3.Zhengda Pan.pdf

.Journal of Non-Crystalline Solids 210 1997 130135

Raman spectra and thermal analysis of a newleadtelluriumgermanate glass system

Zhengda Pan ), Steven H. MorganCenter for Photonic Materials and Deices, Physics Department, Fisk Uniersity, Nashille, TN 37208, USA

Received 7 June 1996; revised 28 August 1996

Abstract

.Differential scanning calorimetry DSC and Raman scattering studies of a new glass system, leadtelluriumgermanate .glasses in the form of 90yx GeO PxTeO P27PbOP10CaO with xs0, 10, 20, 30, and 40, are reported. The glass2 2

samples were fabricated using a conventional melt-quenching method. The Raman spectra and possible glass structures arediscussed for different TeO contents. The results indicate that increasing TeO content up to 40 mol% in the glass system2 2decreases the glass transition temperature and melting temperature, and suppresses the crystallization tendency in the fiberpulling temperature range. The leadtelluriumgermanate glass, GTPC, possesses a larger refractive index and a smallermaximum phonon energy than that of a leadgermanate glass, 63GeO P27PbOP10CaO, and shows a better thermal2

.stability compared to a tellurite glass, 75TeO P20ZnOP5Na O TZN . These improved properties could be beneficial for2 2fabricating rare-earth doped fiber devices.

1. Introduction

The properties and structures of leadgermanateand tellurite based glasses have been the subject of

w xseveral recent investigations 15 . Germanate andtellurite based glasses generally have better mechani-cal strength, chemical durability, and thermal stabil-ity than fluoride based glasses, and therefore aremore promising than halide glasses for practical useas long optical fibers. The smaller maximum vibra-

y1 .tional energy 750 to 850 cm of germanate andtellurite glasses compared to that of silicate ;1150

y1 .cm glasses, results in a smaller multiphonon de-cay rate of doped rare-earth ions, compared to that in

w xsilicate based glasses 14 .

) Corresponding author. Tel.: q1-615 329 8537; fax: q1-615329 8634; e-mail: [email protected].

GeO has long been recognized as a good glass2w xformer 6 . GeO based glasses exhibit better in-2

.frared IR transmission than SiO based glasses due2to the larger size and the heavier mass of germaniumwhen compared to that of silicon. Leadgermanatebased glass systems have been previously reportedhaving a large glass forming region while maintain-

w xing a high infrared transmittance 7,8 . A specificglass of the composition 63GeO 27PbO10CaO2 . yGPC was noted to have less OH contamination,but crystallized in the fiber pulling temperature rangew x2 .

TeO is not able to form a glass itself, but it2w xforms glasses with modifying oxides 9 . Tellurite

glasses have a wide transmission region 0.35 to 5.mm , the lowest maximum phonon energy among

the common oxide glasses, and a large refractiveindex, compared to both silicate and fluoride glasses

0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. .PII S0022-3093 96 00603-5

( )Z. Pan, S.H. MorganrJournal of Non-Crystalline Solids 210 1997 130135 131

w x3 . The large refractive index and the small phononenergy are desirable for radiative transitions of rare-

w xearth ions 1 . The PbOTeO GeO glass was first2 2reported as a low-loss infrared transmitting glassw x10 . Few studies on PbOTeO GeO glasses have2 2been reported. One recent article reports on thethird-order non-linearity of PbOTeO GeO2 2glasses, where GeO was added to improve the glass2

w xforming ability and thermal stability 11 .In this article, we report on the thermal properties,

refractive index, and Raman spectra of a new leadtelluriumgermanate glass system in the form of .63yx GeO PxTeO P27PbOP10CaO with differ-2 2ent TeO content, x. Based on the glass properties2studied, we propose a new glass composition, leadtelluriumgermanate glass, GTPC, as a possible can-didate material for rare-earth doped fiber devices.

2. Experiment

Glasses were prepared from anhydrous oxidepowders of GeO , PbO, and TeO , and anhydrous2 2carbonate powder of CaCO . The PbO and CaCO3 3were common reagent grade, GeO was of 99.9999%2

.purity Eagle Picher Research Lab , and TeO was2 .of 99.99% purity Alfa, Johnson Matthey . The glass

.compositions were in the form of 63yx GeO P2xTeO P27PbOP10CaO with xs0, 10, 20, 30, and240. The CaO was added to reduce the OHy contami-

w xnation 2 . The batch powders of 30 g were thor-oughly mixed, and melted in fused silica crucibles ina temperature range of 780 to 11008C. The liquidswere held for 1.5 h and cast onto a copper plate andpressed by another copper plate from the top, form-ing a glass plate about 2 mm in thickness. Theglasses were subsequently annealed just below theglass transition temperatures for 50 min and thencooled to room temperature in the furnace.

The Raman spectra were taken using a right anglescattering geometry on a double-grating spectrometer .Spex Model 1403 with a cooled photomultiplier .RCA3134 and photon-counting system. The excita-tion source of Raman measurements was an argon

.ion laser Coherent Innova 90 operating at 514.5 nm .and 50 mW. Both parallel polarized HH and cross

.polarized HV spectra were recorded.The refractive index, n, was measured using an

.ellipsometer type 43603-200E, Rudolph Research .

The thermal properties were analyzed by differential .scanning calorimetry DSC on a scanning calorime-

.ter PerkinElmer DSC-4 . A glass sample, about 60mg, was heated at a rate of 108Crmin, from roomtemperature to 6008C. The sample pans used werematched pans made of stainless steel.

3. Results

3.1. DSC results

Fig. 1 shows the DSC curves for leadtelluriumgermanate glasses with different TeO contents. The2glass transition temperature, T , gradually decreasedgwith increasing TeO content. The 63GeO P27PbO2 2

.P10CaO GPC sample has a sharp exothermal peakat 5718C, indicating a strong crystallization. Withincreasing TeO content, the exothermal peak is2gradually suppressed. For samples with 30 mol%TeO , there is no exothermal peak in the temperature2region up to 5908C, but there is a bump near 6008Cwhich is close to the glass melting point. For sam-ples with 40 mol% TeO , there is a broad bump2around 5608C, which may indicate some crystalliza-

.Fig. 1. DSC curves of 67y x GeO PxTeO P27PbOP10CaO2 2glasses.

( )Z. Pan, S.H. MorganrJournal of Non-Crystalline Solids 210 1997 130135132

Table 1 .Thermal properties and refractive index of 63y x GeO PxTeO2 2

P27PbOP10CaO glasses with different TeO content x2Properties xs0 xs10 xs20 xs30 xs40

.T C 505 470 420 385 365g .T C 560 560 570 no nox .T C ;940 ;600 ;610m

.n 5461 A 1.79 1.82 1.86 1.91 1.95

T : glass transition temperature, T : crystallization onset tempera-g xture, T : glass melting temperature, n: refractive index.m

tion tendency. The melting temperatures were testedby placing samples in an electric furnace, graduallyincreasing the temperature to the point that the sam-ple becomes liquid. The melting temperature Tm

decreases from about 9408C for GPC glass without.TeO to near 6008C for glasses with 30 mol% to 402

mol% TeO . Table 1 lists the measured thermal2properties and refractive index of glasses with differ-ent TeO content.2

3.2. Raman spectra

Figs. 2 and 3 illustrate the parallel-polarized andcross-polarized Raman spectra of leadtelluriumgermanate glasses with different TeO content, x,2

.Fig. 2. Parallel-polarized Raman spectra of 67y x GeO PxTeO2 2P27PbOP10CaO glasses.

.Fig. 3. Cross-polarized Raman spectra of 67y x GeO PxTeO P2 227PbOP10CaO glasses.

respectively. These spectra are composed of threespectral regions: Low-frequency region F 250

y1 . y1 .cm ; intermediate region 250 to 600 cm ; and y1 .high-frequency region )600 cm . The Raman

spectrum of GPC sample agrees well with the re-ported spectrum of the same composition in a previ-

w xous study 4 . With increasing TeO content and2decreasing GeO content, the evolution of Raman2

.spectra show the following features: i The highfrequency Raman band decreases in frequency from

y1 . .820 to 750 cm and increases in intensity; iiintermediate bands at 430 and 525 cmy1 decrease inintensity, while a weak band at 330 cmy1 grows; .iii a polarized shoulder at the high frequency sideof the low-frequency band grows. These spectralfeatures reflect the structure change of the sampleswhen tellurium oxide content increases.

4. Discussion

4.1. Thermal analysis

.Clear glasses were formed for 63yx GeO P2xTeO P27PbOP10CaO system with xs0, 10, 20,230 and 40. These glasses appear to be of goodoptical quality, with no visual evidence of devitrifi-


cation, however, attempts to make glass with 50mol% TeO were unsuccessful because of phase2separation.

The DSC results show that substitution of GeO2with TeO decreases the glass transition temperature,2T , and melting temperature, T . The T decreasedg m gfrom 5058C for sample GPC to 3658C for a samplewith 40 mol% TeO . The corresponding melting2temperature decreased from circa 9408C to near6008C. The T and T values of GPC samples areg m

w xconsistent with that reported in a previous study 2 .Our DSC curve of a GPC sample shows an exother-mal peak at 5718C with a crystallization onset tem-perature T s5608C. With increase of TeO content,x 2this exothermal peak shifts to higher temperaturesand is gradually suppressed. No exothermal maxima,therefore no crystallizations were observed for glasswith 30 mol% TeO up to 5908C. A criterion,2Hrubys ratio, has been frequently used to measure

w xglass stability 12,13 . Hrubys ratio involves theglass transition temperature T , the melting tempera-gture T , and the crystallization onset temperature T ,m x

w xand has been defined as 12

T yTx gK s .G T yTm x

According to this formula, the larger the difference,T yT , and the smaller the temperature interval,x gT yT , the stronger is the inhibition to processes ofm xnucleation and crystallization, and consequently thegreater the glass forming tendency for the system. Inour leadtelluriumgermanate glass system, a higher

.TeO content up to 40 mol% decreases T and T2 m g .T and T are approximately related by 2r3 ruleg mand increases T , both lead to an increase in Hrubysxratio. These results indicate that incorporation ofTeO into the leadgermanate glass network im-2proved the glass thermal stability. The improvedglass stability is desirable to achieve a large workingrange during operations such as preform preparation

w xfor fiber drawing 2,3 . The optimized TeO content2 .in the 63yx GeO PxTeO P27PbOP10CaO sys-2 2

tem is 30 mol%. This new glass is labeled GTPC.We also compared our GTPC glass to a tellurite

.glass 75TeO P20ZnOP5Na O TZN which was2 2previously reported as a new candidate for fiberdevices and had a glass transition temperature at

2998C and a crystallization onset temperature at .4178C, which gives a working range T yT sx g

w x1188C 3 . The GTPC glass has a much larger work- .ing range )2008C without crystallization, there-

fore shows a better thermal stability than TZN glass.We found that the TZN glass and binary leadtel-lurite glasses have a tendency to crystallize duringthe conventional melting-quench process.

4.2. Raman scattering and glass structure

y1 .The low-frequency F250 cm Raman scatter-ing is attributed to the collective modes of local

wstructures and heavy metal vibrational modes 14x16 . There is almost no change with increasing TeO2

.content for the depolarized component Fig. 3 , whichhas been attributed to the collective modes of local

w xstructures 15 . However, the polarized component . y1Fig. 2 has changed, the intensity near 90 cm hasdecreased while a shoulder at the high frequency

y1 .side ;130 cm has grown with the increase ofTeO content. The polarized band near 130 cmy1 is2attributed to the PbO symmetric stretching modesin PbO pyramids while the component at near 904cmy1 is due to the vibration of PbO bonds with a

w xmore ionic bonding nature 1416 . Our low-frequency Raman results therefore suggest that thebonding strength between lead cation and oxygenanion has been enhanced in GTPC glass as comparedto GPC glass.

Raman scattering in the intermediate region 300y1 .to 580 cm has generally been attributed to the

deformation vibrational modes of glass networkw xstructure with bridged oxygen 1720 . The Raman

bands at 430 to 525 cmy1 are attributed to the bondrocking modes of the glass network units with bridged

.anion i.e., bridged GeOGe in germanate glassw x17 . In a glass structure with mixed glass formers,the connection between structure units could benon-symmetric bridged oxygens andror non-bridged

w xoxygens. Previous studies 7,21 have indicated that,with increased non-symmetric bridged oxygensandror non-bridged oxygens in glass network, theRaman scattering increases in the high frequencyregion and decreases in the intermediate region,which is consistent with our observed Raman scatter-ing features. The weak band at 330 cmy1 from

( )Z. Pan, S.H. MorganrJournal of Non-Crystalline Solids 210 1997 130135134

GTPC samples has been assigned to symmetric2y w xbending vibration of TeO units 19 .3

The Raman scattering in the high-frequency re- y1 .gion )600 cm has been attributed to the stretch-

ing vibrational modes of the glass network formerw x1720 . The Raman spectrum of the GPC sampleagrees well with the reported spectrum of the same

w xcomposition in a previous study 4 . Lead oxideenters the glass as an intermediate between a net-work former and a network modifier, increases thenumber of non-bridging oxygens in the glass net-

w xwork, and weakens the GeO bond 1,15 . As aresult, the GeO stretching band shifted to 820cmy1, less than that of the pure GeO glass which2

y1 w xhas a maximum phonon band around 860 cm 17 .The Raman spectrum of tellurite glasses has two

y1 w xhigh frequency bands at 745 and 665 cm 11 .These two Raman bands are well understood. The665 cmy1 band is from the TeO stretching vibra-

.tion of TeO trigonal bipyramids tbp units while4the 745 cmy1 band is attributed to that of TeO3

. w xtrigonal pyramids tp units 11,20 . It has beensuggested that the relative intensities of the Ramanbands assigned to tbp and tp units stand for the

w xfractions of these two units in glass 22 . The Ramanspectra of our GTPC samples with 30 to 40 mol%TeO show a strong high-energy phonon band peaked2at 750 cmy1. The Raman band from GeO stretch-ing vibration was red-shifted and merged under thehigh-frequency side of the broad 750 cmy1 band,and a small shoulder at 665 cmy1 appears at thelow-frequency side which is from TeO tbp units.4

. .The large intensity ratio of I 750 rI 665 indicatesthat tellurium oxide is mostly in the form of tp unitsin the GTPC sample. Previous studies suggest that

. ythese tp units are in the O Te 5O O or1r2y w xO Te5O configurations 19,20,22 . These fea-2r2

tures of the Raman spectra suggest that a mixedtellurium oxide and germanium oxide glass networkhas been formed in GTPC sample, where the tel-lurium oxide is mostly in the tp units and germanium

w xoxide in the distorted tetrahedral units 23 . Theinterconnections could be the non-symmetric bridgedoxygens andror non-bridged oxygens. The positionof the highest-phonon band is important because themultiphonon decay of rare-earth ions in a glassdepends on the maximum-phonon energy of the host

w xglass 24,25 . Our Raman spectra indicate that the

maximum-phonon band in GTPC sample is at 750cmy1, smaller than the 820 cmy1 in GPC sample.We also measured the infrared reflectance spectrumof GTPC sample, the maximum infrared band peakedat 650 cmy1 with a shoulder extending to about 750cmy1. Because the rate of multiphonon decay issensitive to the maximum vibrational energy of theglass, a smaller multiphonon decay rate of rare-earthions in GTPC glass compared to GPC glass is there-fore expected.

5. Conclusion

We have presented the DSC and Raman data on anew leadtelluriumgermanate glass system, 63y

.x GeO PxTeO P27PbOP10CaO with xs0, 10, 20,2 230, and 40. Our results indicate that incorporatingtellurite oxide into the lead germanate oxide networkimproves the glass thermal stability and decreasesthe maximum phonon energy. The new GTPC glassshowed a better thermal stability than both a leadgermanate glass, GPC, and a tellurite based glass,TZN, reported previously. The improved thermalstability is desirable for fiber fabrication. In addition,a smaller maximum phonon energy and a largerrefractive index are also desirable for reducing multi-phonon decay rate and enhancing the radiative transi-tion of rare-earth ions in the host glass, respectively.

Acknowledgements

The authors would like to acknowledge the finan-cial support of NASA through grant NAGW-2925.

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