAS NMR spectra were measured using a 9.4 T NMR spectrometer (Larmor frequency 162.3) with a 4 mm probe. The spinning speed was 12.5 kHz. The pulse length was 1.2 μs (π/4), and the recycle delay was 60 s which was sufficient to enable relaxation at this field strength. The Larmor frequency was 162.3 MHz for 31P nuclei. The chemical shift of 31P nuclei is given relative to 85% H3PO4 at 0 ppm. Raman spectra were measured on bulk samples at room temperature. The spectra were recorded in back-scattering geometry under excitation with Ar-laser radiation (514.5 nm) at a power of 5 mW. The spectral slit width was 1.5 cm− 1 and the total integration time was 100 s. 3. Results We have prepared and characterized 30 samples of the ZnO–MoO3– P2O5 ternary system by slow cooling of the corresponding melt in air to room temperature and investigated their amorphous character by X-ray diffraction. The experiments enable to present the glass-forming region on the composition diagram shown in Fig. 1. Open circles denote glassy samples while half-open circles denote glass-crystalline samples and closed circles denote crystalline samples. Glass-forming region in the MoO3–P2O5 glasses was taken from Bridge and Patel [22], who reported glass formation in the range of 0–83 mol% MoO3 and in ZnO–P2O5 system from Koudelka and Mošner [16]. As can be seen, in the ternary system glasses can be obtained even with the P2O5 content as low as 15 mol%. The glass samples containing MoO3 were of blue color showing on the presence of Mo+ V species in the glasses which was confirmed by EPR spectroscopy. The EPR studies also showed that the ratio of Mo+ V/ Mototal is lower than 0.03. Table 1 contains basic characterization data of the prepared glasses — density, molar volume, glass transition temperature, crystallization temperature and their difference Tc −Tg which we take as a measure of the thermal stability of glasses. For structural studies we have chosen four compositional series to facilitate the evaluation of structural changes in the glasses with
Fig. 1. Glass formation region of the ternary system ZnO–P2O5–MoO3.
composition. The first series (A) has equimolar content of ZnO/P2O5 in the series (100 − x)[0.5ZnO–0.5P2O5]–xMoO3. The other three series have a constant content of one of the components, (B) 10 mol% of ZnO in the series of 10ZnO–yMoO3–(90 − y)P2O5, (C) 20 mol% MoO3 in the series of zZnO–20MoO3–(80 − z)P2O5 and (D) 50 mol% of P2O5 in the series of (50 − t)ZnO–tMoO3–50P2O5. All the studied compositional series are marked in the ternary diagram (see Fig. 1). In the (A) series (100−x)[0.5ZnO–0.5P2O5]–xMoO3 (0b xb 70) we were able to prepare glasses in the compositional range of 0–70 mol% MoO3. 31P MAS NMR spectra of the (A) series are shown in Fig. 2. The NMR spectrum of zinc metaphosphate glass (x=0) is characterized by the dominant signal of Q2 units with the chemical shift δ=−31 ppm. A small contribution of Q1 resonance at −5 ppm indicates a small deviation from the metaphosphate composition [23]. When MoO3 is added, new signals appear on the low field side of the spectra (towards positive Table 1 Compositions, density, ρ, molar volume, VM, glass transition temperature, Tg, and crystallization temperature, Tc, of ZnO–MoO3–P2O5 glasses. Samples
ZnO MoO3 P2O5 ρ±0.02 VM ±0.5 Tg ±2 °C Tc ±5 °C Tc −Tg [g cm−3] [cm3 mol− 1] [°C] [°C] [°C] mol%
Series A 50 45 40 35 30 25 20 15 Series B 10 10 10 10 10 10 Series C 10 20 30 40 50 Series D 50 40 30 20 10 –
– 10 20 30 40 50 60 70 20 30 40 50 60 70 20 20 20 20 20 – 10 20 30 40 50
50 45 40 35 30 25 20 15 70 60 50 40 30 20 70 60 50 40 30 50 50 50 50 50 50
2.90 3.08 3.20 3.34 3.44 3.54 3.65 3.78 2.72 2.95 3.03 3.11 3.31 3.56 2.72 2.74 3.00 3.20 3.72 2.90 2.96 3.00 3.02 3.03 3.03
38.6 37.3 36.9 36.4 36.2 36.1 35.9 35.5 50.1 46.2 45.1 44.0 41.4 38.5 50.1 47.5 41.5 36.9 30.1 38.6 39.9 41.5 43.2 45.1 47.2
460 452 448 443 430 413 388 368 – 482 538 487 415 391 – 386 492 448 449 460 440 492 514 536 530
571 – 663 589 537 523 498 425 – – – – – – – – – 663 553 571 – – – – –
111 – 215 146 107 110 110 57 – – – – – – – – – 215 104 111 – – – – –
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Fig. 3. Concentrations of Qn species derived from the deconvolutions of the 31P MAS NMR spectra of the (100 − x)[0.5ZnO–0.5P2O5]–xMoO3 glasses. The lines connecting the points are just guides for the eye. Dashed lines depict the theoretical model explained in the text.
31
Fig. 2. 31P MAS NMR spectra of the (100 − x)[0.5ZnO–0.5P2O5]–xMoO3 glasses.
chemical shifts), indicating a decrease of the phosphate polymerization. The resonances centered on −17 to −20 ppm and on −7 to −10 ppm are assigned to Q1 and Q0 sites, respectively. By fitting the spectra we obtained the evolution of the relative number of structural units in these glasses with composition showed in Fig. 3. Raman spectra of the (A) glass series are shown in Fig. 4. The spectrum of zinc metaphosphate glass is characterized by the dominant band at 1207 cm− 1 (symmetrical stretching vibration of non-bridging oxygen atoms in Q2 units [24]), medium band at 701 cm− 1 (symmetrical stretching vibration of bridging oxygen atoms between Q2 units [24]) and the weak band at 1253 cm− 1 (asymmetrical stretching vibration of non-bridging oxygen atoms in Q2 units [24]). Even a small amount of MoO3, as low as 2.5 mol%, induces an appearance of the bands at 966 and 891 cm− 1, caused by the presence of Mo–O bonds in the glass structure. These bands are assigned to terminal stretching vibration in MoO6 octahedra [7]. We assume that the band at 976 cm− 1 should be ascribed to the symmetric stretching vibration of MoO6 tetrahedra, rather than to the vibrations of Mo–O or Mo O bonds only [7,14]. Characteristic spectral features of phosphate units decrease in intensity with increasing MoO3 content due to a much higher Raman scattering efficiency for Mo–O vibrations than that for P–O vibrations [25]. A high Raman scattering efficiency of the vibrations of molybdenum species is reflected by a rapid increase in the intensity of bands at ∼968 and ∼896 cm− 1 within the concentration region of 0–40 mol% MoO3. The band at ∼ 968 cm− 1 is dominant in the spectra within the concentration region of 10–30 mol% MoO3, but in the spectra of glasses with x ≥ 50 mol% MoO3 the band at 891 cm− 1 broadens and shifts its maximum to lower wavenumbers with increasing MoO3 content. This is due to the progressive formation of interconnected molybdate species with increasing number of interconnected molybdate species in MoO3rich glasses. Characteristic features of phosphate structural units (at ∼705 and 1202 cm− 1) disappear in the spectra of glasses with x ≥ 30 mol% MoO3. The small band at ∼395 cm− 1 has the highest intensity in the glasses with 20–50 mol% MoO3.
P MAS NMR spectra of the (B) series 10ZnO–yMoO3–(90 − y)P2O5 (20 b y b 70) are shown in Fig. 5. The y = 20 spectrum shows resonances at −15, −30 and −45 ppm, which are characteristic of Q1, Q2 and Q3 sites, respectively [26]. NMR spectra show on the steady transformation of structural units in the direction Q3 → Q2 → Q1 → Q0 with increasing MoO3 content. In the spectrum of the glasses containing more than 50 mol% P2O5 we can see two resonances at −12 ppm and −1 ppm which are not apparent in other spectra of this glass series. Their signals are included in the distributions of Qn units as a part of the number of Q1 and Q0 units, respectively. The evolution of the relative number of structural units with composition is shown in Fig. 6. Q2 units dominate the NMR spectra within the region of 20–40 mol% MoO3, but with a
Fig. 4. Raman spectra of the (100 − x)[0.5ZnO–0.5P2O5]–xMoO3 glasses.
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Fig. 7. Raman spectra of the 10ZnO–yMoO3–(90 − y)P2O5 glasses. Fig. 5. 31P MAS NMR spectra of the 10ZnO–yMoO3–(90 − y)P2O5 glasses.
further increase of MoO3 content in this glass series, their relative quantity falls rapidly down leading to a temporary maximum in the number of Q1 units which are then replaced by isolated Q0 units. Raman spectra of the (B) series are shown in Fig. 7. In the glass with 20 mol% MoO3 there is a dominant band at 975 cm− 1 accompanied by 2 small bands at 936 and 1007 cm− 1. There are also 2 small bands at 689 and 1189 cm− 1 ascribed to the vibrations of phosphate units νs(P–O–P) and νs(PO2), respectively [24]. A weak band at 403 cm− 1 shifts a little to lower wavenumbers with increasing MoO3 content, and its relative intensity is maximal in the spectra of glasses with 30–50 mol% MoO3. With increasing MoO3 content a new band at 884 cm− 1 appears in the Raman spectra, which broadens and shifts its maximum to lower wavenumbers. Thermal stability of glasses of this series is very high, they do not crystallize on heating and the compositional dependence of the glass transition temperature reaches a maximum at the glass with 50 mol% P2O5 (see Table 1).
Fig. 6. Concentrations of Qn species derived from the deconvolutions of the 31P MAS NMR spectra of the 10ZnO–yMoO3–(90 − y)P2O5 glasses. The lines connecting the points are just guides for the eye.
31 P MAS NMR spectra of the (C) glass series zZnO–20MoO3–(80 − z) P2O5 (20 b z b 50) are shown in Fig. 8. The z = 20 spectrum, shown previously in series B, reveals resonances at −15, −30 and −45 ppm, which are characteristic of Q1, Q2 and Q3 sites, respectively [26]. The spectra show relatively small changes within the concentration region of 10–30 mol% ZnO, where the signal of Q2 units at −31 ppm dominates the spectra. A further increase in ZnO content to 40 and 50 mol% results in an appearance of signals of Q1 and Q0 units, and a rapid decrease in the number of Q2 units. The changes in the relative number of Qn units in this glass series are shown in Fig. 9.
Fig. 8. 31P MAS NMR spectra of the zZnO–20MoO3–(80 − z)P2O5 glasses.
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Fig. 9. Concentrations of Qn species derived from the deconvolutions of the 31P MAS NMR spectra of the zZnO–20MoO3–(80 − z)P2O5 glasses. The lines connecting the points are just guides for the eye.
Raman spectra of the (C) glass series are shown in Fig. 10. Only small changes in the spectra can be observed within the composition region 10–30 mol% ZnO, but drastic changes can be seen in the spectra when ZnO content increases from 30 to 50 mol% and P2O5 content decreases from 50 to 30 mol%. The most pronounced change is a dramatic increase in the intensity of the broad Raman band at 889 cm− 1. The Raman band characteristic of vibrations of Q2 units at 1191 cm− 1, νs(PO2), reaches its maximum at the glass with 30 mol% ZnO. Thermal stability of glasses of this series is very high, they do not crystallize on heating and the compositional dependence of the glass transition temperature reaches a maximum at the glass with 50 mol% P2O5 (see Table 1). 31 P MAS NMR spectra of the (D) series (50−t)ZnO–tMoO3–50P2O5 are shown in Fig. 11. When zinc oxide is replaced by MoO3 in zinc metaphosphate (t=0), new signals at −2 ppm (Q0 units), −17 ppm (Q1 units) and −49 ppm (Q3 units) appear in the spectra. A further resonance appears on the t=50 spectrum, which is tentatively assigned to Q0 with a different quantity of Mo second neighbours than the −2 ppm resonance. We can see a steady decrease of Q2 units and a slow increase of Q1 and Q0 units with increasing MoO3 content (see Fig. 12). Raman spectra of the series (D) are shown in Fig. 13. The spectrum of the parent zinc metaphosphate glass was commented above. With increasing MoO3 content new vibrational bands appear within the region of 800–1000 cm− 1. A dominant band at 970 cm− 1 shifts up to 988 cm− 1 in the spectrum of the glass with 50 mol% MoO3. This band is accompanied by a weak band at 932–938 cm− 1. A middle-strength band at 880–888 cm− 1 increases its relative intensity with increasing MoO3 content. In the Raman spectra of Mo-containing glasses a weak band appears at 399–402 cm− 1, the intensity of which increases also with increasing MoO3 content. In this series thermal stability of glasses with MoO3 is high, as they do not crystallize on heating. Their glass transition temperature values generally tend to increase nonmonotonously with MoO3 content.
Fig. 10. Raman spectra of the zZnO–20MoO3–(80 − z)P2O5 glasses.
broad range. Nevertheless, the resolution of 31P NMR spectra of glasses is limited by the large width of the resonances, due to the distribution of structural parameters like bond length and bond angles. Improved structural information on phosphate glass network structure was obtained by the use of 2D NMR pulse sequences that enable to observe
4. Discussion The data given in Table 1 cover also density and molar volume of the studied glasses. For the observed compositional changes in the density, molar weight of replaced oxides plays the major role. For the changes in the molar volume the changes in the number of atoms affect decisively the observed compositional trends. Previous 31P MAS NMR studies of binary and ternary phosphate glasses [23,26] have shown that it is possible to get quantitative information on structural units formed by phosphorus atoms, because the chemical shift of the different Qn units formed by PO4 tetrahedra are observed over a
Fig. 11.
31
P MAS NMR spectra of the (50 − t)ZnO–tMoO3–50P2O5 glasses.
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Fig. 12. Concentrations of Qn species derived from the deconvolutions of the 31P MAS NMR spectra of the (50 − t)ZnO–tMoO3–50P2O5 glasses. The lines connecting the points are just guides for the eye. 31
P–31P homonuclear and 31P–X (X= 27Al, 11B, and 23Na) heteronuclear correlations. These methods enable to evaluate the spatial proximity of different sites, or their direct connection using through bond correlation detection [27]. For the present study we did not use correlation methods since the resolution of spectra was too low due to overlapping of resonances. We decided to decompose the spectra using the minimum number of individual component that enabled to reproduce the spectrum lineshape. Gaussian lineshape was used, in accordance with the expected distribution of chemical shift in the glasses. The envelope curves of the bands obtained by the decomposition of the 31P MAS NMR spectra coincide with the measured spectra. The individual components were assigned to Qn sites (where n is the number of bridging oxygen of a given PO4 site), although one must notice that they include sites with variable connectivity (i.e. with a variable nature of second neighbours). After the decomposition, the chemical shift and width of the different Qn sites show systematic variation with the glass compositions, which is due to the evolution of bonding on the PO4 sites (covalency, nature of second neighbours, bond length and angles). In the present study, we did not manage to interpret these variations, we limited the discussion to the
Fig. 13. Raman spectra of the (50 − t)ZnO–tMoO3–50P2O5 glasses.
evolution of the proportion of the Qn sites with the composition of the four glass series. A relative fraction of Qn units obtained by the deconvolution of the spectra was estimated with the error of ∼5%. The coordination of molybdenum in the molybdato-phosphate glasses is still a matter of discussion. Chowdari et al. [7] in their studies of the Li2O–P2O5–MoO3 glasses came to the conclusion that in the 50P2O5–50MoO3 glass molybdenum forms MoO6 octahedra characterized in the Raman spectra by the stretching vibration at ∼983 cm− 1. With increasing additions of Li2O the MoO6 octahedra are converted partly to MoO4 tetrahedra characterized by the vibrational band at 945 cm− 1. Santagneli et al. [11] obtained 95Mo NMR spectra for the investigation of molybdenum coordination in (100 − x)NaPO3–xMoO3 glasses and on the basis of chemical shift trends they came to the conclusion that both MoO6 and MoO4 units are present in these glasses. On the other side, Poirier et al. [28] studied (100 − x)NaPO3–xMoO3 glasses by X-ray absorption spectroscopy and from the course of XANES spectra they concluded that Mo atoms form only MoO6 octahedra in these glasses. In the structure of all crystalline compounds described in the P2O5–MoO3 system, i.e. MoO2(PO3)2 [29], (MoO2)2P2O7 [30], MoOPO4 [31] and Mo2O2P4O13 [32] molybdenum atoms form MoO6 octahedra regardless of the molybdenum valence. Glasses of the series 10ZnO–yMoO3–(90−y)P2O5, lying close to the P2O5–MoO3 tie-line, reveal in their Raman spectra a characteristic strong vibrational band at 975–992 cm− 1. We assume that these glasses have similar coordination of Mo atoms as the crystalline compounds with a similar composition and thus we ascribed this strong band to the vibrations of MoO6 octahedra. The observed values are also close to those observed by Morgan and Magruder [33] in the Raman spectra of MoO2 (PO3)2 (975 cm− 1) and (MoO2)2P2O7 (984 cm− 1) glasses containing MoO6 octahedra. As the dominant peak vibration of molybdate units in the Raman spectra of ZnO–MoO3–P2O5 glasses lies close to these values, we suppose that MoO6 units are formed in these glasses throughout the whole glass-forming region. Raman spectra of all the studied glasses containing molybdenum and phosphorus contain also a weak band at 386–403 cm− 1. Its relative strength is the highest at the glasses with a medium content of MoO3 and P2O5 and therefore we ascribe it to the vibration of Mo–O–P bonds in the ZnO–MoO3–P2O5 glasses. MoO6 octahedra reveal a strong tendency towards clustering and only in the glass 10ZnO–70P2O5–20MoO3 we assume the presence of isolated MoO6 octahedra. This conclusion was made on the basis of the absence of Raman band at 884 cm− 1 (Fig. 7), ascribed to the vibrations of Mo–O–Mo bonds [6,14,25]. We suppose that the structure of this glass is built up of phosphate chains interconnected by bridging oxygens or by –O–Mo–Obridges of MoO6 octahedra as well as in Mo2O2P4O13 [32]. The structure of this crystalline compound is indeed formed by MoO6 octahedra linked via PO4 units and has a similar ratio of Mo/P as the P2O5-rich glass. Structural data obtained in our (A) glass series (100−x)[0.5ZnO– 0.5P2O5]–xMoO3 can be compared with those obtained by Santagneli et al. [14] on similar NaPO3–MoO3 glasses. Some common features can be found with our (A) glass series: In the low to medium content of MoO3 (x=0–30 mol% MoO3), the evolution of the relative proportion of Q2 units (Fig. 3) decreases with MoO3 addition in a way similar to [14]. In sodium metaphosphate glasses, the local structure around Mo6+ ions was described as MoO4 tetrahedra in the form of Mo(=O)2(O1/2)2, each species forming two Mo–O–P links to phosphorus. The similarity of our Raman data with those of sodium phosphate glasses enables us to assume similar bonding relations in the studied zinc molybdato-phosphate glasses. In the low to medium content of MoO3 (x=0–30 mol% MoO3) the evolution of Qn units in (A) series, corresponds with the model of Santagneli [14], which supposes that the majority of the molybdenum species form MoO4 tetrahedra. The same distribution will be valid also for MoO6 octahedra forming two Mo–O–P bonds. This model results in the Qn distribution shown in Fig. 3 by the dashed line. In the region of high MoO3 content the number of Qn species deviates from the model of Santagnelli [14] in such a way that the number of Q1 units is higher and Q0 lower than predicted by the model. This is due to the higher clustering of MoO6
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octahedra in ZnO-based glasses as confirmed also by the Raman spectra (Fig. 4). Glass transition temperature in the (A) glass series (100−x)[0.5ZnO– 0.5P2O5]–xMoO3 decreases with increasing MoO3 content (see Table 1). This result is in good agreement with the strength of chemical bonds in diatomic molecules of P–O, and Mo–O [34] determined by spectroscopic methods. When we are comparing their strength we can find a higher value for the P–O bonds (599.1 kJ/mol), whereas a lower value was reported for the Mo–O bonds (560.2 kJ/mol). Thus the replacement of P–O bonds by Mo–O bonds results in a decrease of bonding forces in the structural network of this glass series. Comparing the maxima of Q1 units in Qn units evolution of (A) series of our Zn-glasses (see Fig. 3) and in Na-glasses [14] we observe that there is a preferential formation of Q1 units in Na-glasses (maximal portion is 80% of all Qn units). In Zn-glasses, the Q1 units (maximal portion is 60%) tend to be transformed to Q0 units, even if there is still a significant number of Q2 units that could be transformed. It follows that Q2 units possessing stronger covalent bonds with zinc atoms are more stable than those charge-compensated by Na+ ions. Moreover, the much stronger Raman band characteristic of Mo–O–Mo vibrations in Zn-glasses (at 849 cm− 1) compared to that in Na-glasses (at 840 cm− 1) shows a higher degree of clustering of MoO6 octahedra in Zn-glasses. In the P2O5-rich glasses in the series (B) 10ZnO–yMoO3–(90 − y) P2O5 and (C) zZnO–20MoO3–(80 − z)P2O5 (20 b z b 50) metaphosphate (Q2) units dominate the structure of glasses with a high P2O5 content, but some Q3 and Q1 units appear in the structure as well (Figs. 6 and 9). It is known that ultraphosphate glasses prepared in air accommodate certain amount of water in the form of P–OH bonds [23]. The two resonances at − 12 ppm and −1 ppm, which appear only at the spectra of glasses containing more than 50 mol% P2O5 (see Figs. 5 and 6), are probably due to the presence of phosphate units containing various amounts of terminal P–OH bonds. Consequently, the amount of phosphate units with two and three bridging oxygens (Q2 and Q3) obtained from fitting the spectra is lowered as showed by Brow [35]. With a decrease of P2O5 content down to 60 mol% the amount of water in the glass lowers and the oxygen atoms, which were used up for the formation of P–OH bonds can now form P–O–P bridges. That is why the number of Q2 units decreases only very slowly with a decrease of P2O5 content within the range of 70–50 mol% P2O5 in the glasses of the (B) and (C) series. Thermal stability of glasses of these series is high, they do not crystallize on heating and their reaches a maximum at the glass with 50 mol% P2O5. In the MoO3-rich glasses in the (B) series, phosphate chains are interrupted by MoO6 units and thus Q2 and Q3 units are gradually transformed to Q1 and further to isolated Q0 units. Thus in the glass with 70 mol% MoO3 there are nearly no linkages between individual phosphate groups and mostly isolated phosphate units in the form of P(OMo)4 can be found in the glass structure (Fig. 6). This is similar to the structure found in sodium phosphomolybdates [14]. Simultaneously, with increasing MoO3 content a band at 884 cm− 1 appears in the Raman spectra, which shows on the formation of Mo–O–Mo bridges between MoO6 octahedra [6,14,25]. This band broadens and shifts to lower wavenumbers in glasses with more than 50 mol% MoO3, which can be caused by the formation of three-dimensional clusters composed of several MoO6 octahedra interconnected by common edges. In the ZnO-rich part of the ternary diagram in the (C) series zZnO– 20MoO3–(80 −z)P2O5 with the constant MoO3 content within the concentration range of 30–50 mol% ZnO the number of Q2 units steeply decreases as phosphate chains are rapidly interrupted (Fig. 9). At the same time a large increase in the number of Mo–O–Mo bonds takes place as manifested by a significant increase in the intensity of the Raman band at 885–889 cm− 1 (Fig. 10). We suppose that the main reason for the observed steep decrease in the number of Q2 units is a decreasing P2O5 content and a consequent decrease in the number of oxygen atoms available for the formation of Mo–O–P bonds.
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The (D) series (50 − t)ZnO–tMoO3–50P2O5 with the constant P2O5 content is very useful for comparison of the differences in the role of ZnO and MoO3 in the glass structure. From the evolution of Qn phosphate units (Fig. 12) with the replacement of ZnO by MoO3 we can see that the number of Q1 units increases at the expense of Q2 units. We assume that Mo–O–P bonds do not tend to replace P–O–P bonds and MoO6 units are preferentially bonded to non-bridging oxygen atoms on PO4. This conclusion is in good agreement with the lower strength of Mo–O bonds (560.2 kJ/mol) than P–O bonds (599.1 kJ/mol) [34] as discussed above. Nevertheless, increasing number of oxygen atoms in the glass structure (1 O atom of ZnO is replaced by 3 atoms of MoO3) results in the transformation of Q2 units into Q1 units. The presence of small signals characteristic of Q0 and Q3 units is probably due to a partial disproportíonation of Q2 and Q1 units [23]. A relatively high number of oxygen atoms in the glasses with 50 mol% P2O5 results also in a lower degree of clustering of MoO6 units as revealed by a relatively small increase in the intensity of the vibrational band at 880–888 cm− 1 (Fig. 13) ascribed to the vibration of Mo–O–Mo bonds. 5. Conclusions A very large glass-forming region was found in the ternary system ZnO–MoO3–P2O5, which made it possible to analyze structural features in this system by combination of 31P MAS NMR spectroscopy and Raman spectroscopy. In the P2O5-rich region the phosphate chains are interconnected to form three-dimensional network in glasses with more than 50 mol% of P2O5. However these glasses tend to accommodate certain amount of water in form of terminal P–OH bonds which lowers their degree of cross-linking. Each species of MoO3 and ZnO make two Mo–O–P or Zn– O–P bonds with the phosphate units and thereby interrupt phosphate chains. Nevertheless, Mo–O–Mo bonds were found to be already present in glasses with 60 mol% P2O5. In the ZnO-rich region the phosphate network is highly depolymerized by a high number of Zn–O–P covalent bonds. Molybdate polyhedra are mutually interconnected in this region by Mo–O–Mo bonds even in the glasses with a small amount of MoO3. In the MoO3-rich region the glass structure is characterized by the formation of three-dimensional clusters of several MoO6 octahedra interconnected by corners and edges. Remaining phosphate units are isolated and form P(OMo)4 units and thus the number of P–O–Mo bonds created by one MoO3 species is lowered in this concentration region under one bond for a species. Acknowledgements The Czech authors are grateful for the financial support from the research project no. P106/10/0283 of the Grant Agency of Czech Republic, from the research project no. 0021627501 of the Ministry of Education of Czech Republic and the research project KAN301370701 of the Academy of Sciences of the Czech Republic. LM thanks the Feder, the USTL, the Région Nord Pas de Calais and the CNRS for funding of NMR spectrometers. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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