Journal of Non-Crystalline Solids 357 (2011) 3345–3350

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Journal of Non-Crystalline Solids

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Glasses and glass-ceramics in the oxyfluoride ternary system Pb(PO3)2-WO3-PbF2Roger Gomes Fernandes a, Camila Pereira a, Roberto Bertholdo a, Fábia Castro Cassanjes a,b, Gael Poirier a,b,⁎a Research Group of Solid State Chemistry, Universidade Federal de Alfenas, Campus de Alfenas, Alfenas-MG, Brazilb Research Group of Materials Chemistry, Universidade Federal de Alfenas, Campus de Poços de Caldas, Poços de Caldas-MG, Brazil

⁎ Corresponding author at: Laboratório InterdiscipliFederal de Alfenas, UNIFAL-MG, Rua Gabriel Monteiro daCEP 37130–000. Tel.: +55 35 32 99 12 61; fax: +55 35

E-mail address: gael.poirier@unifal-mg.edu.br (G. Po

0022-3093/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.jnoncrysol.2011.06.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 March 2011Received in revised form 8 June 2011Available online 5 July 2011

Jel Classifications:81.05.Kf81.10.Jt

Keywords:Glass;Glass-ceramic;Phosphate;Tungsten;Lead;Crystallization

Glasses were prepared by the melt-quenching method in the ternary system Pb(PO3)2-WO3-PbF2 and dopedwith Er3+ in order to prepare luminescent transparent glass-ceramics. This work focused on thermal andstructural characterization of tungsten lead-phosphate glasses and crystallization study for preparingtransparent glass-ceramics. Thermal properties such as thermal stability and crystallization behavior uponheating were investigated by DSC in function of PbF2 content. For low PbF2 concentrations, only onecrystallization peak due to Pb3(PO4)2 is observed whereas samples containing more than 15% of PbF2 presentanother exothermic event at lower temperatures related with precipitation of PbF2, Pb2P2O7 and Pb2OF2.Structural investigations by Raman spectroscopy suggest that PbF2 modifies the tungsten-phosphate networkthrough the formation of P―F and P―O―Pb bonds but the average network connectivity remains almostconstant. A crystallization study has been performed by DSC to investigate the dominant crystallizationmechanisms in these glasses and it has been established that Pb3(PO4)2 is nucleated on the surface whereasPbF2, Pb2P2O7 and Pb2OF2 crystallize dominantly from the glassy bulk. Transparent glass-ceramics containingnanosized PbF2 crystallites were also prepared by suitable heat-treatment on the glass sample containing20% of PbF2 and Raman microscopy of these glass-ceramics supports the crystallization mechanismsdetermined by DSC.

nar de Química, UniversidadeSilva, 700, Alfenas-MG, Brazil,32 99 12 62.irier).

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Rare earth doped-fluoride glasses have been intensively investi-gated in the past because of their very lowphonon energy compared tooxide glasses and resulting improved luminescence properties [1–3].However, their poor chemical and thermal stability as well as highmanufacturing costs limited their applications in integrated optics andoptical telecommunications. For that reasons, rare earth dopedoxyfluoride glasses containing heavy metal fluoride nanocrystalshave been developed to combine physical and chemical properties ofoxide glasses and high luminescence properties of rare earth ionsincorporated inside the fluoride nanocrystals in these materials [4–6].Such heterogeneous materials have been reported in silicate andgermanate based matrix [7,8]. Although phosphate glasses exhibitseveral advantages over other oxide glasses such as low meltingtemperatures, high solubility of transition metal and rare earth ionsand low processing costs [9,10], these glasses also exhibit a relativelypoor chemical and thermal stability [11,12]. In the past few years,tungsten-phosphate glasses were investigated because of their newproperties obtained by incorporation of tungsten ions which signifi-cantly modifies the phosphate network by increasing connectivity

and stronger chemical bond strength [13–15]. These new materialscombined very high chemical and thermal stability [16], non-linearoptical properties [17,18], photochromic [19,20], redox properties [21]and exhibit a relatively low phonon energy (920 cm−1) comparedwith other phosphate glasses (1100–1200 cm−1). Lead tungsten-phosphate glasses were also investigated since they present the sameinteresting properties than tungsten-phosphate glasses but leadincorporation results in a higher refractive index of the glass matrix[22].

In this work, PbF2 has been incorporated for the first time in a leadtungsten-phosphate glass matrix doped with Er3+ and the thermaland structural properties studied in function of PbF2 content by DSCand Raman spectroscopy. Dominant crystallization mechanism (bulkor surface) has been investigated for each crystalline phase formedduring heating by DSC and these results used to prepare transparentglass-ceramics containing β-PbF2 nanocrystals. Raman microscopy onthe surface also confirmed the crystallizationmechanisms determinedby DSC. Even though Er3+ luminescence was not studied in this work,the glassmatrixwas dopedwith Er2O3 since the rare earth ions can actas nucleating agents and may strongly influence the crystallizationbehaviors and mechanisms.

2. Experimental part

Lead metaphosphate Pb(PO3)2 was prepared by precipitation inaqueous solution: aqueous solutions of Pb(NO3)2 1 M and NaPO3 1 M

Fig. 1. (a) DSC curves of glass samples in function of composition and (b) deconvolutionof the second crystallization peak for composition 23PbF2.

3346 R.G. Fernandes et al. / Journal of Non-Crystalline Solids 357 (2011) 3345–3350

were mixed and the precipitate filtered under vacuum and driedat 80 °C for 48 hours. The resulting compound was characterized byX-ray diffraction as being lead metaphosphate Pb(PO3)2. Glasssamples were synthesized by the conventional method. Powderedstarting materials lead metaphosphate Pb(PO3)2, tungsten oxide WO3

from Alpha (99.8% pure), lead fluoride PbF2 from Aldrich (99.8% pure)and erbium oxide Er2O3 from Aldrich (99.9% pure) were mixed inan agatemortar andmelted at 800 °C for 10 min in a covered platinumcrucible to minimize fluoride losses due to sublimation. Finally themelt was cooled in a brass mold preheated 20 °C below the glasstransition temperature Tg. Annealing was implemented at thistemperature for several hours in order to minimize mechanical stressresulting from thermal gradients upon cooling. Glass samples weresynthesized in the (79.75–0.8x) Pb(PO3)2-(20–0.2x) WO3-x PbF2-0.25Er2O3 system with x=0, 5, 10, 15, 20 and 23 molar%. The molarconcentrations, glass transition and crystallization temperatures aswellas glass stability parameter Tx–Tg are presented in Table 1 for eachsample.

DSC curves presented in Fig. 1were obtained using a TA InstrumentsQ20 calorimeter. The glass samples were grinded and sieved with afinal grain size between 25 μm and 49 μm. DSC scans were performedon 10 mg sample masses between 300 and 700 °C at a heating rate of10 °C/min in gold pans under N2 atmosphere. The estimated error onthe temperature is±2 °C for glass transition and onset of crystallizationwhich are obtained from tangents intersection and ±1 °C for theposition of the crystallization peak which is determined by thetemperature at the top of the peak. X-ray diffraction patterns ofcrystallized glass samples were performed in order to determinecrystalline phases associated with the crystallization peaks labeled(1) and (2) observed in DSC curves. X-ray diffraction measurementswere carried out with a D5000 Siemens diffractometer using Cu Kαradiation. The 2θ investigation region was in the range of 15–65° with asteppass of 0.05° anda step timeof 1 s. Raman scatteringmeasurementswere recorded between 200 and 1400 cm−1 on bulk samples using aJobin-Yvon Horiba-HR800 instrument, operating with a He/Ne-laserat 632.8 nm. For the crystallization study, each glass compositionwas grinded and the resulting powder sieved into three different grainsize groups: 38–49 μm, 106–125 μm and 212–500 μm. These threedifferent groups were described by the average grain size: 43 μm,115 μm and 355 μm. DSC measurements of these glass powders wereperformedon a constantmass of 10 mgat 10 K/minbetween300 °Cand700 °C using the same TA Instruments Q20 calorimeter and the crys-tallization peak intensity determined in function of glass grain size foreach glass composition. Sample 20PbF2 was heat-treated at 410 °Cduring 75, 150, 180 and 360 min and the UV-visible spectra of the as-quenched and treated samples recorded between 350 and 900 nmusing a Thermo Scientific Evolution 60 spectrophotometer with aresolution of 0.5 nm.

3. Results

Transparent colorless glass samples were prepared by the melt-quenching method in the system (79.75–0.8x) Pb(PO3)2-(20–0.2x)WO3-x PbF2-0.25 Er2O3 with x=0, 5, 10, 15, 20 and 23 molar% and

Table 1Samples molar compositions and characteristic temperatures.

Samples Molar concentration (%) Characteristic temperatures (°C)

Pb(PO3)2 WO3 PbF2 Tg Tx2 Tx1 Tx–Tg

0PbF2 80 20 0 385 °C – 530 °C 145 °C5PbF2 76 19 5 388 °C – 541 °C 153 °C10PbF2 72 18 10 390 °C – 546 °C 156 °C15PbF2 68 17 15 389 °C 497 °C 573 °C 108 °C20PbF2 64 16 20 390 °C 476 °C 590 °C 86 °C23PbF2 61.6 15.4 23 392 °C 450 °C 601 °C 58 °C

labeled 0PbF2, 5PbF2, 10PbF2, 15PbF2, 20PbF2 and 23PbF2 respec-tively as described in Table 1. DSC curves of these glass samples arepresented in Fig. 1 and the characteristic temperatures (glasstransition and crystallization) and thermal stability parameter Tx–Tg summarized in Table 1. The two main important results that couldbe extracted from Fig. 1 and Table 1 are the almost constant value ofTg and distinct crystallization behavior of the studied glasses withincreasing PbF2 content. Samples containing less than 15% of PbF2exhibit only one symmetric crystallization peak labeled (1) whereassamples with at least 15% of PbF2 present two crystallization peakslabeled respectively (1) for the peak at higher temperatures and(2) for the asymmetric peak at lower temperatures. Deconvolution ofpeak (2) is presented in the inset of Fig. 1 and suggests that thisexothermic event is related with three different crystalline phases.Based on the progressive evolution of DSC curves with composition, ithas been assumed that peaks (1) and (2) are related with the samecrystallization phases in all samples. This assumption has been verifiedby X-ray diffraction. Sample 0PbF2 was heat-treated at 570 °C for 2hours and the resulting X-ray diffraction pattern attributed to leadorthophosphate Pb3(PO4)2 as shown in Fig. 2. Samples 5PbF2 and 10PbF2 heat-treated above the crystallization peak (1) presented thesame crystalline pattern. For peak (2) identification, sample 23PbF2washeat-treated above peak (2) at 500 °C for 1 hour. The X-ray diffractionpattern in presented in Fig. 3 and attributed to three crystalline phases

Fig. 2. X-ray diffraction pattern of sample 0PbF2 heat-treated at 570 °C for 2 hours anddiffraction peaks of crystalline reference Pb3(PO4)2.

Fig. 3. X-ray diffraction pattern of sample 23PbF2 heat-treated at 500 °C for 1 hour anddiffraction peaks of crystalline references Pb2P2O7, β-PbF2 and Pb2OF2.

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identified as lead fluorideβ-PbF2, lead pyrophosphate Pb2P2O7 and leadoxyfluoride Pb2OF2. This result is in agreement with DSC observationsand deconvolutions described below. Similar treatments above peak(2) in samples 15PbF2 and 20PbF2 resulted in the same attributions.

Raman scattering spectra of the studied glass samples wererecorded to monitor the structural evolution with composition andare shown in Fig. 4. The first important observation is that PbF2incorporation slightly decreases the glass maximum phonon energyfrom 920 cm−1 to 875 cm−1 which can be considered very low forphosphate-based glasses. This evolution is important for improvedrare earth luminescence in phosphate matrix. Raman bands observedat 1100 cm−1 and 730 cm−1 are attributed to symmetric stretchingvibrations of Q2 metaphosphate units and P–O–P linkages respectivelyand are consistentwith the presence of linearmetaphosphate chains forall compositions [23–25]. The Raman band centered at 1020 cm−1 andincreasing with PbF2 content is attributed to symmetric stretchingvibrations of Q1 pyrophosphate units suggesting a progressive depoly-merization of the metaphosphate chains [26]. Dominant features at875 cm−1 and 815 cm−1 are commonly attributed to fluorophosphateunits OPO2F- and OPOF2[27–30] whereas the weak Raman band near530 cm−1 is due to terminal P―F bonds [30]. The increase of thesebandswithPbF2 content is in agreementwith the formationof P-Fbondsin the glass network. Finally, bending vibrations of P―O―Pb bondsare observed at 370 cm−1[31].

Fig. 4. Raman spectra of glass samples in function of composition.

The dominant crystallization mechanism (bulk or surface) wasinvestigated for each crystallization peak by monitoring the crystal-lization peak intensity in function of glass grain size as describedearlier by Ray et al. [32]. This determination method is based on asimple concept which claims that surface crystallization is favored forsmaller grain sizes and can be identified by a decrease of thecrystallization peak intensity with increasing grain size. On the otherhand, dominant bulk crystallization can be identified by an increase ofthe peak intensity with increasing grain size. Such determination isfundamental for technological applications since the preparation oftransparent glass-ceramics obtained by controlled crystallizationrequired a dominant bulk mechanism. This simple methodology wasapplied in this work for the first time to our knowledge in fluoropho-sphates glasses as well as in glass compositions exhibiting severalcrystallization events. As shown in Fig. 5, the intensity of the exo-thermic event labeled (1) decreases with increasing grain size for allcompositions suggesting that lead orthophosphate Pb3(PO4)2 isnucleated at the surface. On the other hand, peak (2) intensityincreases with grain size particularly for samples 20PbF2 and 23PbF2as shown in Fig. 6, suggesting that crystalline phases β-PbF2, Pb2P2O7

and Pb2OF2 are nucleated in the volume of the glass. Based on thesepreliminary results, a special attention has been devoted to theobtaining of transparent glass-ceramics for further optical character-izations. Sample 20PbF2was chosen for its relatively high PbF2 contentand satisfying thermal stability. This glass sample was heat-treatedat 410 °C for 75, 150, 180 and 360 min in order to evaluate thepossibility of obtaining transparent glass-ceramics from this compo-sition. UV-visible absorption of these samples has been recorded toprobe the transparency in function of heat-treatment time and torelate the growth of a crystalline phase with light scattering (Fig. 7).Sharp absorption bands are observed in the visible and near infraredrange and are related with internal f-f electronic transitions of Er3+

ions. It can be also observed from these results that sample heat-treated for 180 min strongly scatters visible light whereas samplesheat-treated for lower times are highly transparent in this wavelengthrange. Such observation is consistent with a crystal growth in theseglasses and formation of micrometric crystals in sample heat-treatedfor 180 min. Transparent samples heat-treated for 75 min and150 minmay contain a crystalline phase with nanocrystals of a size muchsmaller than the visible light wavelength. X-ray diffraction measure-ments were realized in these heat-treated samples to identify theappearance of crystalline phaseswith heat-treatment time (Fig. 8). It isobserved that glass sample 20PbF2 heat-treated for 75 min appearstotally amorphous within the detection limit whereas sample heat-

Fig. 5. Evolution of the crystallization peak intensity in function of glass particle size forpeak 1 (associated with crystallization of Pb3(PO4)2).

Fig. 6. Evolution of the crystallization peak intensity in function of glass particle size forpeak 2 (associated with crystallization of β-PbF2, Pb2P2O7 and Pb2OF2).

Fig. 8. X-ray diffraction of sample 20PbF2 heat-treated at 410 °C for 75 min, 150 minand 180 min.

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treated for 150 min exhibits weak and broad diffraction peaks at 26°,30°, 43° and 51°. These diffraction features are consistent withcrystallization of the β-PbF2 phase according to ICSD file no 201114.In addition, the broadening of the β-PbF2 diffraction peaks suggeststhat the crystalline phase is composed of nanosizedβ-PbF2 crystallites.This assumption is consistent with the transparency of the sampleobserved in Fig. 7 and from the picture in Fig. 8 (inset). Crystallite sizedistribution is also under investigation by Transmission ElectronMicroscopy. For longer heat-treatments (180 min), other crystallinephases associatedwith the first crystallization event (labeled peak(2))are observed and were already attributed with lead pyrophosphatePb2P2O7 and lead oxyfluoride Pb2OF2. Finally, Raman scatteringmicroscopy was performed on the surface of these heat-treatedsamples and the resulting spectra presented in Fig. 9 along with apicture of the analyzed area in function of heat-treatment time inFig. 10. These opticalmicrographswere obtained using themicroscopeused for Raman microscopies. It appears that sample 20PbF2 heat-treated for 75, 150 and 180 min exhibit the same Raman spectrawith broad bands typical of the amorphous state. Specially, samplesheat-treated for 150 and 180 minwhich are glass-ceramics containingβ-PbF2 and β-PbF2, Pb2P2O7 and Pb2OF2 respectively are apparentlytotally amorphous at the surface. Only for much longer heat-treatments(360 min), sharp Raman bands usually observed for crystallinecompounds as well as small crystals are observed at the sample surface.

Fig. 7. UV-visible spectra of sample 20PbF2 heat-treated at 410 °C for 0 min, 75 min,150 min and 180 min.

Raman spectrum of glass sample heat-treated for 360 min is consistentwith the appearance of Pb2P2O7 at the surface.

4. Discussion

In this work, the thermal and structural behaviors of new glasscompositions in the ternary system Pb(PO3)2-WO3-PbF2 have beeninvestigated in order to prepare optically useful transparent glass-ceramics. For this reason, the glass compositionswere dopedwith Er3+

since Er3+ incorporation can modify thermal and crystallizationbehaviors as described elsewhere for the preparation of glass-ceramics[33]. However, the luminescent properties of Er3+ in the glasses andglass-ceramics were not investigated in this work. Thermal resultsobtained by DSC and presented in Fig. 1 and Table 1 brought severalinformations useful for structural considerations and crystallizationtendencies. The almost constant value of Tg with increasing PbF2content allows to infer the structural effect of Pb2+ and F− on theoriginal tungstenphosphate covalent network. Since theglass transitiontemperature is directly related with the network connectivity andchemical bond strength, it can be assumed that PbF2 incorporation doesnot effectively influence these network properties. It is suggested thatpart of the terminal P―O−bonds are progressively replaced by terminalP―F resulting in an almost constant connectivity. The appearance ofcrystalline phases β-PbF2, Pb2P2O7 and Pb2OF2 precipitated at low

Fig. 9. Raman spectra of the surface of glass sample 20PbF2 heat-treated at 410 °C for75 min, 150 min, 180 min and 360 min and crystalline reference Pb2P2O7.

Fig. 10. Picture of the sample surface after heat-treatments at 410 °C for (a) 150 min, (b) 180 min and (c) 360 min.

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temperatures for high PbF2 concentrated samples results in a decreaseof the glass thermal stability as presented in Table 1. The Tx–Tg valuedecreases to 58 °C for sample 23PbF2 explaining why higher PbF2concentrations cannot be vitrified in this ternary system. Incorporationof PbF2 results in a thermally non-stable glass network with a hightendency to form β-PbF2, Pb2P2O7 and Pb2OF2 phases and a consequentlow tendency to form Pb3(PO4)2. It can be also inferred that a tungsten-phosphate glassy phase remains even after the second crystallizationevent since no crystalline phases containing tungsten could be detectedin these samples. Further structural investigations by Raman spectros-copy are in agreement with the preliminary hypothesis extracted fromthermal analysis: PbF2 modifies the tungsten-phosphate networkthrough P―F and P―O―Pb bonds which at high PbF2 concentrationspromote the crystallization of leadfluoride and oxyfluoride compounds.Further structural investigations are under study by solid-state NMRof 31P and 19F to support these structural models.

Determination of crystallization mechanisms for each crystalliza-tion peak revealed that PbF2 incorporation is able to induce bulkcrystallization in tungsten-phosphate glasses and can allow preparingtransparent glass-ceramics since internal nuclei should be formed forthese compositions by suitable heat-treatments. Such heat-treat-ments were performed above glass transition temperature in sample20PbF2 and confirmed the hypothesis of three crystalline phasesformed during the exothermic event (2) and also point out that thefirst crystalline phase to be formed is β-PbF2. These investigationsperformed on sample 20PbF2 indicate that heat-treatment at 410 °Cfor 150 min is suitable to produce transparent glass-ceramics con-taining nanocrystals of β-PbF2. Heat-treatment times must be preciselycontrolled since longerheat-treatments at the same temperature inducethe precipitation of other crystalline phases and result in non-transparent glass-ceramics. Finally, Raman microscopy measurementsperformed at the surfaceof theseheat-treated samples are in agreementwith crystallization mechanisms suggested previously in this work.Glass-ceramic samples containing crystalline phases β-PbF2, Pb2P2O7

and Pb2OF2 appear totally amorphous at the surface, supporting thehypothesis that these three crystalline phases are nucleated primarilyinside the bulk and not from the surface.

5. Conclusion

Glass samples doped with Er3+ were prepared by the melt-quenching method in the ternary system Pb(PO3)2-WO3-PbF2 andtheir thermal and structural properties probed in function of PbF2concentration. Crystallization behavior of these glasses is stronglydependent of PbF2 content. Samples containing less than 15 mol% ofPbF2 exhibit only one crystallization peak in the range 300–700 °Crelated with lead orthophosphate Pb3(PO4)2 precipitation. For higherPbF2 concentrations (15%, 20% and 23%), a second crystallizationpeak appears at lower temperatures and was attributed to precipi-tation of lead fluoride β-PbF2, lead orthophosphate Pb2P2O7 and leadoxyfluoride Pb2OF2. Raman investigations suggested that PbF2modifies the glass network through formation of P―F and P―O―Pbchemical bonds. A crystallization study performed by probing thecrystallization peak intensity in function of glass grain size suggeststhat Pb3(PO4)2 crystallizes dominantly by the surface whereas β-PbF2,

Pb2P2O7 and Pb2OF2 crystallizations occur inside the glassy bulk. Heat-treatments on sample 20PbF2 and further characterizations pointedout that suitable heat-treatments enable the formation of β-PbF2 inthe glass and result in transparent glass-ceramics. Raman microscopyon the surface of these glass-ceramics allowed confirming thedominant bulk crystallization for the first three crystalline phasesβ-PbF2, Pb2P2O7 and Pb2OF2 which is a necessary condition for theobtaining of transparent-glass ceramics. This work combined for thefirst time to our knowledge the determination of crystallizationmechanisms and further preparation of transparent glass-ceramics intungsten fluorophosphates glasses. Further studies include lumines-cence properties of Er3+ in as-quenched glasses and correspondingglass-ceramics.

Acknowledgments

The authors would like to thank funding agencies FAPEMIG, FINEP,CNPq and CAPES for financial support, Profs. Drs Sidney J.L Ribeiro andYounes Messaddeq from UNESP-Araraquara-Brazil for Raman micros-copies and Prof. Dr. Rodrigo Fernando Costa Marques from UNIFAL-MG for crystalline phase determinations.

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