Journal of Non-Crystalline Solids 357 (2011) 1463–1468
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Journal of Non-Crystalline Solids
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NMR investigation of the crystallization mechanism of LaF3 and NaLaF4 phases inaluminosilicate glasses
Francisco Muñoz a,⁎, Araceli de Pablos-Martín a, Nicolas Hémono a, María Jesús Pascual a, Alicia Durán a,Laurent Delevoye b, Lionel Montagne b
a Instituto de Cerámica y Vidrio (CSIC), Kelsen 5, 28049 Madrid, Spainb Unité de Catalyse et Chimie du Solide, Université des Sciences et Technologies de Lille, Ecole Nationale Supérieure de Chimie de Lille, 59655 Villeneuve d'Ascq, France
Article history:Received 30 November 2009Received in revised form 10 November 2010Available online 6 December 2010
Keywords:Glass-ceramics;Oxyfluoride glasses;Crystallization;Nano-crystals;LaF3;NaLaF4;Nuclear magnetic resonance
The structure of transparent F-containing nano-crystalline glass-ceramics has been investigated by means ofNuclear Magnetic Resonance as well as X-ray Diffraction as a function of the processing conditions. LaF3 orNaLaF4 are crystallized depending on the glass composition: low silica content and low modifier to aluminaratios lead to crystallization of LaF3, while increasing SiO2 and decreasing Al2O3 results in NaLaF4 nano-crystals. NMR results show that fluorine in glasses is involved in Al–F–Na or La–F environments, the laterconstituting pre-nucleation sites. After annealing, crystallization of either LaF3 or NaLaF4 crystals takes place,which is correlated with an increase in the intensity of fluorine resonance of La–F environment and a decreasein the Al–F–Na one. The description of the local environment around F, Al and Na is in accordance with acrystallization model of these materials. In low silica containing glasses, it starts from La–F sites andprogresses thanks to fluorine diffusion from Al–F–Na bonds until crystal growth is limited through theincrease in the viscosity of the glass matrix around crystals. In high silica glasses, long annealing times giverise to crystallization of NaF after NaLaF4 crystals have formed and a silica-enriched region around theminhibits further crystal growth.
Fluoride and oxyfluoride glasses have attracted the attention ofresearchers due to their unusual optical properties, e.g. low phononenergy, optical transparency, rare-earth ion solubility, leading topotential applications such as high power lasers or up-conversiondevices [1–3]. In transparent oxyfluoride glass-ceramics, the opticalactive ion may be incorporated into a fluoride crystalline phase, thusoffering a better alternative to both fluoride glasses and crystals.
It is well known that substitution of oxygen by fluorine in glassesresults in a drastic modification of their properties, e.g. decreasing inviscosity and modifying surface tension, which may result in anenhanced tendency to phase separation and, consequently, crystalli-zation. The processing of fluorine-containing glass-ceramics is agrowing field of research due to the advantages of glass-ceramicmaterials as hosts for fluorides of rare-earth ions. Processing offluorine-containing glass-ceramics has been studied throughoutnano-crystallization of different phases, such as (Cd,Pb)F2 [4] or LaF3[5]. A large concentration of nano-crystals with narrow sizedistribution is required in order to minimize the scattering losses inphotonic applications, which can be done by controlling the
nucleation and crystal growth within the glass matrix. The crystalli-zation studies are usually focused on isochemical systems, in whichboth crystals and glass matrix have the same composition. Rüsselexplained the nano-crystallization of CaF2 in silicate glasses by theincrease in the viscosity of the remaining glass matrix around nuclei[6], which is an example of a non-isochemical system. Here, crystal–glass interphase acts as a barrier to diffusion and notably deceleratesthe crystal growth velocity leading to nano-crystals with size in therange from 10 to 50 nm. Rare earth-doped nano-glass-ceramics in theNa2O–Al2O3–SiO2–LaF3 system, where LaF3 phases crystallize, havebeen investigated through their crystallization behavior, structureand the fluorescence properties [5,7–9].
A previous study has been published on the preparation of glass-ceramics in the system Na2O–Al2O3–SiO2–LaF3 [10]. For the composi-tions studied, 20Na2O·30Al2O3·40SiO2·10LaF3 and 15Na2O·20Al2-O3·55SiO2·10LaF3, LaF3 nano-crystals are formed with size below20 nm after annealing at Tg+100 K for 72 h. The X-ray diffractionanalysis of 40 mol% SiO2 glass-ceramics, obtained after annealing at595 and 645 °C during 20 h, resulted in crystal sizes, as calculated byusing Scherrer equation, of 10±1 nm for both temperatures with aslight increase in the area under the diffraction peaks of LaF3 for thehighest temperature. On the other hand, XRD of the 55 SiO2 mol%glass-ceramics (treated at 620 °C – 20 h and 700 °C – 20 h) presentedan increase in the crystal size, from 12 to 18 nm, with increasingtreatment temperature, accompanied by an important increase in the
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area under the diffraction peaks of LaF3. The mean crystal size andcrystalline fraction increase with annealing temperature, as well asthe glass transition temperature of the glass-ceramics, and thecrystallization of the LaF3 phase is influenced by the phase separationoccurring in the glass matrix.
The aim of this work has been to contribute to the elucidation ofthe crystallization mechanism in LaF3-containing aluminosilicateglasses through 19F, 27Al and 23Na Nuclear Magnetic Resonanceexperiments. 29Si NMR analyses did not bring any useful informationbecause nucleation and crystallization processes do not affectsignificantly the matrix structure, as will be shown in the presentreport. 135La NMRwas unsuccessful owing to the low La concentrationin the samples.
Some of the glasses studied in this work were doped with differentquantities of Tm2O3 and the location of Tm3+ ions in the nano-crystalsof the corresponding glass-ceramics was confirmed [11]. The lowphonon energy environment of Tm3+ ions enhances laser emissionintensities respect to the doped parent glass. Further work focused onthe Up-conversion emission in the Tm2O3 doped systems is inprogress.
2. Experimental procedure
2.1. Elaboration of glasses and glass-ceramics
Glasses in the system Na2O–K2O–Al2O3–SiO2–LaF3 have beenprepared by melt-quenching procedure. Three glass compositionshave been formulated with increasing SiO2 contents: 18Na2O·30Al2-O3·40SiO2·12LaF3 (40Si–12La glass), 15Na2O·20Al2O3·55SiO2·10-LaF3 (55Si–10La) and 8Na2O·8K2O·7Al2O3·70SiO2·7LaF3 (70Si–7La).Batches for 100 g of glass were obtained using reagent grade SiO2
(Saint Gobain, 99.6%), Al2O3 (Panreac), Na2CO3 (Panreac, 99.5%),K2CO3 (Puriss. p.a.) and LaF3 (Panreac, 99%) in an electric furnace. Thebatches were calcined in covered platinum crucibles up to 1200 °Cand then melted during 2 h (40Si and 55Si) and 1 h (70Si) between1400 and 1600 °C, depending on composition. The melts werequenched in air onto brass plates. The oxyfluoride glass-ceramicswere obtained through controlled crystallization of the parent glasses,using cut cubic samples, by heat treating at temperatures between Tgand Tg+100 K for a constant treatment time of 20 h, as well as forincreasing treatment times at a fixed temperature above Tg.
2.2. Characterization of the glasses and glass-ceramics
The glasses were analyzed by X-ray Fluorescence Spectroscopy(XRF) with a Panalytical spectrometer. All oxides were determinedemploying the melting method with Li2B4O7, and fluorine analysiswas performed on pressed pellets of powdered glass (8 g) in order toavoid fluorine volatilization. Glass transition temperature (Tg) wasdetermined by dilatometry in a Netzsch Gerätebau dilatometer,model 402 EP, using a 10 K min−1 heating rate in air. The estimatederror on Tg is ±2 °C. Table 1 presents the nominal and analyzedcomposition of the glasses, in mol%, as well as the glass transitiontemperature, determined by dilatometry, in °C.
Powder XRD analyses were carried out with a D-5000 Siemensdiffractometer using monochromatic Cu Kα radiation (1.5418 Å)between 23° and 43° with a step time of 8 s and 0.05° in size.
Table 1Nominal and analyzed composition of the 40Si–12La, 55Si–10La and 70Si–7La glasses, in m
Glass Na2O Nom. Na2O An. K2O Nom. K2O An. Al2O3 Nom.
19F Magic Angle Spinning (MAS) Nuclear Magnetic Resonance(NMR) spectra were recorded on a Bruker ASX 400 spectrometeroperating at 376 MHz (9.4 T) using a 2.5 mm probe. The pulse lengthwas 1.3 μs and 30 s delay time was used. A total number of 128 scanswere accumulated with a spinning rate of 30 kHz. This very highspinning rate enabled to decrease the homo-nuclear dipolar interac-tions that broaden 19F spectra. Solid CaF2 was used as secondaryreference with a chemical shift of −0.108 ppm with respect to CFCl3reference.
27Al MAS NMR spectra were recorded by using a Bruker Avance800 spectrometer operating at 208.6 MHz (18.8 T) using a 3.2 mmprobe. The use of a high field NMR spectrometer enabled to decreasethe broadening due to second order quadrupolar interaction (27Alnuclear spin I is 5/2). The pulse length was 1 μs and 1 s delay timewasused. A total number of 64 scans were accumulated with a spinningrate of 20 kHz. A 0.1 M [Al(H2O)6](NO3)3 solution was used as thechemical shift reference.
23Na MAS NMR spectra were recorded on a Bruker ASX 800spectrometer operating at 423.6 MHz (18.8 T). 23Na is a quadrupolarnucleus (I=5/2), hence the use of a high field NMR spectrometerenabled to decrease the broadening due to second order quadrupolarinteraction. The pulse length was 1 μs and 1 s delay time was used. Atotal number of 320 scans were accumulated with a spinning rate of20 kHz. A 0.1 M NaCl solution was used as the chemical shiftreference.
All relaxation delays used for the different NMR experiments wereverified to be long enough to enable relaxation.
3. Results
Fig. 1 shows the XRD patterns of the 70Si–7La nano-glass-ceramicsobtained at 530 °C at different annealing times (20, 50 and 72 h). It isobserved that NaLaF4 (PDF file No. 75-1923) is the only phaseappearing in the glass-ceramics with an increase in crystal size from17 nm (530 °C – 20 h) to 26 nm (600 °C – 20 h) together with anincrease in the area under the peak of NaLaF4. 70Si–7La glass alsoshows crystallization of NaF for annealing times of 50 and 72 h, whichis seen through the XRD analysis of the 70Si7La 530 °C – 50 h sampleby using 10 s step time and 0.03 step size (see inset in Fig. 1).
Fig. 2 shows the 19F MAS NMR spectra of two of the oxyfluorideglasses and two of their corresponding glass-ceramics obtained bythermal treatment during 20 h. Fig. 2a gathers the spectra of the 40Si–12La glass and glass-ceramics obtained at 595 and 645 °C and Fig. 2bthose of the 55Si–10La glass and glass-ceramics obtained at 620 and700 °C. The 19F NMR spectra of both glasses are very similar, as well asthose of the respective glass-ceramic series.
In the 40Si–12La and 55Si–10La glass spectra, one resonance isobserved around −180 ppm. According to Kiczensky et al. [12], theNa5Al3F14 phase has a single 19F chemical shift of−182 ppm, assignedto fluorine in Al–F–Na(4) environment, i.e. with 1 Al and 4 Na atomsin the first coordination sphere. Considering that 19F chemical shift isvery sensitive to a modification of fluorine local environment [13], theobserved chemical shift in our glasses is thus assigned to the presenceof fluorine in Al–F–Na groups.
A second 19F resonance, with smaller amplitude but much broader,is observed between 50 ppm and −40 ppm, with a maximum at9 ppm. According to literature data [14], this resonance should beattributed to fluorine with different local environments in LaF3
ol%, and their glass transition temperature.
Al2O3 An. SiO2 Nom. SiO2 An. LaF3 Nom. F2 An. (wt.%) Tg (°C)
Fig. 1. XRD patterns of the 70Si–7La glass-ceramic samples obtained after treatment at530 °C during 20, 50 and 72 h. The inset shows the XRD pattern of 70Si–7La 530 °C –
50 h glass-ceramic sample between 35° and 45°, by using 10 s step time and 0.03 stepsize, where a crystallization peak of NaF can be seen.
* * *
* *** **
* *** **
-300-200-100010020019F chemical shift (ppm)
40Si-12La glass
40Si-12La595OC - 20 h
40Si-12La645OC - 20 h
Inte
nsity
(a.
u.)
* * *
* *** **
* *** **
55Si-10La glass
55Si-10La620OC - 20 h
55Si-10La700OC - 20 h
-300-200-1000100200
Inte
nsity
(a.
u.)
19F chemical shift (ppm)
b
a
Fig. 2. 19F MAS-NMR spectra of the 40Si–12La glass and glass-ceramics obtained aftertreatment at 595 °C and 645 °C during 20 h (a) and 55Si–10La glass and glass-ceramicsobtained after treatment at 620 °C and 700 °C during 20 h (b). (*) denotes spinning sidebands.
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clusters. The large width of this resonance suggests that LaF3 nucleiare already present within the glass though still with a high degree ofdisorder. Finally, a third and very small contribution has been found at−139 ppm, which is due to probe background.
19F NMR of 40Si–12La and 55Si–10La glass-ceramics show theresonance at −185 ppm attributed to Al–F–Na groups and threeadditional bands at −22, 18 and 26 ppm, which are assigned tofluorine atoms in LaF3 crystal [15]. According to the tysonite structureof LaF3, (P3 ̄c1), the site proportion is 1:2:6. In the NMR spectra, tworesonances (18 and 26 ppm) have a similar intensity, while the third(−22 ppm) presents a much larger one. An estimation of the amountof fluorine at −22 ppm results in double of the sum of fluorine atomsat 18 and 26 ppm, in accordance with the crystal data.
Though all spectra of 40Si–12La and 55Si–10La glass-ceramics aresimilar, it is worth noting that LaF3 crystallization does not involve allfluorine atoms, since residual Al–F–Na groups are observed after alltreatment conditions. Then, fluorine atoms involved in La–F bonds canbe considered as the crystallization nuclei, which will develop afterthermal treatment above Tg of the glass, leading to crystallization ofLaF3 as seen by both XRD and NMR. An estimation of the amount offluorine belonging to each group before crystallization treatments hasbeen made through spectral decomposition by means of dmfitsoftware [16]. In 40Si–12La glass, fluorine in LaF3 accounts for 34%while fluorine in Al–F–Na represents around 66%. In 40Si–12La glass-ceramics obtained at 595 °C during 20 h and 645 °C for 20 h glass-ceramics, LaF3 represents 74 and 77%, respectively, and Al–F–Na 26and 23% of F atoms, respectively. All percentages are assumed to bewithin ±5%.
Fig. 3 depicts the 19F NMR spectra of 70Si–7La glass and glass-ceramics obtained from treatments at 530 and 600 °C during 20 h (a),and 70Si–7La glass-ceramics obtained from treatments at 530 °Cduring 20, 50 and 72 h (b). The spectrum of 70Si–7La glass (Fig. 3a)shows a weak and broad resonance at −180 ppm and a larger onecentered on −46 ppm. The first resonance is assigned, as previously,to fluorine in Al–F–Na groups, while the larger one spreads over a very
broad chemical shift range, meaning a broad distribution of fluorineenvironments. Thermal treatment of this glass results in thecrystallization of NaLaF4. Indeed, the spectra of the 70Si–7La glass-ceramic samples show resonances at−24,−54 and−60 ppm, whichare assigned to different fluorine sites in crystalline NaLaF4 [17]. Aftercrystallization, small amounts of Al–F–Na groups remain within theglassy phase, but their relative intensity with respect to those ofNaLaF4 resonance bands decreases with both temperature andtreatment time. As a new feature of this composition, crystallizationof NaF arises in glass-ceramics obtained by treatment at 530 °C during50 and 72 h, detected through a new narrow resonance at−225 ppm.We noticed that NaF crystallization is more clearly detected by NMRthan by XRD, probably due to a very low amount of this phase.
Fig. 4 shows the 27Al spectra of the 55Si–10La glass and itscorresponding glass-ceramics obtained after 20 h at 620 and 700 °C.They present a main resonance at 60 ppm, attributed to four-foldcoordinated aluminum atoms, and all three 27Al MAS NMR spectra
-300-200-100010020019F chemical shift (ppm)
-300-200-100010020019F chemical shift (ppm)
70Si-7La glass
70Si-7La530OC - 20 h
70Si-7La600OC - 20 h
Inte
nsity
(a.
u.)
Inte
nsity
(a.
u.)
70Si-7La530OC - 20 h
70Si-7La530OC - 50 h
70Si-7La530OC - 72 h
**
*
*
**
*
*
*
*
*
* * *
** *
***
b
a
Fig. 3. 19F MAS-NMR spectra of 70Si–7La glass and glass-ceramics obtained aftertreatment at 530 °C and 600 °C during 20 h (a), and 70Si–7La glass-ceramics obtainedafter treatment at 530 °C during 20, 50 and 72 h (b). (*) denotes spinning side bands.
*
*
*
-60-40-2002040608010027Al chemical shift (ppm)
55Si-10La glass
Inte
nsity
(a.
u.)
Al(IV)
Al(VI)Al(V)
55Si-10La620OC - 20 h
55Si-10La700OC - 20 h
Fig. 4. 27Al MAS-NMR spectra of the 55Si–10La glass and glass-ceramics obtained aftertreatment at 620 and 700 °C during 20 h. (*) denotes spinning side bands.
02040608010027Al chemical shift (ppm)
70Si-7La glass
70Si-7La530OC 20 h
70Si-7La565OC 20 h
70Si-7La530OC 50 h
70Si-7La530OC 72 h
Inte
nsity
(a.
u.)
Fig. 5. 27Al MAS-NMR spectra of the 70Si–7La glass and glass-ceramics obtained aftertreatment at 530 °C for increasing times.
1466 F. Muñoz et al. / Journal of Non-Crystalline Solids 357 (2011) 1463–1468
show also two very small resonances at 30 and 0 ppm, which areassigned to five and six-fold coordinated aluminum, respectively [18].A rough quantification of the aluminum species has allowed toestimate that an approximately constant amount below 5% ofaluminum atoms are involved in either five- or six-fold coordinationpolyhedra in all three cases.
Fig. 5 shows the 27Al MAS NMR spectra of the 70Si–7La glass and aseries of glass-ceramics obtained under different thermal treatments.All show a main resonance at 60 ppm, assigned to four-foldcoordinated aluminum atoms and, similarly to 55Si–10La glass andglass-ceramics, all samples present two much smaller resonancescentered at ca. 30 and 0 ppm, attributed to Al (V) and Al (VI) species,respectively, which account for less than 3% of the total amount ofaluminum atoms. There is no chemical shift variation and nodifference appears in the glass-ceramics with respect to the baseglass spectrum. Thus, it may be concluded that no major change of thealuminum environment occurs during crystallization of 70Si–7La
glass, despite part of fluorine atoms must migrate from Al–F–Nagroups into either LaF3 or NaLaF4.
Fig. 6 presents the 23Na MAS NMR spectra of 70Si–7La glass andglass-ceramics obtained after thermal treatment at 600 and 650 °Cduring 20 h. The spectrum of the 70Si–7La glass shows one singleresonance with Gaussian line shape typical of the distribution of 23NaNMR parameters (chemical shift and quadrupolar constant) inglasses. The spectra of glass-ceramic samples present the same
40 20 0 -20 -4023Na Chemical shift (ppm)
70Si-7La glass
70Si-7La 600OC - 20 h
70Si-7La 650OC - 20 h
Fig. 6. 23Na MAS-NMR spectra of the 70Si–7La glass and glass-ceramics obtained afterthermal treatment at 600 °C and 650 °C during 20 h.
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resonance, and a new narrow resonance is visible at−13 ppm, whichis attributed to sodium in NaLaF4 crystals [14]. A spectral decompo-sition has been performed to estimate the proportion of sodiumtaking part of the crystallization. The 23Na best fit simulations havebeen obtained using a Czjcek (Gaussian Isotropic Model) modelimplemented into dmfit [16]. This model allows for a betterdescription of the dissymmetry observed on the 23Na line shape forthe glassy phase. Thus, increasing the precision of the relativequantification of amorphous versus crystalline phases. An 2nd orderquadrupolar for the resonance of the crystallized sodium species. Thesodium atoms involved in the crystalline phase represent 3±0.5% ofthe total sodium in 70Si–7La-600 and 70Si–7La-650 samples,respectively. Taking into account the relative sodium content in thesample, the amount of NaLaF4 crystalline phase can be estimated to be1.3 wt.%.
4. Discussion
Glass composition has demonstrated to have an influence not onlyon the crystallization kinetics but also on the nature of the crystallinephases. The comparison of XRD results in 40Si–12La, 55Si–10La [seeref. 10] and 70Si–7La (this work) compositions indicate thatincreasing SiO2 content and decreasing Al2O3, together with animportant increase in the ratio (Na2O+K2O)/Al2O3, from 0.6 in 40Si–12La to 2.28 in 70Si–7La, leads to the crystallization of the doublesodium lanthanum fluoride phase, instead of the LaF3 one. It is alsoworth mentioning that sodium is incorporated within the crystallinephase not for the highest but, surprisingly, for the glass with thelowest Na2O concentration, i.e. 6.73 mol%. Furthermore, looking at theglass transition temperature of the glasses (Table 1), it is found that70Si–7La glass has the lowest Tg at 511 °C. The mixed-alkali effect islikely contributing to decrease both Tg and viscosity at thetemperature of glass-ceramic processing, thus facilitating the incor-poration of sodium into the crystalline phase. On the other hand, thelower amount of lanthanum in the 70Si–7La glass could provokesodium to take part in the crystallization of NaLaF4 phase instead ofLaF3 one. Hémono et al. have pointed out that crystallization of LaF3crystals in 40Si–12La and 55Si–10La glass compositions is precededby phase separation [10]. Thus, a lower amount of lanthanum wouldprevent the formation of LaF3–enriched phase–separated droplets andthe lower viscosity of the 70Si–7La glass will facilitate sodiumdiffusion into NaLaF4 crystal. All these factors are also the clue forthe slower and less efficient crystallization process of the 70Si–7Lacomposition.
Bhattacharyya et al. have recently shown through energy-filteredTransmission Electron Microscopy imaging techniques that lantha-num and silicon enriched regions exist in the 40Si–12La glass in theform of phase-separated droplets [19]. Further thermal treatment ofthe glass resulted in the growing of LaF3 crystals inside the droplets
while silicon relocates itself around the crystals forming a shell andleading to a final stage where further crystal growth is inhibited. Thismechanism of inhibition of crystal growth has also been postulated byBocker et al. [20,21] and experimentally proved through advancedTransmission Electron Microscopy techniques by Battacharyya et al.[22] in BaF2-containing silicate glasses. Such a crystallizationmechanism has been proposed to proceed somewhat differently tothe one proposed in CaF2-containing glasses by Rüssel [6], whoproposed that CaF2 precipitates from an homogeneous glass (notpreceded by a phase separation). The separation of silica induces anincrease of the viscosity near the crystals, thus producing a diffusionbarrier which hinders further crystal growth.
From the 19F NMR results in the three glass compositionspresented in this work, two important features can be pointed outconcerning the crystallization mechanism. First, an important amountof amorphous LaF3 or NaLaF4 is already present in the glasses aftermelting. Second, fluorine diffusion occurs from Al–F–Na sites in theglass into either LaF3 or NaLaF4 crystals.
The broad resonances observed within the 19F chemical shift rangeof LaF3 and NaLaF4 species in the glasses agrees with the presence ofLa-rich regions observed by TEM in 40Si–12La glass [10,19]. We canthus conclude that amorphous La–F-containing nuclei are formedduring melting for all compositions. However, it is not possible tostate fromNMR experiments whether La-rich regions appear togetherwith silica separation (observed by TEM), even though Si–F bonds canbe excluded in practice owing to SiF6 volatility. Moreover, 29Si NMRresults did not enable us to confirm the presence of a silica enricheddiffusion barrier around LaF3, as proposed in [19]. This silica-enrichedregion is indeed expected to hinder further crystal growth aroundLaF3 or NaLaF4 crystals, since silica would not be compatible withfluorine diffusion toward the crystallized region. Crystallizationresults for 70Si–7La glass have shown that for long treatment times,even for the lowest treatment temperature, NaLaF4 formation isfollowed by NaF precipitation, which was not observed for lower SiO2
compositions. This can be explained taking into account the model ofa silica shell formed around the crystals. The 19F NMR spectrum of the70Si–7La-530 °C-20 h sample shows evidence for both NaLaF4 crystalsand amorphous fluorine involved in Al–F–Na groups (Fig. 3).Increasing treatment time to 50 h gives rise to additional crystalliza-tion of NaF, and 72 h treatment shows an even larger amount of NaFcrystals. Fluorine diffusion from Al–F–Na groups into NaLaF4 crystalshas thus occurred only up to 20 h, or for less than 50 h. Then, NaFprecipitates because fluorine cannot diffuse anymore to form NaLaF4crystals, despite at that particular treatment temperature, the latterare thermodynamically favored.
5. Conclusions
Crystallization of LaF3 and NaLaF4 phases has shown to depend onglass composition and structure. Annealing above Tg of aluminosili-cate glasses with up to 55mol% SiO2 and above 20mol% Al2O3 resultsin crystallization of LaF3, while higher silica contents with loweralumina ones produce NaLaF4 nano-crystals. Fluorine in the glassesappears to be involved in Al–F–Na bonds as well as La–F ones of eitherLaF3 or NaLaF4 as crystallization nuclei. Annealing of the glassesinduce crystal growth with temperature and time through fluorinediffusion from Al–F–Na groups in the glass into crystals. Aluminumatoms are in four-fold coordination in all glasses and glass-ceramicsstudied and, meanwhile no changes are taking place in the sodiumcoordination environment of LaF3 crystallized glass-ceramics, 23NaNMR allowed to estimate that the amount of crystalline NaLaF4 in70Si–7La glass-ceramics is around 1.3 wt.%.
The mechanism of nano-crystallization is explained to proceed asfollows: thermal treatment of the aluminosilicate glasses induces thediffusionoffluorine from theglassmatrix (Al–F–Nabonds) into theLa–Fbond-enriched nuclei, giving rise to a decrease in the concentration of
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Al–F–Na and the increase in La–F bonds. [SiO4] structural units mustrearrange in order to facilitate crystal growth and once a silica-enrichedregion around the crystals is formed, further fluorine diffusion andcrystal growth are hindered. Then, for long treatment times, as observedin the highest silica containing glasses, crystallization of NaF takes place,i.e. when no more fluorine can diffuse into NaLaF4 crystals to continuetheir growth.
Acknowledgements
This work has been supported through the INTERCONY project,contract No. NMP4-CT-2006-033200, within the FP6 of the EuropeanUnion, and the Integrated Action HF2007-0101 (MICINN-Ministère del'Education). The FEDER, Région Nord Pas-de-Calais, Ministère del'Education Nationale de l'Enseignement Supérieure et de la Recherche,CNRS, and USTL are acknowledged for the funding of NMR spectro-meters. The authors also greatly acknowledge the assistance of B. Revelin recording theNMR spectra at theNMR centre of the University of Lille.A. De Pablos-Martín thanks her JAE-doctoral fellowship from CSIC.
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