The Structural, Thermal, and Optical Analyses of MulticomponentGermanium Oxide Glasses for Engineering Mid-Infrared Fiber

Chemical Sensing

Xin Jiang,w Joris Lousteau, and Animesh Jha

Institute for Materials Research, University of Leeds, Clarendon Road, LS2 9JT, U.K.

We demonstrate the suitability of multicomponent germaniumoxide (GeO2)-based glasses for engineering on-line chemicalsensors operating in the 2–5 lm spectral region. Bulk glasssamples based on the oxide and oxy-fluoride multicomponentcompositions GeO2–PbO–Na2O, GeO2–PbO–Na2O–Ga2O3,and GeO2–PbO–Na2O–Ga2O3–PbF2, respectively were ana-lyzed for comparing thermal, structural, and optical properties.These glasses were melted using in an oxygen-rich atmosphereabove 12001C, and annealed after casting before undertakingdetailed thermal analysis. Glass compositions with especiallyhigh fluoride and lead concentrations are reported to have theglass transition temperature above 3501C, which makes themsuitable for operation in the 1501–2001C temperature range forchemical sensing and process metrology. The effects of compo-sition on the thermal, structural, and optical properties of theseglasses are studied using differential thermal analysis, Ramanand infrared reflection spectroscopy, and prism coupling tech-niques. The compositional and wavelength dependence of re-fractive indices are also analyzed from which the value of thezero dispersion wavelength was determined.

I. Introduction

IT is generally known that most chemical bonds, such as theO–H, C–H, C–O, and S–H, have stronger fundamental vib-

rational signatures in the mid-infrared (IR) range than theircorresponding harmonics or overtones in the near-IR region.1

These characteristic absorptions are normally used for identify-ing chemical species, for example, benzene (C6H6), methanol(CH3OH), ethanol (C2H5OH), acetone ((CH3)2CO), etc.2–4 It istherefore for chemical sensing operated in the mid-IR region isof great interest. In order to characterize the fundamental modesof molecular vibrations, optical fiber sensors, based on mid-IRfibers, may be able to determine the concentrations, both qual-itatively and quantitatively, by following the Lambert–Beer’slaw equation.5 Such light-based sensors remain immune fromexternal influences from electromagnetic fields in the environ-ment, and can be compact and rugged, and fabricated cost-effectively.6 On the other hand, synthesis of IR glass materialsremains low cost and the well-developed techniques for silicafibers can be used for reference of IR fiber fabrication. In spite ofsuch a promise of potentials for a fiber optic chemical sensor,chemical sensor-based fiber optics are not as popular as thestandard optical or data communication device. This is becausethe mid-IR fiber fabrication technology is quite underdevelopedand is full of challenges. This is because the raw materials usedfor fiber fabrication are often impure and contribute to the

overall absorption loss in the fiber, which then obscures the mid-IR spectral range for any potential metrological applications. Inthe last 20–30 years, significant efforts have been made in de-veloping fluoride fibers, based on ZrF4–BaF2–LaF3–AlF3–NaF(ZBLAN), heavy metal oxide based on tellurium oxide (TeO2)and germanium oxide (GeO2), and chalcogenide AsGeSeTe andGeS2 glass fibers.

7–9 Among these IR glasses, GeO2-based heavymetal oxide glasses have the advantages such as the ease of fiberdrawing, extended visible to IR transmission window from 0.35 to5.5 mm, high resistance against mechanical and environmentaldamage, and high glass transformation temperature (Tg) over3501C.7–10 In particular, the high Tg of the GeO2 glasses makesthem suitable for chemical sensing and metrological applicationsin the 1501–2001C range.9 However, in spite of the attractiveproperties of GeO2 glasses, limited glass property and fiber engi-neering research have been carried. McPherson and Murray11

studied several series of GeO2 glasses synthesized for surgical lasertransmission within the 2.5 and 3.0 mm regions. Wang et al.12 for-mulated a PbO–GeO2–ZnO glass and attempted to draw fibers bystretching with a soft silicate cladding layer; however, the condi-tions for drawing were not precisely defined due to the lack ofviscosity data. Furthermore, Wang and colleagues pointed outthat a large number of surface defects were observed, which con-tributed to a high scattering loss in the fabricated fibers.

In this study, we have systematically investigated the thermal,optical, and structural properties of a number of different GeO2

glass-forming multicomponent compositions: GeO2–PbO–Na2O, GeO2–PbO–Na2O–Ga2O3, and GeO2–PbO–Na2O–Ga2O3–PbF2. The incorporation of PbF2 in the quarternaryoxide compositions, shown above, is particularly relevant forenhancing the mid-IR transparency by lowering the overall ab-sorption loss from OH� ions. The relationships between theeffects of glass compositions on thermal, optical, and structuralproperties will facilitate engineering of low-loss fibers for mid-IRchemical sensing applications. For this purpose, glass propertieswere investigated through differential thermal analysis (DTA),Raman, and Fourier transform infrared (FTIR) spectroscopy.Materials dispersion properties and the refractive index (RI), asa function of wavelength, were also measured from visible tonear-IR region.

II. Experimental Procedure

Details of the compositions of the glass prepared are presentedin Table I. The compositions can be regrouped in four differentseries.

Four different types of multicomponent compositions wereexplored in detail for structural, thermal, and optical charac-terisations of GeO2-based glasses. These were ternary composi-tions based on (70�x)GeO2–30PbO–xNa2O (x5 0, 5, 10, 15)and (90�x)GeO2–xPbO–10Na2O (x5 10, 20, 30, 40), in whichthe effects of Na2O and PbO, respectively, were investigated. Inorder to develop an understanding of the thermal stability ofmulticomponent compositions described above, partial substi-tutions of Na2O and PbO were carried out in the above ternary

J. Ballato—contributing editor

This work was financially support by the GlaxoSmithKline and the EPSRC multi-corefiber (EP/C515234/1) project.

wAuthor to whom correspondence should be addressed. e-mail: xin.jiang@mpl.mpg.de

Manuscript No. 27345. Received January 7, 2010; approved May 5, 2010.

Journal

J. Am. Ceram. Soc., 93 [10] 3259–3266 (2010)

DOI: 10.1111/j.1551-2916.2010.03913.x

r 2010 The American Ceramic Society

3259

compositions, and new quaternary compositions were:(90�x)GeO2–xPbO–6Na2O–4Ga2O3 (x5 20, 30), 55GeO2–30PbO–11Na2O–4Ga2O3, and 56GeO2–31PbO–9Na2O–4Ga2O3. Once again, the effects of Na2O and PbO wereinvestigated for comparison with the ternary compositions.Finally, for controlling the residual concentrations of OH�

ions in the resulting glass compositions after melting, the PbF2

compound was incorporated in the starting quaternary mixturesfor removing OH� ions during melting, as explained inErnsberger.13 The new quinary composition range was:56GeO2–(31�x)PbO–9Na2O–4Ga2O3–xPbF2 (x5 3, 9, 15).

The minimum purity of the chemical precursors used for theglass preparation is 99.99% purity. Glass batches of 10 g foreach sample were weighed and mixed in ambient condition. Theoxides mixture was transferred into a platinum (Pt) crucible,which was then placed inside a resistance furnace, followingwhich it was heated up to 12501C. The starting chemicals weremelted for 2 h for all oxide compositions, whereas between 2 and4 h were necessary for all PbF2-incorporated compositions. Themelt was cast into a brass mold preheated at 101C below theglass transformation temperature (Tg) for quenching. The glassformed via quenching was then annealed at Tg�101C tempera-ture for 2 h in an annealing furnace, after which the furnace wasturned off to allow the sample to cool down slowly at a rate of0.51C/min to room temperature. The as-prepared bulk samplesappeared homogeneous and free of bubbles under optical mi-croscopy. Each annealed sample was then cut and fine polisheddown to 0.6 mm using a set of polishing papers with an averagegrain size at 30, 20, 10, and 6 mm, followed by polishing sus-pensions with 3–0.6 mm particulates. The final thickness of thesamples following grinding and polishing was 4 mm.

The characteristic temperatures of a glass are the glass trans-formation (Tg), the onset of the crystallization (Tx), and the glassmelting (Tm) temperatures, which were determined using a com-mercial Perkin-Elmer DTA 7 differential thermal analyzer(Cambridge, UK; DTA). Before any measurements, the DTAanalyzer was first calibrated for a baseline correction scan usinginert alumina powder in the reference and sample holders in thetemperature range from 501 to 8001C. Fragments of 20 mg foreach glass sample were weighed and placed inside the sampleholder, also made of an alumina crucible. The glass fragmentswere then covered with inert alumina powder for providinggood heat transfer between the glass fragments and its sur-rounding, because over a large temperature range from 2001 to8001C, the heat transfer characteristic rapidly changes from theconduction at lower temperatures to radiative mechanism athigher temperatures than 5001C. The scanning speed was 101C/min. The values of Tg and Tx were determined by measuring theonset values of the glass transition and crystallization peaks,respectively,14 as illustrated in Fig. 1. During DTA scans, the

values of Tm were only recorded below 8001C, which is why inTable I, for several compositions the values of Tm are not re-ported. Also, there was risk of damage to the DTA cell above8001C due to the preferential volatilization of PbO, which thenreacts with the Pt casing and thermocouples, and thereforedamages the Pt and Pt-alloy components of the cell.

The analysis of glass structure was carried out using Ramanand IR reflection spectroscopic techniques, which only yieldvibrational spectra of the structural units present in a compo-sition. The Raman spectra were measured using a Renishaw in-Via Raman spectrometer (New Mills, Gloucestershire, UK) inthe 200–1000 cm�1 region, excited with a 514 nm Ar1 ion laser.A Bruker Vertex 70 infrared spectrometer (Coverntry, UK) wasused to obtain the IR reflection spectra in the range from 1000 to400 cm�1 in a scanning resolution of 4 cm�1.

The optical transmission window of the GeO2 glasses wasmeasuredwith FTIR andUV/visible absorption spectrophotom-eters, and based on which, the total intrinsic loss was calculated.The FTIR and UV/visible spectra were recorded in the range of1–25 mm and 0.2–0.6 mm using a Bruker Vertex 70 spectrometerand a Perkin-Elmer Lambda 19 spectrometer, respectively.

The RIs were measured using a Metricon 2010 Prism Coupler(Pennington, NJ) at wavelengths of 0.532, 0.633, 1.321, 1.476,1.55, and 1.56 mm. Owing to the nonuniformity in the flatness of

Table I. Characteristic Thermal Properties of the Germanate Glasses Series S1–S4w,z

Sample ID Glass composition (mol%) Tg (721C) Tx (721C) Tm (721C)y dT5Tx�Tg HR (Tx�Tg)/(Tm�Tx)

S1-1 70GeO2–30PbO 452 593 736 141 0.98S1-2 65GeO2–30PbO–5Na2O 428 555 722 127 0.76S1-3 60GeO2–30PbO–10Na2O 392 570 729 178 1.12S1-4 55GeO2–30PbO–15Na2O 368 596 709 228 2.02S2-1 80GeO2–10PbO–10Na2O 475 624 — 149 —S2-2 70GeO2–20PbO–10Na2O 441 570 741 129 0.75S2-3 60GeO2–30PbO–10Na2O 392 570 729 178 1.12S2-4 50GeO2–40PbO–10Na2O 356 509 698 153 0.81S3-1 70GeO2–20PbO–6Na2O–4Ga2O3 450 — — — —S3-2 60GeO2–30PbO–6Na2O–4Ga2O3 421 610 788 189 1.06S3-3 55GeO2–30PbO–11Na2O–4Ga2O3 387 — — — —S3-4 56GeO2–31PbO–9Na2O–4Ga2O3 387 — — — —S4-1 56GeO2–28PbO–9Na2O–4Ga2O3–3PbF2 391 — — — —S4-2 56GeO2–25PbO–9Na2O–4Ga2O3–6PbF2 389 — — — —S4-3 56GeO2–22PbO–9Na2O–4Ga2O3–9PbF2 386 634 745 237 2.14

wThe results presented in this table complement the limited data of the table from our previous paper.10 zThe scanning rate of 101C/min was used. yTm was not recorded for

glasses with Tm48001C and for glasses with the absence of devitrification peaks.

300 350 400 450 500 550 600 650 700 750

440 450 460 470 480

Tm

TxTg

Hea

t Flo

w E

ndo

up

Temperature [°C]

Temperature [°C]

Hea

t Flo

w [a

.u.]

End

o up

−>

DTA traces of theglass 70GeO2-30PbO Tg

Identification of thegTg

Fig. 1. A differential thermal analysis trace of a germanate glass with acomposition (mol%) 70eO2–30PbO, when scanned at a rate of 101C/min.The graph identifies the characteristic temperatures Tg, Tx, and Tm,which are the glass transition, onsets of crystallization and melting,respectively. The inset shows the method for determining the Tg.

3260 Journal of the American Ceramic Society—Jiang et al. Vol. 93, No. 10

glass samples after polishing, the measured RIs varied in therange of 70.0004 in each sample, when the measurements wererepeated on other positions of the same sample.

III. Results and Discussions

(1) Thermal Properties

GeO2 is well known as a glass former15 in which two differentglass network-forming structures exist, namely the tetrahedralGeO4 with a fourfold and the octahedral GeO6 with a sixfoldcoordination of oxygen.13 The composition dependence of Tg issummarized in Table I, from which it is observed that in a ter-nary glass, with the increasing concentrations of GeO2 from 55to 70 mol%, the Tg increases from 3681 to 4521C. Figure 1shows the DTA traces of the glass S1-1 and the method for de-termining the Tg. A similar trend in the increase in Tg for thesecond glass ternary composition was observed, in which theconcentrations of PbO were varied. As it can be seen in Table I,the increase in Tg is from 3561 to 4751C for the 50–80 mol%GeO2 composition range. In general, it can be summarized thatwithin the glass-forming region, the glass transition temperatureincreases with the concentrations of GeO2 because with a largerfraction of GeO4 and GeO6 structural units in the three-dimen-sional network, the network manifests more continuity thanwhen the alkali concentrations increase and terminate the bridg-ing oxygen (BO) sites.

The magnitude of the values of temperature differenceDT5Tx�Tg and the Hruby parameter HR 5 (Tx�Tg)/(Tm�Tx)are used for determining the stability of a glass against devitri-fication.14,16,17 The magnitude of DT is an important factor thatdetermines the volume nucleation rate Iv, because

Iv / exp�DTkBT

� �

where kB is the Boltzmann constant and T is any arbitrary tem-perature between Tx and Tg.

16 On the other hand, the magnitudeof DTm 5 (Tm�Tx) differential determines the growth rate ofcrystals (Uc) from preexisting nuclei, which is proportional to

Uc /ðDTmÞ2

T

" #

suggesting that as DTm decreases, Uc decreases in a parabolicmanner. The implications of DT and DTm are therefore apparentin the definition of HR.

17 For this reason, a comprehensivecomparison of HR and DT for different glasses are made inTable I, showing that those ternary, quaternary, and quinaryglasses with magnitudes of HR more than 1 are comparativelymore stable than those compositions that have values o1. Ofspecial importance is the composition S1-4 having a DT valueover 2201C and an HR parameter of 2.0, which we further de-veloped for fiber engineering, as reported elsewhere.10 Such asignificant difference in HR and DT has been possible throughthe structural modification of S1-4 composition, having a con-tinuous GeO4 tetrahedron network ,which may also have beenmodified by the PbO linear chain as shown in Eq. (1a) and PbO4

tetrahedra in Eq. (1b). The S1-4 was further modified in S3-4and S4-3 with the incorporation of Ga2O3 with the presence ofGaO4 tetrahedra. The composition S3-4 and S4-3 are very stableas we did not observe significant crystallization.9,15 The incor-poration of Ga2O3 and PbO in the GeO4 structure is explainedin Eqs. (1)–(3).13 For ternary compositions based on the formu-lation: (70�x)GeO2–30PbO–xNa2O, the alkali ion Na1 entersinto the GeO2 glass network by terminating the oxygen sites,which are the nonbridging oxygen sites (NBOs), as explained inEq. (2). The presence of larger number of NBO sites in thestructure appears to lower the glass transition temperature andcorresponding viscosity with increasing concentrations ofNa2O.15 By comparison, the Eq. (3) explains how a GaO4

tetrahedron unit may be able to restore the NBOs in the pres-ence of the presence of Na1 ions. The local charge neutrality dueto the difference between [GaO4]

5� and [GeO4]4� can be re-

stored through the presence of Na1 ion around a NBO in thevicinity of GaO4. The role of Ga31 as a network contributor infourfold and fivefold coordinations and as a modifier in sixfoldcoordination of oxygen is well known in the literature.18 FromTable I, it should be noted that when Ga2O3 was incorporated inthe compositions, e.g., in samples S3-1 to S3-4, the DT could notbe established accurately at the scan rate of 101C/min. By re-peating the thermal scans at lower heating rates at 21 and 51C/min, we observed a characteristic absence of any devitrificationpeaks for samples S3-1, S3-3, S3-4, S4-1, and S4-2 which sug-gested that these glasses exhibit much reduced rates of crystal-lization on reheating than the two sets of ternary compositions,shown in Table I.

O�Gej

j�O�Ge

j

j�Oþ PbO

! O�Gej

j�O� Pb�O�Ge

j

j�O (1a)

O�Gej

j�O�Ge

j

j�Oþ PbO

! O�Gej

j�O� Pb

j

j�O�Ge

j

j�O (1b)

O�Gej

j�O�Ge

j

j�OþNa2O! 2 O�Ge

j

j�O�:::Naþ

� �(2)

(2) Raman and IR Reflection Spectroscopy

The analysis of the structure of GeO2 glasses, listed in Table I,was performed using Raman and IR spectroscopic techniques.Major optical phonon bands are observed in the range of 300–900 cm�1. The spectra shown in Fig. 2 were first normalizedwith respect to the amplitude of the highest peak and thendeconvoluted into five symmetrical Gaussian peaks, A, B, C, D,and E, as shown in Fig. 2. The fitted peak positions were basedon data reported in the literature.15,17–29 The peaks B, C, D, and Ecan all be assigned to vibrations of the tetrahedron GeO4 glass

300 400 500 600 700 800 900

0.20.40.60.81.0

E

Nor

mal

ized

Inte

nsity

Wavenumber [cm–1]

A BC D

0.20.40.60.81.0

E

DCBA

0.00.20.40.60.81.0

ED

CBA

0.00.20.40.60.81.0

E

70GeO -30PbO

65GeO -30PbO-5Na O

60GeO -30PbO-10Na O

55GeO -30PbO-15Na OD

CBA

Fig. 2. Relative intensities of Raman peaks for compositions describedin the glass series 1, (70�x)GeO2–30PbO–xNa2O (x5 0, 5, 10, 15mol%), excited by an Ar1 laser at 514 nm. Each spectrum was decon-voluted into five symmetrical Gaussian peaks marked as A, B, C, D, andE, based on data reported in the literature.15,19–29

October 2010 The Structural, Thermal and Optical Analyses of Multicomponent GeO2 Glasses 3261

structure, whereas the octahedron GeO6 vibrational mode at688 cm�1, reported by Scott,19 was not observed in the preparedglasses. The high-energy Raman peak E atB820 cm�1 is due tothe asymmetric stretching vibrations of O–Ge–O in GeO4 tet-rahedral structural units.18,28 Peak D at B760 cm�1 is assignedto the Ge–O� asymmetric stretching vibrations with NBOs.21

The peaks B (506 cm�1) and C (563 cm�1) are due to thestretching mode contributions from Ge–O–Ge BO’s vibra-tions.21–23 The peak A (B330 cm�1) is ascribed to the antisym-metric vibration of Ge–O–Pb bond.21 The intensities of thesepeaks are directly related to the proportion of each bonding vi-bration reflected in the glass structure. It is evident from Fig. 2that the intensities of peaks B (506 cm�1) and C (563 cm�1) in-crease as the concentration of GeO2 increases. The latter indi-cates that the three-dimensional GeO4 glass network is morecontinuous with the increased Ge–O–Ge linkages, and subse-quently contributes to the increasing intensities of correspond-ing Raman active bands. By comparison, the peak D, which hasthe highest intensity in glass S1-4, originated from the nonbridg-ing oxygen glass network and becomes distinct with the reducedGeO2 concentration. Similarly, the peak E (820 cm�1) is reducedin glasses having low GeO2 concentrations. Canale et al.24

reported that in a GeO2–PbO glass-forming system, when theconcentration of PbO is lower than 40 mol%, the Pb21 ions actas the glass modifiers occupying tetrahedral interstitial sites andcreating nonbridging oxygen Ge–O�. However, when the con-centration of PbO is raised over 40 mol%, Pb21 works as theglass formers by forming covalent bonds with oxygens,23–25 asexplained in Eqs. (1a) and (1b). In Eq. (1a), the bridging of di-valent PbO as a chain is shown, whereas the correspondingPbO4 tetrahedron shared between the two GeO4 tetrahedra isshown in Eq. (1b). The substitution of a GeO4 with a PbO4 unitis discussed by Mydlar et al.30 and Varsheneya et al.31 in thecontext of PbSiO3 and Pb2SiO4 structures

2 O�Gej

j�O�:::Naþ

� �þGa2O3

! O�Gej

j�O�Ga

j

j� O

Naþ

�Gej

j�O�Ga

j

j� O

Naþ

(3)

The optical phonon range between 400 and 550 cm�1 are as-signed to the symmetric vibrations of fourfold coordinatedGe(4)–O–Ge(4) units in three- and four-membered ring struc-tures.29 The ring formation in the context of Eq. (1b), whichdemonstrates structural changes in the presence of PbO4, sug-gests that the GeO4–PbO4–GeO4 symmetric vibration modes inthe Raman spectra will tend to smear out due to the asymmetryassociated in the PbO4 units. The range of symmetric vibrationsexpected from such dissimilar polyhedron pairing can be esti-mated from the linear harmonic model31 for GeO4 and PbO4:

nR ¼1

2p

ffiffiffif

m

s

where f is the force constant in a cation–anion pair and m is thereduced cation–anion mass. When nR for GeO4 and PbO4 werecompared and normalized with respect to the highest phononenergy in the 820–800 cm�1 range for GeO4, the frequency rangefor the ring structures fall in the 400–410 cm�1 range. This rangein both Figs. 2 and 3 are completely smeared; however, there areempirical evidences pointing out that as the Pb-ion concentra-tions increase, the apparent intensities increase quite signifi-cantly. For this normalization process, the values of interionicdistances between Ge41 and O2� and Pb21 and O2� have beentaken from Varsheneya et al.,31 and they are equal to 0.170 and0.230 nm, respectively.

A comparison of the Raman spectra of glasses from glass se-ries 2 to 4 is shown in Fig. 3. All Raman spectra were decon-voluted into five distinct peaks, as explained in Fig. 2, as

assigned with corresponding vibrational modes explained inTable II. The Raman spectrum of glass 50GeO2–40PbO–10Na2O is consistent to that of the glass 55GeO2–30PbO–15Na2O, both of which have a raised intensity of vibrationmode at 760 cm�1. The high concentrations of PbO in these twoglasses contribute to create a number of NBOs in the glassstructure and hence raise the intensity of peak C. The Ramanspectra of glasses S3-2 and S4-2 are distinctly different as com-pared with that of glass S2-4, where the intensity of peak B, thevibration mode of BO, is increased significantly with the addi-tion of Ga2O3. The increased intensity of peak B explains thatthe abundance of BO sites in the presence of potentially fourfoldGa31, which might be contributing to the extended three-di-mensional glass network (Eq. (3)). The Raman analysis for thedependence of the intensities of peak B with Ga31 ion concen-trations also appears to be consistent with the thermal stabilitydata for the series 2 glasses, shown in Table I in Section III(1).

IR reflection spectra are shown in Fig. 4. Two main bands arelocated at 550 and 810 cm�1, with the appearance of a shoulderband at about 730 cm�1. The 810 cm�1 band is due to theasymmetric stretching vibration of Ge–O–Ge bridges and it wasshifted to a lower frequency with the increase of Na2O concen-tration.27,28 The shoulder at 730 cm�1 emerges with the increas-ing PbO concentration and combines with the main 810 cm�1

band to form a broad IR active band from 650 to 850 cm�1.This band appears clearly when the concentration ratio of GeO2

and PbO is increased to values within the range of 1.5–2.0.Kamitsos et al.29 reported that a band at 770 cm�1 in xRb2O–(1�x)GeO2 glasses and assigned this band to the stretchingmode of the nonbridged Ge–O� groups. Similarly, we suggestthat the emergence of the 730 cm�1 arises from the Ge–O�

bonds. This frequency difference may be explained by the Col-umbic forces between Pb41 and O2� are higher than that be-tween Pb21 and O2�, and therefore correspond to aproportionally higher vibrational energy. With the increasingconcentrations of PbO, more Pb21 ions enter into the glass net-work and contribute to the continuity of network, as shown inEqs. (1a) and (1b). The IR active band, centerd at 550 cm�1, wasreported due to the symmetric stretching of the Ge–O–Gelinks.28 As observed from Fig. 4, this band does not change sig-nificantly in the position or intensity with composition. In brief,several points can be concluded. First, the addition of Na2Ofrom 0 to 15 mol% contributes to shift the 810 cm�1 band to alower frequency at 790 cm�1 as compared with the glasses S1-1and S1-4. Second, the increase in the intensity at 730 cm�1 isascribed to the increasing concentration of PbO. Third, there isno obvious difference in the IR reflection spectra when replacingthe PbO content with its fluoride counterpart, PbF2. Fourth, the

300 400 500 600 700 800 9000.20.40.60.81.0

0.20.40.60.81.0

0.20.40.60.81.0

E

C

BA

Nor

mal

ized

Inte

nsity

Wavenumber [cm–1]

E

D

DCBA

ED

CB

56GeO -22PbO-9Na O-4Ga O -9PbF

60GeO -30PbO-6Na O-4Ga O

50GeO -40PbO-10Na O

A

Fig. 3. Relative intensities of Raman peaks of glasses from series 2, 3,and 4. 50GeO2–40PbO–10Na2O, 60GeO2–30PbO–6Na2O–4Ga2O3, and55GeO2–22PbO–9Na2O–4Ga2O3–9PbF2, excited by an Ar1 laser oper-ating at 514 nm. The spectral deconvolution was performed in a similarmanner as in Fig. 2.

3262 Journal of the American Ceramic Society—Jiang et al. Vol. 93, No. 10

IR active band at 550 cm�1 which was assigned to the symmetricstretching of the Ge–O–Ge links, has no distinct change in theposition or intensity with different glass compositions.

A summary of the main conclusions arising from the struc-tural analysis discussed above is given below:

(1) The increase in the concentrations of Na2O leads to therising in the number of nonbridging Ge–O� groups, whichcan be observed from the intensity of the Raman active bandat 760 cm�1.

(2) The high-frequency band at 810–820 cm�1 in Ramanspectra is due to the asymmetric stretching vibration in tetra-hedral GeO4 units. This band is reduced in glasses having lowerGeO2 concentrations.

(3) The Raman spectra do not show any vibrational modesfor the octahedral GeO6 groups

15,19 in the region of 688 cm�1 inthe compositions considered herein.

(4) The intensity of the Raman active bands at 563 and 506cm�1, which were assigned to the vibrations of Ge–O–Ge withBO, increase with the concentrations of GeO2 and PbO.

(5) The addition of Ga2O3 helps in BO sites, which is evi-dent from the comparison of the intensities of peaks at 506 and563 cm�1 in the Raman spectra in Fig. 3.

(6) The role of PbO in the glass structure is complex and itappears from the vibrational spectroscopic analysis that it maypredominantly assuming the role of network contributor in thepresence of alkali, as depicted in examples, shown in Eqs. (1)to (3).

(7) The IR spectroscopic analysis complements the Ramandata in Figs. 2 and 3. In IR, the modes due to the vibrations ofGe–O–Ge reduce with the decreasing GeO2 concentrations. Themodes due to the vibrations of Ge–O–Ge are reduced when theGeO2 concentration decreases. From the binary composition inGeO2–PbO structure analysis, we observe the role of PbO in thenetwork termination by creating O2� sites, for which the vibra-tional modes instead of being amplified in the IR spectra, appearto have reduced because IR spectra represent the symmetric andasymmetric modes of dipoles. As a Pb–O dipole is weaker than aGe–O, the former is significantly reduced in comparison with themodal intensities in the corresponding Raman spectra, becausethe larger polarizability of Pb21 increases the Raman scatteringintensity.

(3) Optical Transmission

The UV/visible and IR absorption spectroscopic analyses werecarried out to determine the glass transmission window and cal-culate the intrinsic loss curves. It has been reported in one of theearlier investigations4 that the GeO2 glasses exhibit transmissionin the 0.36 and 5.5 mm. The total intrinsic loss as a function ofwavelength was then calculated for the glasses 70GeO2–20PbO–10Na2O, 65GeO2–30PbO–5Na2O, and 55GeO2–30PbO–11Na2O–4Ga2O3. The minimum loss value is around 0.003dB/km at around 4.1–4.2 mm for the three glasses. However,this is the theoretical estimation and it only provides informa-tion about the medium, free from any extrinsic contributions.When the fibers are fabricated, the extrinsic defects accumulateand contribute to the overall loss above the intrinsic loss floor,which may be 103–106 times larger depending upon the process-ing conditions.10

(4) RI and Material Dispersion

(A) Wemple–DiDomenico Equation: The measured RIsof all GeO2 reported in Table I glasses are given in Table III.The relationship between the RI and the glass compositions canbe analyzed through the Wemple–DiDomenico theory.32–34 Theequation is based on a single electronic oscillator in the regionfrom UV to near-IR

n2 � 1 ¼ EdE0

E20 � E2

(4)

E ¼ hn ¼ hc

l(5)

Table II. Vibrational Mode Assignments for Raman Peaks of Glasses from Series 1–4 and their Comparison withReported Data

15,17–29

Assigned vibrational mode in GeO2 glass

Reported Raman

peaks of GeO2

glasses nR (cm�1)

Glass nR (cm�1)

S1-1 S1-2 S1-3 S1-4 S2-4 S3-2 S4-3

Asymmetric stretching vibrations of O–Ge–O in tetrahedral GeO4 820 807 810 819 821 803 795 809Asymmetric stretching vibrations of Ge–O� with nonbridging oxygen 760 751 739 752 758 781 734 746Vibrations of bridging oxygen Ge–O–Ge 563 569 563 581 579 588 578 582

506 510 504 527 545 523 529 538Ge–O–Pb bridging vibrations 330 323 321 331 343 306 338 316

400 500 600 700 800 900 1000

56GeO2-22PbO-9Na2O-4Ga2O3-9PbF2

60GeO2-30PbO-6Na2O-4Ga2O3

50GeO2-40PbO-10Na2O

70GeO2-20PbO-10Na2O

55GeO2-30PbO-15Na2O

80GeO2-10PbO-10Na2O

60GeO2-30PbO-10Na2O

65GeO2-30PbO-5Na2O

Ref

lect

ance

Wavenumber [cm–1]

70GeO2-30PbO

Fig. 4. Infrared reflection spectra of selected GeO2 glasses from theglass series 1: (70�x)GeO2–30PbO–xNa2O (x50, 5, 10, 15 mol%); glassseries 2: (90�x)GeO2–xPbO–10Na2O (x5 10, 20, 40 mol%); glass series3, 60GeO2–30PbO–6Na2O–4Ga2O3, and glass series 4: 56GeO2–22PbO–9Na2O–4Ga2O3–9PbF2.

October 2010 The Structural, Thermal and Optical Analyses of Multicomponent GeO2 Glasses 3263

where n is the RI, E is the photon energy, E0 is the effectiveoscillator energy or the electronic energy bandgap, Ed is thedispersion energy or oscillator strength. In other words, thevalue of E0 is determined by the energy gap between the con-duction band and valence band of a material, and can be cal-culated through Eq. (6), where Eg is the bandgap energy, and Cis a constant of about 1.5.34

E0 ¼ CEg (6)

The dispersion energy Ed is related to the bond strength betweentwo constituent elements in this material. Both energy levels aredependent on the materials compositions and their RIs.

By modifying Eq. (4),33 it can be shown:

1

n2 � 1¼ E0

Ed� E2

Ed � E0(7)

From Eq. (7), it is clear that the values of 1/(n2�1) will changelinearly with E2, suggesting that a linear regression analysis

can be performed when such a plot is constructed and the cor-responding values of E0 and Ed can be obtained from the slopeand intercept of the line function of 1/(n2�1) and E2, respectively.Figure 5 shows the plot of the values of 1/(n2�1) as a function ofE2 for the glass series 1. The calculated E0 and Ed of all compo-sitions are given in Table III. In the first four ternary composi-tions, the single oscillator energy E0 drops from 7.82 to 7.38 eVwith the addition of Na2O, corresponding to the decrease of RIfrom 1.9078 to 1.8757 (measured at 0.532 mm) for glasses S1-1 toS1-4. By comparison, the dispersion energy Ed decreased from18.82 to 16.72 eV. However, in the second ternary composition,the trend observed in the first set of ternary compositions is nolonger applicable. The energy E0 decreases from 8.43 to 7.02 eVwith an increase in the concentrations of PbO concentration from10 to 40 mol%. The corresponding increase in Ed was from 16.24to 17.64 eV for all ternary, quaternary, and quinary composi-tions, shown in Table I; we propose that the lower values of RIscorrespond to large values of E0/Ed, whereas the value of 1/EdE0

determines how fast the RI can vary with wavelengths. In aGeO2–PbO–Na2O system (glass series 1 and 2), the change of themolar percentage of constituents led to an alteration of the E0

and Ed values, and subsequently modified the corresponding RIs.However, the relationship between the RI and wavelength stillobeys the general rule discussed above. Moreover, if we com-pared two glasses S2-2 and S3-1 in Table I, the RI reduces from1.8257 to 1.8142 (at 0.532 mm) with the addition of Ga2O3, whilethe two glasses have E0 values of 7.99 and 8.17 eV and similar Ed

values of 17.08 and 17.05 eV. The comparison qualitatively pointsout that Ga2O3 might be incorporated in the GeO4 network, asthe values of Ed are not modified significantly in Ga2O3-contain-ing and Ga2O3-free glasses in Table I. Finally, from glass series 4,we found that both the bandgap and dispersion energy were re-duced with the addition of fluorine.

(B) Sellmeier Equation and Materials Zero-DispersionWavelength: From Table III, it is clear that the RI n isdependent on the wavelength l. The materials dispersionproperties can be further analyzed through the Sellmeier equa-tion. From Eq. (8), the dependence of materials dispersion onwavelength can be determined by using a two-pole Sellmeierequation,35 which was used to fit the measured RI data

n2 ¼ Aþ B

1� C=l2þ D

1� E=l2(8)

Table III. Refractive Indices of the Germanate Glasses at Various Wavelengths, and the Calculated E0 and Ed UsingWemple Analysis

Sample ID

Refractive index n (70.0004)

E0 (eV) Ed (eV)0.532 mm 0.633 mm 1.321 mm 1.476 mm 1.55 mm 1.56 mm

Series 1S1-1 1.9078 1.8888 1.8537 1.8524 1.8511 1.8516 7.82 18.82S1-2 1.8954 1.8764 1.8402 1.8402 1.8398 1.8396 7.74 18.23S1-3 1.8847 1.8638 1.8267 1.8267 1.8247 1.8250 7.48 17.24S1-4 1.8757 1.8520 1.8160 1.8160 1.8142 1.8140 7.38 16.72

Series 2S2-1 1.7965 1.7452 1.7214 1.7218 1.7202 1.7200 8.43 16.24S2-2 1.8257 1.8098 1.7790 1.7786 1.7765 1.7763 7.99 17.08S2-3 1.8847 1.8638 1.8267 1.8267 1.8247 1.8250 7.48 17.24S2-4 1.9554 1.9289 1.8843 1.8843 1.8827 1.8825 7.02 17.64

Series 3S3-1 1.8142 1.7975 1.7688 1.7690 1.7674 1.7672 8.17 17.05S3-2 1.8815 1.8617 1.8261 1.8259 1.8236 1.8235 7.61 16.65S3-3 1.8724 1.8505 1.8130 1.8122 1.8119 1.8122 7.39 17.50S3-4 1.8828 1.8622 1.8249 1.8236 1.8226 1.8222 7.43 17.05

Series 4S4-1 1.8800 1.8590 1.8223 1.8220 1.8209 1.8203 7.49 17.15S4-2 1.8722 1.8501 1.8112 1.8106 1.8093 1.8091 7.25 16.28S4-3 1.8629 1.8449 1.8057 1.8037 1.8019 1.8017 7.28 16.19

0 1 2 3 4 5 6

0.38

0.40

0.42

0.44

1/(n

2 –1)

E2 [eV]

70GeO –30PbO 65GeO –30PbO–5Na O 60GeO –30PbO–10Na O 55GeO –30PbO–15Na O

Fig. 5. Relationship between 1/(n2�1) and E2 for Wemple–DiDo-menico analysis of the glass series 1, (70�x)GeO2–30PbO–xNa2O(x50, 5, 10, and 15 mol%).

3264 Journal of the American Ceramic Society—Jiang et al. Vol. 93, No. 10

where l is the wavelength in mm and A, B, C, D, and E areconstants.

Based on the fitted Sellmeier parameters (e.g., glass S1-1:A5 1.92995, B5 1.49438, C5 0.03592, and D5 1.01736,E5 121), continuous RI data as a function of wavelengthwere obtained and are shown in Fig. 6 (glass series 1). The RI,as a function of wavelength, is obtained in the wavelengthrange from 0.3 to 2 mm, which provides useful information fora suitable glass pair for core-clad preform and subsequent fiberfabrication.

Based on the n�l relationship, the materials dispersion co-efficients of the first set of ternary glasses were calculated by thefollowing equation36:

Dm ¼ �lc

d2n

dl2

� �(9)

where l is the wavelength, c is the speed of light in vacuum,and d2n

dl2is the second derivative of the RI. As shown in Fig. 7, the

dispersion coefficients of the glasses are plotted as a function ofwavelength. The zero dispersion wavelengths of these GeO2

glasses occur in the vicinity of 1.82 mm, as compared with thezero dispersion at 1.3 mm for SiO2 glasses.

IV. Conclusions

We have investigated novel IR GeO2-based glasses forengineering optical IR fibers for chemical sensing at elevated

temperatures. These GeO2 glass possess high glass transitiontemperatures 43501C, which is suitable for chemical sensingand metrological applications in the 1501–2001C. The DTAresults prove that by incorporating Ga2O3 into the ternaryGeO2–PbO–Na2O system, the thermal stability of GeO2 glassesimprove dramatically by reducing the crystallization behavior,which is apparent from the thermal analysis data in Table I.Based on the Raman and IR reflection data, the optical phononstructure was obtained. In order to further understand the ma-terials dispersion properties, RIs were analyzed for the charac-terization of band gap and oscillator energies in the light of Eq.(4). The materials dispersion coefficient was also calculated andthe zero-dispersion wavelength of these glasses was found to beat 1.8270.02 mm.

References

1M. F. Churbanov, ‘‘Recent Advances in Preparation of High-Purity Chalco-genide Glasses in the USSR,’’ J. Non-Cryst. Solids, 140, 324–30 (1992).

2J. S. Sanghera, F. H. Hung, L. E. Busse, P. C. Pureza, and I. D. Aggarwal,‘‘Infrared Evanescent Absorption Spectroscopy of Toxic Chemicals Using Chal-cogenide Glass Fibers,’’ J. Am. Ceram. Soc., 78, 2198–202 (1995).

3J. Heo, M. Rodrigues, S. J. Saggese, and G. H. Sigel, ‘‘Remote Fiber-OpticChemical Sensing Using Evanescent-Wave Interactions in Chalcogenide Glass Fi-bers,’’ Appl. Opt., 30, 3944–51 (1991).

4S. Simhony, E. M. Kosower, and A. Katzir, ‘‘Novel Attenuated Total InternalReflectance Spectroscopic Cell Using Infrared Fibers for Aqueous Solutions,’’Appl. Phys. Lett., 49, 253–4 (1986).

5J. D. J. Ingle and S. R. Crouch, Spectrochemical Analysis. Prentice Hall, En-glewood Cliffs, NJ, 1988.

6K. T. V. Grattan and B. T. Meggitt, Optical Fiber Sensor Technology: Funda-mental. Springer, Berlin, 2000.

7N. S. Kapany and R. J. Simms, ‘‘Recent Developments of Infrared Fiber Op-tics,’’ Infrared Phys., 5, 69–80 (1965).

8T. Katsuyama and H. Matsumura, Infrared Optical Fibers. Adam Hilger, Bris-tol, 1978.

9J. A. Savage, ‘‘Materials for Infrared Optics,’’ Mater. Sci. R, 2, 99–137 (1987).10X. Jiang, J. Lousteau, B. Richards, and A. Jha, ‘‘Investigation on Germanium

Oxide-Based Glasses for Infrared Optical Fiber Development,’’ Opt. Mater., 31,1701–6 (2009).

11D. M. McPherson and S. C. Murray, ‘‘Germanate Glass for Mid-InfraredMedical Optical Fiber’’; U.S. Patent 5,491,767, 1996.

12J. Wang, J. R. Lincoln, W. S. Brocklesby, R. S. Deol, C. J. Mackechnie, A.Pearson, A. C. Tropper, D. C. Hanna, and D. N. Payne, ‘‘Fabrication and OpticalProperties of Lead-Germanate Glasses and a New Class of Optical Fibers Dopedwith Tm3,’’ J. Appl. Phys., 73, 8066–75 (1993).

13F. M. Ernsberger, ‘‘Molecular Water in Glass,’’ J. Am. Ceram. Soc., 60, 91–2(1977).

14W. G. Jordan and A. Jha, ‘‘A Review of the Role of DSC Analysis in theDesign of Fluorozirconate Glasses for Fiber Optic Applications,’’ J. Therm. Anal.,42, 759–70 (1994).

15J. Wong and C. A. Angell, Glass Structure by Spectroscopy. Marcel DekkerInc., New York, 1976.

16X. Liu, S. Shen, and A. Jha, ‘‘Investigation on the Kinetics of Devitrificationof GeS2-Based Glasses,’’ J. Mater. Res., 20, 856–63 (2005).

17A. Hruby, ‘‘Evaluation of Glass-Forming Tendency by Means of DTA,’’Czech. J. Phys., 22, 1187–93 (1972).

18G Lucazeau, S Barbier, and AM Loiresu-Loza’ch, ‘‘Spectres Vibrationnels,Transitions et Electronique et Structures a Courtes Distances Dans Les Verre:Sulfures de Terres Rares—Sulfures de Gallium,’’ Mater. Res. Bull., 12, 437–47(1977).

19J. F. Scott, ‘‘Raman Spectra of GeO2,’’ Phys. Rev. B, 1, 3488–93 (1970).20F. Gan, Optical and Spectroscopic Properties of Glass. Springer-

Verlag and Shanghai Scientific and Technical Publishers, Berlin and Heidelberg,1992.

21H. Verweij and J. Buster, ‘‘The Structure of Lithium, Sodium and PotassiumGermanate Glasses, Studied by Raman Scattering,’’ J. Non-Cryst. Solids, 34, 81–9(1979).

22Z. Pan and S. H. Morgan, ‘‘Raman Spectra and Thermal Analysis of a NewLead–Tellurium–Germanium Glass System,’’ J. Non-Cryst. Solids, 210, 130–5(1997).

23A. Trukhin and B. Capoen, ‘‘Raman and Optical Reflection Spectra of Ger-manate and Silicate Glasses,’’ J. Non-Cryst. Solids, 351, 3640–3 (2005).

24J. E. Canale, R. A. Vondrate, K. Nassau, and B. C. Cornilsen, ‘‘Character-ization of Various Glasses in the Binary PbO-GeO2 and Bi2O3-GeO2 Systems,’’J. Can. Ceram. Soc., 55, 50–6 (1986).

25S. J. L. Ribeiro, ‘‘Structural Studies in Lead Germanate Glasses: EXAFS andVibrational Spectroscopy,’’ J. Non-Cryst. Solids, 159, 213–21 (1993).

26V. N. Sigaev, ‘‘Structure of Lead Germanate Glasses by Raman Spectro-scopy,’’ J. Non-Cryst. Solids, 279, 136–44 (2001).

27V. N. Morozov, ‘‘Vibrational Spectra of Glasses of the PbO–GeO2 System,’’Zh. Prikl. Spektrosk., 8, 830–3 (1968).

28D. Di Martino, L. F. Santos, A. C. Marques, and R. M. Almeida, ‘‘Vibra-tional Spectra and Structure of Alkali Germanate Glasses,’’ J. Non-Cryst. Solids,293–295, 394–401 (2001).

1.00 1.25 1.50 1.75 2.00 2.25 2.50–500

–400

–300

–200

–100

0

100

Mat

eria

ls d

ispe

rsio

n [p

s km

–1 n

m–1

]

Wavelength [µm]

Material dispersion coefficient70GeO2-30PbO65GeO2-30PbO-5Na2O60GeO2-30PbO-10Na2O55GeO2-30PbO-15Na2O

Fig. 7. Material dispersion coefficients of GeO2 glass series 1:(70�x)GeO2–30PbO–xNa2O (x5 0, 5, 10, and 15 mol%).

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

1.80

1.85

1.90

1.95

2.00 nS1-1

nS1-2

nS1-3

nS1-4

fitting of nS1-1

fitting of nS1-2

fitting of nS1-3

fitting of nS1-4

Ref

ract

ive

inde

x n

Wavelength [µm]

Fig. 6. Refractive indices as a function of wavelength for the glass series1, (70�x)GeO2–30PbO–xNa2O (x50, 5, 10, and 15 mol%).

October 2010 The Structural, Thermal and Optical Analyses of Multicomponent GeO2 Glasses 3265

29E. I. Kamitsos, Y. D. Yiannopoulos, M. A. Karakassides, G. D. Chryssikos,and H. Jain, ‘‘Raman and Infrared Structural Investigation of xRb2O–(1�x)GeO2

Glasses,’’ J. Phys. Chem., 100, 11755–65 (1996).30M. F. Mydlar, N. J. Kreidl, J. K. Hendren, and G. T. Clayton, Phys. Chem.

Glasses, 11 [6] 196–204 (1970).31A. K. Varsheneya, Fundamentals of Inorganic Glasses, Chapter 5, 2nd edition,

pp. 131–2. Society of Glass Technology, Sheffield, 2006.32S. H. Wemple and M. DiDomenico Jr., ‘‘Behavior of the Electronic

Dielectric Constant in Covalent and Ionic Materials,’’ Phys. Rev. B, 3, 1338–51(1971).

33H. Yayama, S. Fujino, K. Morinaga, H. Takebe, D. W. Hewak, and D. N.Payne, ‘‘Refractive Index Dispersion of Gallium Lanthanum Sulfide and Oxysul-fide Glasses,’’ J. Non-Cryst. Solids, 239, 187–91 (1997).

34S. Kasap, H. Ruda, and Y. Boucher, Cambridge Illustrated Hand-book of Optoelectronics and Photonics. Cambridge University Press, Cambridge,2009.

35G. Ghosh, ‘‘Sellmeier Coefficients and Chromatic Dispersions for Some Tellu-rite Glasses,’’ J. Am. Ceram. Soc., 78, 2828–30 (1995).

36D. N. Payne and W. A. Gambling, ‘‘Zero Material Dispersion in OpticalFibers,’’ Electron. Lett., 11, 172–5 (1975). &

3266 Journal of the American Ceramic Society—Jiang et al. Vol. 93, No. 10