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Journal of Non-Crystalline Solids 352 (2006) 4101–4111

Ex situ XRD, TEM, IR, Raman and NMR spectroscopy ofcrystallization of lithium disilicate glass at high pressure

T. Fuss a,*,1, A. Mogus-Milankovic b, C.S. Ray c, C.E. Lesher d,R. Youngman e, D.E. Day a

a Ceramic Engineering Department and Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65409, USAb Rudjer Boskovic Institute, 10 000 Zagreb, Croatia

c Marshall Space Flight Center, NASA, Huntsville, AL 35812, USAd Department of Geology, University of California-Davis, Davis, CA 95616, USA

e Corning Incorporated, Science & Technology Division, Corning, NY 14831, USA

Received 28 October 2005; received in revised form 11 June 2006Available online 11 September 2006

Abstract

The structure of Li2O Æ 2SiO2 (LS2) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 �C, respec-tively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle �7� lowerthan that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressureform of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four dif-ferent Si sites. NMR results for 6 GPa indicate the presence of Q4 species with (Q4)Si–O–Si(Q4) bond angles of �157�. This is the firstreported occurrence of Q4 species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si arefound.� 2006 Elsevier B.V. All rights reserved.

Keywords: Crystals; Raman scattering; Pressure effects; Nuclear magnetic (and quadrupole) resonance; FTIR measurements; Silicates; Medium-rangeorder; Short-range order; X-ray diffraction

1. Introduction

The effect of pressure on the viscosity and structure ofsilicate melts has been for last decade an area of great inter-est to geologist and material scientists [1–4]. For geologist,understanding of pressure induced structural changes inmelt is critical for understanding the behavior and proper-ties in melts within the Earth, while materials scientists areinterested in pressure induced changes in melt propertiesthat occur in industrial processing that involves application

0022-3093/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2006.06.038

* Corresponding author. Tel.: +1 508 351 7194; fax: +1 508 351 7723.E-mail address: Tihana.Fuss@saint-gobain.com (T. Fuss).

1 Author is currently employed by Saint-Gobain High PerformanceMaterials at their Northborough R&D facility.

of pressure. Today it is a common practice to study pres-sure induced changes in melt structure via studies of thestructure of a fast quenched glass obtained from a meltprocessed at high pressure, high temperature conditions[5–9].

High pressure–high temperature research conducted onpotassium and sodium silicate melts showed that those sys-tems, when subjected to high pressure, go through a seriesof phase transformations that result in series of new highpressure crystalline phases [5]. Most of those phases showpresence of five or six coordinated Si, which, combinedwith the reduction of Si–O–Si bond angles, is consideredto be one of the main mechanisms accommodating densifi-cation of silicate glasses [2,4,5,9]. The effect of pressureon the structure of lithium silicate melts has not been stud-ied in detail. Preliminary work on lithium disilicate system

Table 2

4102 T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111

has shown a possible pressure induced preferential crystal-lization of lithium metasilicate phase over lithium disilicatecrystalline phase [10].

The present investigation deals with the effect of pres-sure on a structure of lithium disilicate glass subjected tohigh hydrostatic pressure (P4.5 GPa). We show that pres-sure can cause large structural changes in lithium disilicateglass. It is also shown that (at T < Tmelting) a pressure of6 GPa is sufficient to change glass crystallization path froman expected lithium disilicate crystalline phase to a newdenser crystalline phase.

2. Experimental

Lithium disilicate glass was prepared by melting a 200 gbatch of Li2CO3 and SiO2 at 1400 �C for 4 h and character-ized as a stoichiometric LS2 glass using XRD, IR andRaman spectroscopy (for more details regarding glasspreparation and characterization see [11]). Pressure of 4.5and 6 GPa was applied to the glass using an 6/8 octahedralmulti-anvil (MA) high pressure apparatus [12]. The pres-sure–temperature–time schedule used for the present exper-iments is shown in Table 1 where T(TC) is the thermocoupletemperature (±2 �C) and t(TC) is the time the sample washeld at pressure P and temperature T(TC) [11]. During heattreatment in the high pressure apparatus the sample expe-rienced both vertical (�24 �C/mm; hot spot located at themid plane of the sample) and radial temperature gradients(hot spot located at one of the samples edges). Temperaturegradient determination showed that in the MA press, thetemperature at the outer surface of a sample (edge) isaffected by both the radial and longitudinal temperature

Table 1Pressure–temperature–time schedules used

Sample Appliedpressure(P/GPa)

Sampletemperature(TTC) ± 2 �C

Holdingtime(tTC) (min)

Sample conditionafter high pressure hightemperature treatment

C1 4.5 608 20 Partially crystallized(medium)a

C2 4.5 608 20 Partially crystallized(medium)a

A1 6 555 20 GlassA2 6 608 20 Partially crystallized

(light)a

A3 6 627 20 Partially crystallized(light to medium)a

A4 6 656 20 Partially crystallized(medium)a

A5 6 656 20 Partially crystallized(medium)a

A6 6 422 60 GlassA7 6 20 60 GlassB1 6 656 60 Partially crystallized

(medium to heavy)a

B2 6 753 60 Partially crystallized(heavy)a

a Crystallinity in the samples was visually estimated based on the lack ofsample transparency.

gradients [11]. In contrast, the temperature in the centerpart of the sample is not strongly influenced by the radialtemperature gradient (temperature gradient in that regionis 6 �C/mm). This area of the sample is therefore definedas the ‘radial isothermal zone’, and represents a circularregion with a diameter of �2 mm whose center coincideswith the radial center of the sample [11].

The samples were analyzed for structural investigation,using IR, Raman, NMR, spectroscopies, XRD andTEM, after being heat-treated at 4.5 or 6 GPa. Due tothe small sample size, prior to the IR and Raman measure-ments, in order to conserve as much of the sample as pos-sible for future XRD, NMR and TEM measurements, eachsample was cut into three disks (top, middle and bottomdisk, each �1 mm thick) with a diamond crystal saw. Disksthat were used for IR and Raman analysis were cut in half,where one half of the disk was ground to a fine powder andused to prepare KBr pellets, while the other half was usedfor Raman spectra measurements. The middle disk wasnormally used for the IR and Raman measurements,except for the C1 sample, whose middle portion was usedfor the IR measurements and top portion was used forthe Raman measurements.

The samples used for the IR and Raman measurementsare described in Table 2. Temperatures given in Table 2are calculated using temperature gradient function (for fur-ther detail see [11]), and the position of the middle disk mea-sured from the top of the sample up to the section fromwhich the disk was made. Temperatures assigned for the

Description of samples used for IR and Raman measurements

IR Spectraa

Sample description TCAL (�C)

C1 1/2 of middle disk 667A1 1/2 of middle disk 611A2 1/2 of middle disk 665A3 1/2 of middle disk 687A4 1/2 of middle disk 714B1 1/2 of middle disk 665B2 1/2 of middle disk 795

Raman Spectrab

Sample name Position in the sample TCAL (�C)

C1 C1 (ISO) 1/2 of top disk (isothermal area) 608C1 (EDGE) 1/2 of top disk (sample edge) 639

A1 A1 (ISO) 1/2 of middle disk (isothermal area) 600A2 A2 (ISO) 1/2 of middle disk (isothermal area) 653A3 A3 (ISO) 1/2 of middle disk (isothermal area) 670

A4 A4 (ISO) 1/2 of middle disk (isothermal area) 700A4 (EDGE) 1/2 of middle disk (sample edge) 731

B1 B1 (ISO) 1/2 of middle disk (isothermal area) 656B1 (EDGE) 1/2 of middle disk (sample edge) 687

a Analysis on the crushed one half of the middle disk (Fig. 1.). Tem-perature given is the average calculated temperature (TCAL) of the samplepart used. Temperature deviation is ±20 �C.

b Analysis on bulk samples (Fig. 1.). Temperature deviation is ±10�.

T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111 4103

IR measurements are the average temperature for the hot-test and the coldest portions on the sample used for theanalysis. The maximum temperature difference betweenthe hot and cold ends of the sample are �40 �C, so it shouldbe taken in consideration that part of the sample used in IRmeasurements experienced a range of temperatures thatvary ±20 �C from the temperature reported in Table 2.

The IR spectra of the samples were obtained with aFTIR spectrometer (Nicolet Research 870 E.S.P) usingthe KBr pellet method. The Raman spectra were measuredusing a 500 mW of 514.5 nm light from Innova 100 coher-ent argon-ion laser and were recorded by a computerizedtype monochromator Dilor model Z 24 at the Rudjer Bosk-ovic Institute, in Croatia. A 90� scattering geometry wasused with the sample oriented at a near-grazing angle.For each half disk that showed edge crystallinity (sampleC1, A4, B1) two Raman spectra were obtained, one fromthe ‘radial isothermal zone’ (noted as (ISO) in Table 2)in the middle of the half disk [11] and a second spectrafrom the edge (noted as (EDGE) in Table 2) of the samemiddle half disk [11]. For samples that showed no, or verylittle edge crystallinity (samples A1, A2 and A3), Ramanspectra were measured only from the ‘radial isothermalzone’ in the middle of the half disk. The temperaturesassigned to the Raman spectra were calculated using thetemperature gradient function [11] and the position of thebeam on the surface of the disk. The temperature uncer-tainty depends only on the precision of positioning of theRaman beam and is estimated to be ±10 �C.

XRD data were acquired using Philips XPERT PROX-ray diffractometer over 2h range from 0� to 70� usinggrazing incidence with scan rate of 1�/min. Grazing inci-dence XRD analysis was preformed on samples C1 (pow-der), C2 (bulk/middle disk), A4 (bulk/middle disk) andB2 (powder) samples. Transmission Electron Microscopy(TEM) was performed using a Philips EM 430T micro-scope with 300 kV electron beam. The microscope wasaligned and conditioned at 300 kV before the experiment.Instrument camera length was calibrated using a LS2 con-trol-crystal powder sample. Known crystalline parametersof this crystal were used to calibrate the camera length to1103 mm. TEM analysis was performed on sample A4.One half of a top disk was ground using a mortal and pes-tle, and the powder (<200 lm) was deposited on Cu grid.The diffraction pattern from the sample was obtained usinga SAD aperture and analyzed using Desktop Microscopistsoftware.

Solid-state 29Si MAS NMR spectra were obtained forsamples C1, A1, A5, B2 (see Table 1) and LS2 control-crystalsample [11] using the Corning Incorporated NMR facilities.Prior to NMR measurements, the samples were ground intoa powder (<200 lm) using a mortar and pestle. Each samplewas analyzed at an operating magnetic field strength of11.7 T, corresponding to a 29Si resonance frequency of99.28 MHz. All data were collected with 4 and 5 mm com-mercial magic-angle spinning (MAS) NMR probes, usingsample spinning speeds of 17 kHz for the LS2 control-crystal

sample and B2 sample, 11 kHz for the C1 sample, and10 kHz for samples A1 and A5. Due to high spinning speed,no sidebands were observed in the NMR spectra of the LS2

control-crystal and the B2 sample, but sidebands wereobserved at �18 and �203 ppm in the spectra for the C1sample and at 12 and �195 ppm in the spectra of samplesA1 and A5. All 29Si MAS NMR spectra were acquired withshort radio-frequency pulses (�p/4) and recycle delays of180–300 s. These data were referenced to an external tetra-methylsilane (TMS) standard. Spectral analysis was per-formed using Origin 7.5 Peak Fitting Module.

3. Results

3.1. IR and Raman spectra

Fig. 1(a) shows the IR spectra of one half of the middledisk of samples A1 and A2 that were processed at 6 GPaand at TCAL of 611 ± 20 �C and 665 ± 20 �C for 20 min,respectively. For comparison, the IR spectrum of a LS2

control-glass sample is shown. The shape of the IR spectrawas analyzed using Origin 7.5 Peak Fitting Module. Theassignment of the IR bands is given in Table 3.

As can be seen in Fig. 1(a), the IR spectra of the LS2

glass do not change dramatically after treatment at6 GPa and high temperature. The slight shift of the777 cm�1 band toward lower frequencies, and the shift ofthe 1049 cm�1 band to higher frequency can be attributedto pressure induced changes in Si–O–Si bond angles [13–16]. Fig. 1(b) shows the IR spectrum of sample A4 thatwas heat treated at 6 GPa and TCAL of 714 �C. The peakposition and their assignment to characteristic vibrationsare given in Table 4. The IR spectra of samples A3, B1and B2, see Table 2, were identical to that for sample A4(in Fig. 1(b)), if difference in intensity is disregarded.

Like the bands in the IR spectrum of crystalline lithiumdisilicate (LS2) and lithium metasilicate (LS) the bands ofsample A4 are also very sharp and well defined. The IRbands and their assignment to different vibrations for sam-ple A4, LS2 and LS are given in Table 4. The IR spectra ofsample A4 shows the most prominent bands at 501, 616,731 and 849 cm�1, see Fig. 1(b) and Table 4. These bandscorrespond to Si–O deformation band at 518 cm�1, sym-metric stretch of Si–O–Si band at 604 and 734 cm�1, andasymmetric Si–O–Si band or �O–Si–O� asymmetric vibra-tions at 846 cm�1, respectively, found in crystalline LS [13–16]. However, as can be seen in Table 4 and Fig. 1(b), thesebands are slightly shifted in the frequency. Without know-ing the crystalline structure of crystal formed in the LS2

glass under 6 GPa pressure, it is difficult to explain the ori-gin of those shifts. A possible explanation is that high pres-sure caused a structural rearrangement in the Si–O–Si bondangles and/or Si–O bond lengths. Even though the IR spec-tra of the high pressure crystalline phase show some simi-larities to the IR spectra for crystalline LS, similaritieswith the spectrum for crystalline LS2 are also present.For example, the band at 773 cm�1 in the high pressure

Table 3IR bands for samples A1 and A2

IR band (cm�1) Assignment of theIR bands [13,16]A2(1/2 middle

disk) – 6 GPaat TCAL 665 �Ca

for 20 min

A1(1/2 middledisk) – 6 GPaat TCAL 611 �Ca

for 20 min

Li2O Æ 2SiO2

control-glass

468 465 473 dOSiO771 774 777 msSiOSi935 937 935 msSiOSi and mSiO�

1055 1050 1049

a Temperature given is the average temperature for one half of the disk.Estimated temperature deviation is ±20 �C of the value given.

1200 1100 1000 900 800 700 600 500

Li2Si

2O

5 control-crystal Li

2SiO

3 crystal

A4 (6 GPa; TCAL

= 714oC for 20 min)

% T

rans

mitt

ance

Wavenumbers (cm-1)

A4

1200 1100 1000 900 800 700 600 500

A1 (6 GPa; TCAL= 611 +/- 20oC for 20 min)

A2 (6 GPa; TCAL= 665+/- 20oC for 20 min) LS2 control glass

% T

rans

mitt

ance

Wavenumber (cm-1)

A1 (6 GPa; TCAL= 611 +/- 20oC for 20 min)

A2 (6 GPa; TCAL= 665+/- 20oC for 20 min) LS2 control glass

Fig. 1. (a) IR spectra of one half of the middle disk of samples A1 and A2that were heat treated at 6 GPa and temperatures TCAL of 611 ± 20 �Cand 665 ± 20 �C for 20 min, respectively. For comparison, the IRspectrum of a LS2 control-glass sample is shown and (b) IR spectrum oflithium disilicate sample A4 that was partially crystallized at 6 GPa andtemperature, TCAL, of 714 �C for 20 min. The spectra for crystallinelithium disilicate and lithium metasilicate [33] are shown for comparison.

4104 T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111

spectra (see Fig. 1(b)) is close to the symmetric Si–O–Siband located at 756 and 782 cm�1 in crystalline LS2.

In the region above 1100 cm�1, the bands at 1118 and1209 cm�1 can correlate with the symmetric Si–O–Si andSi–O� vibrations that are in crystalline LS2 at 1108and 1212 cm�1, respectively. The spectra of the high pres-sure crystal formed in sample A4 show two bands at802 cm�1 and 1162 cm�1 whose origin is not known at thistime, but may be related to the structural deformationcaused by the high pressure.

Fig. 2(a) shows the Raman spectrum obtained from theisothermal portion of the one half of the middle disk thatwas heat treated at 6 GPa at temperature TCAL = 600 ±10 �C for 20 min (A1(ISO) in Table 2). The spectrum ofsample A2 that was taken from the isothermal area inthe one half of the middle disk heat treated at 6 GPa andTCAL = 653 ± 10 �C for 20 min (A2(ISO) in Table 2) is alsoshown in Fig. 2(a). The Raman spectra measured from theisothermal portion of samples A3, A4 and B1, see Table 2,were identical to the spectra of samples A1 and A2 shownin Fig. 2(a). As can be seen from Fig. 2(a), the Raman spec-tra consist of dominant vibration bands close to 600, 950and 1078 cm�1. The band at about 600 cm�1 for glassesheat treated at 6 GPa can be correlated to the 583 cm�1

band present in the LS2 control glass whose spectrum isshown for comparison in Fig. 2(a). The band at 583 cm�1

in the LS2 glass is attributed to Si–O–Si stretching vibra-tions and the degree of polymerization of silicate units[17,18]. However, this band is also related to the Si–O–Sibend vibrations. Since the frequency of the Si–O–Si vibra-tion depends on the Si–O–Si angle, the shift of this band tohigher frequency, in the 6 GPa samples, is attributed to areduction of the Si–O–Si bond angle [17–19]. The shift ofthis band to higher frequencies is similar to that reportedby Xue and Sherriff [18] for the densification of sodiumdisilicate glass at pressures up to 10 GPa. Xue and Sherriff[18] also suggested that this shift might be due, not only tothe reduction of bond angles but also, as was proposed byMatson et al. [17], to a higher concentration of three mem-ber siloxane rings whose vibration is expected around600 cm�1.

Results in Table 5 and Fig. 2(a) also suggest that pro-cessing at 6 GPa and high temperature might have causedthe 474 cm�1 peak in LS2 control-glass to ‘transform’ into a �500 cm�1 shoulder. The peak at 474 cm�1 in lithiumdisilicate glass has been observed previously [17,20] and hasbeen attributed to a feature characteristic only to SiO2. It isinteresting to note that among alkali disilicate composi-tions only the lithium containing silicate glasses have thislow frequency band.

The Raman spectra of samples heat treated at 6 GPashown in Fig. 2(a) have two characteristic bands at�950 cm�1 and �1077 cm�1 that are assigned to Si–Ostretching vibration in Q2 and in Q3 tetrahedra, respectively[17,19,20]. The width at the half maximum for the1077 cm�1 peak for all 6 GPa quenched samples is�10 cm�1 larger than the width of this band in the control

Table 4Frequencies in the IR spectra of partially crystallized samples

IR band (cm�1) Assignment of theIR band [13–16]

IR band (cm�1) Assignment of theIR band [13–16]A4 (1/2 middle disk) – 6 GPa

at TCAL 714 �C/20 minLi2Si2O5

control-crystalLi2SiO3 crystal [33]

470 dOSiO501 518 Si–O deformation548 548 dOSiO616 635 msSiOSi 604 msSiOSi731 734 msSiOSi

773 756 msSiOSi782 msSiOSi

802849 846 masSiOSi m�asOSiO� m�s OSiO�

937 936 msSiOSi and mSiO� 932973

1040 1022 msSiOSi and mSiO� 1054

1118 1108 msSiOSi and mSiO�

11621209 1212 msSiOSi and mSiO�

T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111 4105

(1 atm) sample. The Raman bands observed in the rangefrom 580 to 770 cm�1 are related to the symmetric Si–O–Si vibrations. The band at 770 cm�1 for A1 and A2 samplesis similar to that in the spectrum of LS2 control-glass at773 cm�1 attributed to the Si–O–Si symmetric stretchingmode and assigned by Lazarev [16]. According to the calcu-lations the mode at 583 cm�1 for LS2 can be classified asinherently antisymmetric rattling of Si against O(BO)(BO means bridging oxygen) [15]. However, this indicatesthe presence of small amounts of silicate tetrahedra withdifferent number of bridging oxygens. This also impliesthat the glasses are more disordered than crystals withrespect to tetrahedral population. This band shifts tohigher frequency at 597 and 600 cm�1 for A1 and A2 sam-ples, respectively.

Further, band at 583 cm�1 and barely detectable band at492 cm�1 are commonly assigned to the symmetric BObending vibrations of four and three membered rings[23]. It can be seen slight that the disappearance of493 cm�1 indicates changes in the ring distribution.

Fig. 2(b) shows the Raman spectra obtained from theedge of the middle half disk (B in Fig. 1) of the sampleA4 that was heat treated at 6 GPa at TCAL = 730 ± 10 �Cfor 20 min (A4(EDGE) in Table 2). For comparison, theRaman spectra of crystalline lithium disilicate and lithiummetasilicate [21] are also shown in Fig. 2(b). The Ramanspectra measured at the edge of the middle disk in the B1sample (B1(EDGE) in Table 2) showed no significant dif-ference when compared to the A4(EDGE) spectra shownin Fig. 2(a). The assignments of the Raman bands in theregion higher to 800 cm�1 for the spectra in Fig. 2(b) aregiven in Table 6 according to Ref. [21].

The high frequency portion of the Raman spectra incrystalline alkali silicates has been thoroughly investigatedby Sprenger [21] and calculated by Dowty [15]. The struc-ture for crystalline lithium disilicate consists of sheets of Q3

silica tetrahedra that are represented in the Raman spectraas a high intensity Q3/333 band located at 1088 cm�1, asshown in Fig. 2(b) and Table 6. (In Qa/bbb the superscript‘a’ in Q denotes the type of central Q-species, and thesuperscript ‘b’ denotes the type of neighboring Q-specieslinked to it [21].) The intensities of the other bands in theregion from 800 to 1200 cm�1 for crystalline LS2 are veryweak and are most probably due to the crystal imperfec-tions, since the crystal spectrum was not obtained from asingle crystal LS2 but from fully crystallized LS2 glass.However, the main Si–O–Si symmetric stretching modeoccurs at 530 cm�1. As noted in [15] calculated atomicmotions in this mode are similar to those in ideal sheetsof silica tetrahedra. The strong band at 425 cm�1 arisesprobably from rocking and symmetric bending vibrationof BO atoms [19,23]. This band is related to the numberof Si–O–Si linkages in silicate units. Comparing the inten-sity of this band to the band at the same position for LScrystal (Fig. 2(b)) it can be seen an increase of the relativenumber of interconnected silicate units.

The lithium metasilicate crystal spectrum in Fig. 2(b)was reproduced from [21]. The spectrum of crystalline LScontains an intense peak at 976 cm�1 that is assigned tothe Si–O stretching vibration in Q2/22 silica tetrahedra,which are the main structural units in crystalline lithiummetasilicate. There is two side bands at 1019 and835 cm�1 which are in the approximate positions of themain symmetric stretching bands for silicate tetrahedrawith different numbers of bridging oxygens [1,15]. Theintense band at 650 cm�1 is related to the Si–O–Si symmet-ric stretching mode [15,22]. Correspondingly, Ramanbands in the region from 200 to 400 cm�1 arise mainly frombending Si–NBO vibrations [22].

The Raman spectra of the crystalline phase formed at6 GPa (sample A4) is quite different from the spectra forcrystalline LS2 or LS crystal, see Fig. 2(b) and Table 6.

200 400 600 800 1000 1200 1400

LS2 control-glass

A1(ISO) (6 GPa; TCAL

= 600+/- 10 oC for 20 min)

Re

lativ

e In

tens

ity

A2 (ISO) (6 GPa; TCAL

= 653+/- 10 oC for 20 min)

LS2 control-glass

A1(ISO) (6 GPa; TCAL

= oC for 20 mi A2 (ISO) (6 GPa; T

CAL= o

Raman shift (cm-1)

300 400 500 600 700 800 900 1000 1100 1200

A4

LS2

LS

Li2SiO3 crystal [21] Li2Si2O5 control-crystal

A4 (EDGE) (6 GPa; TCAL=730+/- 10oC for 20 min)

Rel

ativ

e In

tens

ity

Raman shift (cm-1)

Q3/333

Q2/22

Fig. 2. (a) Raman spectra of sample A1 processed at 6 GPa and TCAL of600 ± 10 �C and sample A2 processed at TCAL 651 �C ± 10 �C for 20 min.The solid line presents the Raman spectrum of the control LS2 glass and(b) Raman spectrum of LS2 sample crystallized at 6 GPa at a temperatureof TTC = 730 ± 10 �C for 20 min. The Raman spectra of crystalline LS[21] and LS2 are shown for comparison.

Table 5Assignment of bands in the Raman spectra of samples A1, A2 andcrystalline LS2

Raman band (cm�1) Assignment of theRaman bands[19,21]

A1 (ISO) 6 GPaTCAL = 600�C/20 min

A2 (ISO) 6 GPaTCAL = 653�C/20 min

Li2O Æ 2SiO2

control-glass

505 (shoulder) 515 (shoulder) 474 Q4?597 600 583 Si–O–Si stretching

and/or bending778 777 773 Si–O–Si stretching948 951 947 Si–O� stretching

1077 1076 1081 Si–O� stretching

Table 6Assignment of bands in the Raman spectra of sample A4 and crystallineLS and LS2

Raman bands (cm�1) Assignment of theRaman bands [21]

A4 (EDGE) 6 GPaTCAL = 730�C/20 min

Li2Si2O5

control-crystalLi2SiO3

crystal[20]

Raman band(cm�1)

Qa/bbbb

844 835 838 Q1/2

934 921 943 Q2/33

981a 976a 980 Q2/22

1027 1015 1019 1015 Q3/322

1068 1101a 1052 Q3/332

1088 Q3/333

a Most intense band.b Notation from Sprenger [21]. Bands are assigned to Si–O stretching

vibrations in SiO4 tetrahedra. In Qa/bbb the superscript ‘a’ in Q denotes thetype of central Q-species, and the superscript ‘b’ denotes the type ofneighboring Q-species linked to it.

4106 T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111

The largest band at 981 cm�1 for 6 GPa crystalline phase islocated in the frequency region that is characteristic for Si–O stretching vibration in Q2/22, accompanied with anotherstrong band at 1068 cm�1 associated with Si–O stretching

vibration in Q3/333. When these bands are compared withthe bands associated with Q2/22 and Q3/333 tetrahedra inthe control lithium disilicate and lithium metasilicate crys-tal, see Fig. 2(b) and Table 6, it can be seen that in theA4(EDGE) sample spectra the Q2/22 band is shifted towardhigher wave numbers, while Q3/333 band is shifted towardsmaller wave numbers. Such a shift in wave numbers hasbeen documented before by Brawer and White [20] forRaman spectra of some alkali disilicate and alkali metasili-cate glasses. The reason for this type of shift is notunderstood.

The IR spectra for the crystallized center portion ofsample C1 (heat treated at temperature of TCAL = 667 ±20 �C for 20 min at 4.5 GPa; Table 2) is shown atFig. 3(a). The IR spectrum for this sample is very similarto the spectrum for crystalline LS2 (dashed line inFig. 3(a)) except for a shoulder at �844 cm�1. This shoul-der has been found at a similar frequency as the 838 cm�1

band associated with the �O–Si–O� in (Q2 species) in crys-talline LS, see Table 6. This shoulder is evidence that Q2

tetrahedra formed either in the glass or in the crystal struc-ture as a function of pressure.

The Raman spectra for an upper portion of sample C1(heat treated at 608 ± 10 �C for 20 min at 4.5 GPa; Table2) is identical to the spectra for the control sample (seeFig. 3(b)).

The Raman spectrum for the top portion of the sampleC1 (heat treated at 639 ± 10 �C for 20 min at 4.5 GPa) ispresented in Fig. 3(b). As can be seen from Fig. 3(b), thespectrum for the sample heat treated at 4.5 GPa closelyresembles the spectrum for crystalline lithium disilicate.The only difference is the presence of the small band at977 cm�1 [21]. Sprenger has assigned this band to Q2/22

species, see Table 4.

3.2. XRD and TEM analysis

Samples A4 (partially crystallized) and B2 (almost fullycrystallized), were analyzed by X-ray diffraction. The XRD

1200 1100 1000 900 800 700 600 500

% T

rans

mitt

ance

LS2 control-glass Li2Si2O5 control-crystal

C1 (4,5 GPa; TCAL=667+/- 20 oC for 20 min)

Wavenumbers (cm-1)

400 600 800 1000 1200

Li2Si2O5 control-crystal

C1(EDGE) ( 4,5 GPa; TCAL=639+/-10 oC for 20 min)r

LS2 control-glass

C1(ISO) ( 4,5 GPa; TCAL=608+/-10 oC for 20 min)

Rel

ativ

e In

tens

ity

Raman shift (cm-1)

Fig. 3. (a) IR spectrum of the one half of the middle disk of sample C1processed at 4.5 GPa and temperature of TCAL = 667 ± 20 �C for 20 min.For comparison the IR spectra of the LS2 glass and crystalline LS2 is shownand (b) comparison of the Raman spectra taken from the isothermalportion of one half of the top disk of sample C1, C1(ISO), and edge of thesame one half of the top disk C1(EDGE), both heat treated at 4.5 to theRaman spectra of LS2 control-glass and control-crystal.

0 10 20 30 40 50 60 70

B2 (powder)

[ 72-1140 Li2SiO3 ]

[ 82-2396 Li2Si2O5 ]

Arb

itara

ry u

nits A4 (bulk, middle disk, edge)

2 degreeθ

Fig. 4. Comparison of XRD spectra of an LS2 glass processed atTTC = 753 �C at 6 GPa for 60 min (sample B2) and sample A4 processedat TTC = 656 �C at 6 GPa for 20 min with the XRD lines for crystallineLS2 [24] and LS [25]. XRD pattern for sample B2 was from a powderedsample (whole sample was used), while XRD pattern for sample A4 wasmeasured from the edge in the middle disk.

Table 7Calculated unit cell parameters and density for lithium metasilicate (LS)crystals at 1 atm, and for the crystals present when the LS2 glass (sampleB2) was crystallized at a pressure of 6 GPa for 60 min at TTC = 753 �C.Crystal parameters for B2 sample were calculated from experimental XRDlines and TEM SAD diffraction pattern

LS crystal(1 atm, [24])

Deformed LS crystal(B2, 6 GPa); XRD

Deformed LScrystal (B2, 6 GPa); TEM

a = 9.36 A a = 9.10 ± 0.05 A a = 9.28 ± 0.5 Ab = 5.39 A b = 5.25 ± 0.05 A b = 5.35 ± 0.5 Ac = 4.67 A c = 4.79 ± 0.05 A c = 4.92 ± 0.5 Aqcal = 2.53 g/cm3 qcal = 2.61 ± 0.06 g/cm3 qcal = 2.44 ± 0.5 g/cm3

T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111 4107

patterns for these two samples are compared with the stan-dard XRD pattern for crystalline LS and LS2 from JCPDSfiles [24,25] in Fig. 4. The XRD patterns for these samplesare similar to that of crystalline LS, but most of the majorpeaks are shifted, showing a deformed LS structure. Thecrystal parameters calculated from the XRD pattern ofsample B2 are given in Table 7 along with the latticeparameters for crystalline LS [25]. The unit cell of the crys-tals present in the 6 GPa sample appears to have a slightlysmaller ‘a’ and ‘b’ and a slightly longer ‘c’ parameters whencompared to the unit cell dimensions that have beenreported [25] for crystalline LS.

The crystalline phase in sample B2 was also analyzedusing Transmission Electron Microscopy (TEM), and the

selected area diffraction (SAD) pattern for this sample isshown of Fig. 5. The diffraction pattern in Fig. 5 wasidentified as being crystalline LS with [110] zone axis.The crystal axes calculated from the diffraction patternare as follows: a = 9.28 ± 0.5 A, b = 5.35 ± 0.5 A and c =4.92 ± 0.5 A. Thus, the TEM analysis confirmed that crys-talline phase in sample B2 has the same diffraction patternas for crystalline lithium metasilicate. Although the accu-racy in calculating unit cell parameters from XRD patternis ten times better (Table 7) than that in measuring fromTEM, the unit cell parameters calculated from XRD andTEM are in very good agreement (see Table 7).

Sample C1 heat treated at a pressure of 4.5 GPa had ameasured density of 2.43 ± 0.01 g/cm3 [11]. XRD analysis(Fig. 6) of sample C1 showed that it contained crystallinelithium disilicate. When this experiment (C2) was repeated,the sample had the density of 2.50 g/cm3 and XRD analysis(Fig. 6) showed that it contained a mixture of crystalline

Fig. 5. (a) TEM (300 keV, L = 1103 mm) Selected area diffraction (SAD)pattern obtained from sample B2 and (b) SAD pattern indexed as [110]lithium metasilicate.

[ 72-1140 Li2SiO3 ]

[ 82-2396 Li2Si2O5 ]

Arb

itara

ry u

nits

C1 (powder)

C2 (bulk, middle disk,edge)

[82-2396 Li2Si2O5]

[72-1140 Li2SiO3]

[ 72-1140 Li2SiO3 ]

[ 82-2396 Li2Si2O5 ]

[82-2396 Li2Si2O5]

[72-1140 Li2SiO3]

[ 72-1140 Li2SiO3 ]

[ 82-2396 Li2Si2O5 ]

[82-2396 Li2Si2O5]

[72-1140 Li2SiO3]

[ 72-1140 Li2SiO3 ]

[ 82-2396 Li2Si2O5 ]

[82-2396 Li2Si2O5]

[72-1140 Li2SiO3]

0 10 20 30 40 50 60 70

2 degreeθ

Fig. 6. Comparison of XRD spectra of an LS2 glass heat treated atTTC = 608 �C at 4.5 GPa for 20 min (samples C1 and C2) with the XRDpattern for crystalline LS2 [24] and LS [25].

Q4(3)

Q4(4)Q4(2)

Q2

Q2

B2 sample6 GPa

Q4(1)

Q3

Q3

Q3

C1 sample4.5 GPa

-70 -80 -90 -100 -110 -120 -130

-70 -80 -90 -100 -110 -120 -130

-70 -80 -90 -100 -110 -120 -130

ppm

ppm

ppm

LS2 control-crystal

Li2Si2O5 control - crystal

C1 sample4 .5 GPa

B 2 sample6 GPa

B2 sample6 GPa

C1 sample4.5 GPa

LS2 control-crystal

C1 sample4.5 GPa

B2 sample6 GPa

Fig. 7. NMR spectra for samples of lithium disilicate glass thatcrystallized at 4.5 GPa (TTC = 608 �C for 20 min) and at 6 GPa(TTC = 753 �C for 60 min). NMR spectra of crystalline LS2 is shown forcomparison.

4108 T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111

lithium disilicate and lithium metasilicate in a ratio ofabout 1:1 (as estimated from the intensity of the XRDpeaks). The exact reason for this inconsistency is notknown, but since the C2 sample had a higher density thanthe C1 sample, it is believed that the pressure applied tothese two samples may have been somewhat different.

3.3. NMR analysis

The 29Si MAS NMR spectra for samples of lithium disi-licate glass that crystallized at 4.5 GPa pressure at TTC of608 �C for 20 min, and 6 GPa pressure at TTC of 753 �C

for 20 min is shown in Fig. 7. The NMR spectrum ofLS2 control-crystal is also shown for comparison. Peakanalysis was preformed using Origin 7.5 Peak Fitting Mod-ule and is presented in Table 8.

According to Englehardt and Michel [26] the NMRpeaks located between �60 and �120 ppm correspond to29Si in tetrahedral sites, where the Q2 sites have an expectedchemical shift of �75 ppm, peaks due to the Q3 tetrahedraare located at �93 ppm and peaks due to the Q4 tetrahedraare located at �107 ppm. Control crystalline LS2 samplepresented in Fig. 7 shows a large peak at �92.1 ppm anda small peak at �93.5 ppm, both of which correspond toQ3 tetrahedra species [26–28].

Table 8Comparison and abundance of 29MAS NMR spectra for the samples processed at 6 and 4.5 GPa

Crystalline Li2Si2O5 (1 atm) 4 and 5 GPa Li2Si2O5 TTC = 608 �C for20 min

6 GPa TTC = 753 �C for 60 min Peak assignment

Chemical shift (ppm) Abundance (±0.5%) Chemical shift (ppm) Abundance (±0.5%) Chemical shift (ppm) Abundancea

�75.0 3.6 �74.5 33.7 ± 1.4 Q2

�92.1 95.2 �92.3 96.4 �90.7 37.5 ± 2.5 Q3

�93.5 4.8 Q3

�103.8 15.3 ± 4.4 Q4

�105.8 5.3 ± 2.1 Q4

�117.1 3.2 ± 0.1 Q4

�118.3 4.8 ± 0.4 Q4

a Increased error is due to the uncertainly in background determination.

A1

A5

Q4

Q3

B2

Q2

-70 -80-50 -60 -90 -100 -110 -120 -130

ppm

Fig. 8. Comparison between NMR spectra for different samples of lithiumdisilicate glass heat treated at 6 GPa. Sample B2 was heat treated atTTC = 753 �C for 60 min and was heavily crystallized. Sample A5 was heattreated at TTC = 656 �C for 20 min and showed medium crystallization;Sample A1 was heat treated at TTC = 555 �C for 20 min and showed novisible signs of crystallinity.

T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111 4109

The 29Si MAS NMR spectrum of sample C1 presentedin Fig. 7 shows two characteristic features: a broadenedpeak at �92.3 ppm due to Q3 tetrahedra and a small peakat �74.8 ppm due to Q2 tetrahedra. It should be noted that,to simulate the area under the Q3 peak at �92.3 ppm, fivedifferent Gaussian peaks had to be used to fit the experi-mental peak. This is an indication of the complexity inQ3 bond angle distribution.

The 29Si MAS NMR spectrum of sample B2 heat treatedat TTC = 753 �C for 60 min at 6 GPa shows six distinctNMR peaks, see Fig. 7. The peak at �74.5 ppm has beenassigned to Q2 tetrahedra while the peak at �90.7 ppm isdue to Q3 tetrahedra. Considerably less negative chemicalshift of Q3 tetrahedra, when compared to 1 atm LS2 crystalQ3 peak, is attributed to the change is Si–O–Si bond angles.

Janes and Oldfield [28] proposed an empirical correla-tion between the chemical shift and the local structurearound the Si ions where the chemical shift becomes lessnegative with decreasing mean Si–O–Si angle. Thus, thechemical shift of �90.7 ppm can be assigned to a meanSi–O–Si angle of 136.5� (shift of �92.1 ppm for 1 atmLS2 crystal NMR is assigned to a mean Si–O–Si angle of139.2�).

The remaining small peaks, in the B2 spectra, between�103.8 and �118.1 ppm are assigned to four different Q4

tetrahedral sites – Q4(1), Q4(2), Q4(3) and Q4(4) respec-tively, see Fig. 7 and Table 8. Chemical shifts for Q4(1)and Q4(2) are characteristic for Q4 sites found in alkali sil-icates [27,28]. Q4(1) peak at �103.8 ppm is broad whichcan be explained by either (a) a large distribution in anglesindicating that it originates from an amorphous part of thesample (glassy phase) (b) via next nearest neighbor distri-bution effects (for example Q4 surrounded by four Q2 spe-cies). The Q4(2) peak at �105.8 ppm shows a slightlyhigher chemical shift that found in quartz (�107 ppm)which is again assigned to slightly lower bond angles. Peaksat �117.1 and �118.1 ppm are also identified as Q4 peaksalthough their position are shifted far upfield for Q4 spe-cies. Using previously mentioned empirical relationshipbetween the chemical shift and bond angle in Q species, amean Si–O–Si angle of 157.7� for Q4(3) and 158.7� forQ4(4) species was estimated. The Si–O–Si bond angles inthese Q4(3) and Q4(4) are �19� larger than the angles thatare usually found in alumina free alkali silicate glasses [28].

Fig. 8 shows a comparison of 29Si MAS NMR spectra ofsamples A1, A5 and B2 all heat treated at 6 GPa. TheNMR spectra of sample A1 (6 GPa, TTC = 555 �C for20 min) shows a broad signal centered at �88 ppm. Thisbroad peak for sample A1 is consistent with the lack ofany evidence of crystallization, as was also determinedusing optical microscope (maximum magnification 400·).

For 1 atm lithium disilicate glass, position of chemicalshift for glass phase coincides with the chemical shift inlithium disilicate crystalline phases [27] and is locatedbetween �92 at �93 ppm. Considerably less negativechemical shift of A1 peak when compared to expected

4110 T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111

1 atm value is attributed to the lowering in average Si–O–Sibond angle by �7�.

The 29Si MAS NMR spectra of sample A5 (6 GPa,TTC = 656 �C for 20 min) in Fig. 8, is typical for a partiallycrystallized sample [27]. The presence of sharp Q2, Q3 andQ4 peaks in the B2 sample can also be seen in sample A5accompanied by strong amorphous peak centered at�88 ppm that is present in A1 sample.

There is no evidence of Si in 5- or 6-fold coordination(Si5 or Si6) in the NMR spectra, as usually seen in highpressure high temperature quenched melts [4,5,27,29]. Nopeaks were detected in the �150 to �250 ppm region wherepeaks for high coordinated Si are expected.

4. Discussion

Structural analysis suggests that densification in the LS2

glass at the temperature region T < Tmelting lowers the glassfree molar volume by deforming the Si–O–Si bond anglesbetween adjacent SiO4 tetrahedra and increasing the distri-bution of the Q-species. The evidence for decrease in Si–O–Si bond angles by �7� can be found in the shift in the posi-tion of a broad amorphous peak at �88 ppm in the NMRspectra of sample A1 (6 GPa) compared to �92 ppm insample C1 (4.5 GPa) and expected position of this peak(�93 ppm) for lithium disilicate glass at 1 atm. It is inter-esting to note that the position of the broad amorphous,glass NMR peak at 6 and 4.5 GPa samples is different.The reduction of the average bond angle in the 6 GPa glassshows that a pressure of 6 GPa is large enough to modifythe glass structure by decreasing the average Si–O–Si bondangle by �7�. Therefore, at 6 GPa pressure, in contrast to4.5 GPa, the glass appears to be in a state where it can nolonger respond to applied pressure just by simply loweringthe free molar volume of the glass but it needs to alter itsbasic structure. Similar pressure induced reduction in Si–O–Si bond angles was found in amorphous SiO2 subjectedto 5 GPa pressure at 600 �C [30].

Further, the shift in the 583 cm�1 band in Raman spec-tra of 6 GPa samples to higher wave numbers is an indica-tion of a decrease in the Si–O–Si bond angle and/or anincrease in the concentration of three member siloxanerings.

The crystallization of a LS2 glass at high pressure has avery dramatic effect on the structure of the crystalline phasethat forms even at temperatures below Tmelting. At a pres-sure of 4.5 GPa, the glass crystallizes to crystalline lithiumdisilicate as indicated by IR, Raman, NMR and XRD. TheIR, Raman and NMR spectra also show the presence of asmall amount of Q2 SiO4 tetrahedra. Since the 4.5 GPasample crystallized as lithium disilicate (consists of onlyQ3 tetrahedra) it is not certain if those newly formed Q2 tet-rahedra are a part of a glassy or crystalline phase wherethey act as some kind of imperfection. Formation of Q2

species could also be a result of pressure induced dispro-portionation reactions 2Q3

M Q2 + Q4, as discussed by[1,6,29,30]. Applying a pressure of 6 GPa to the LS2 glass

changes the crystallization path of the initial glass. Theoverall results suggest that the lithium disilicate glass (den-sity = 2.32 ± 0.01 g/cm3) densified under the pressure of6 GPa to 2.53 g/cm3 [11] crystallizes as a single new high

pressure phase with a calculated density of 2.61 g/cm3.XRD and TEM analysis of sample B2 (Fig. 8), show thatphase a diffraction pattern resembling a slightly deformedlithium metasilicate crystalline phase.

Based on the XRD and TEM data and glass stoichiom-etry, the NMR spectra of that same sample (B2) shouldshow only one sharp Q2 peak associated with crystallinelithium metasilicate, a broad amorphous Q4 peak originat-ing from glassy SiO2 remaining after crystallization, and abroad amorphous Q3 peak associated with Q3 tetrahedrafrom the residual glass.

The NMR spectra of sample B2, as well as the spectra ofthe A5 sample, shows not only the expected presence ofcrystalline Q2 (found at �74.5 ppm), and amorphous Q4,(found at �103.8 ppm), but also the presence of four morepeaks attributed to Q3 and Q4 tetrahedra. The NMR peakassociated with Q3 tetrahedra is sharp (crystal-like) andtherefore it cannot be correlated with residual glass. Theabundance of Q tetrahedra in 6 GPa sample (Table 7)shows that the Q2 and Q4 tetrahedra are present in a ratioof 1:1. Data collected from IR and Raman spectra confirmsthe presence of both Q2 and Q3 tetrahedra.

This new crystalline phase has a complex structure con-taining Q2, Q3 and Q4 tetrahedra and yields a XRD andTEM diffraction pattern very similar to that of crystallinelithium metasilicate. The question raised here is – is it pos-sible to have one crystal that will have the same symmetry(and therefore diffraction pattern) as LS crystal but will beformed out of three different tetrahedral species? This sce-nario is possible if crystal that is formed is a crystalline lith-ium metasilicate solid-solution.

Formation of lithium metasilicate solid solution (LS-ss)in the lithium silicate glasses was previously postulated byHasdemir et al. [31] and Baker et al. [32]. According tothese authors LS-ss forms by statistical exchange of fourLi+ ions from LS crystal lattice, by one Si4+ and three cat-ion vacancies. This statistical substitution and distributionof silicon does not change crystals symmetry (and thereforeits XRD pattern), but does, due to the change in latticeparameters, cause a systematical shift in peak positions.Presence of this shift is in [31,32] considered as a conforma-tion for a formation of a solid solution.

When compared to a XRD pattern of 1 atm lithiummetasilicate crystal XRD pattern obtained from crystalsformed at 6 GPa shows a shift in peak positions character-istic of solid solution formation. Based on this shifts calcu-lated change in d spacing for this high pressure crystal(Table 7) is almost four times larger than those reportedby [31,32] observed for LS-ss that form under 1 atmconditions.

While this type of systematical substitution will notchange crystal symmetry it will change the number ofbridging and non bridging oxygens in the crystal. If in

T. Fuss et al. / Journal of Non-Crystalline Solids 352 (2006) 4101–4111 4111

the process of solid solution formation four Li ions aresubstituted by one Si, solid solution crystals will have asmaller number of NBO (and therefore Q2 tertahedra)and higher number of BO (Q3 and Q4 tetrahedra) thatthe ‘regular’ LS crystal. Conformation of this can be foundin presented IR, Raman and NMR results. The reason whyXRD and TEM techniques are indifferent toward thesechanges is because those two techniques are not sensitivetoward the change in the distribution of bridging andnon bridging oxygen unless this changes crystal symmetry.Therefore obtained IR, Raman and NMR results here alsoserve as a conformation that when lithium disilicate glass issubjected to the pressure of 6 GPa it crystallize as a lithiummetasilicate solid solution.

5. Conclusion

At temperatures below 750 �C application of hydrostaticpressure of 6 GPa lowers the average bond Si–O–Si bondangle and changes glass crystallization path from a lithiumdisilicate crystalline phase to what is believed to be a lith-ium metasilicate solid solution. The new high pressurephase that forms at 6 GPa has a complex structure withat least four different Si sites. NMR of the 6 GPa sampleshowed the presence of Q4 species with (Q4)Si–O–Si(Q4)bond angles of �157�. This is the first time that Q4 specieswith such large bond angles have been reported in aluminafree alkali silicate glass. No evidence for five or six coordi-nated Si was found.

Glass densified at 6 GPa has an average Si–O–Si bondangle �7� lower than that found in glass subjected to4.5 GPa.

Acknowledgments

The work was supported by National Aeronautics andSpace Administration (NASA), Grant # NAG8-1465 andNational Science Foundation (EAR-0001245). We wouldalso like to thank Professor V.M. Fokin, Professor J. Sch-meltzer and Professor E.D. Zanotto for helpful discussions.

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