Chalcohalide glasses for infrared fiber optics

Chalcohalide glasses for infrared fiber optics

Jong Heo, MEMBER SPIEFiber Optic Materials Research ProgramRutgers UniversityP.O. Box 909Piscataway, New Jersey 08855

Jasbinder S. SangheraNaval Research Laboratory4555 Overlook Avenue, Code 6505Washington, D.C. 20375

John D. MackenzieUniversity of California, Los AngelesDepartment of Materials Science and

Engineering405 Hilgard AvenueLos Angeles, California 90024

CONTENTS1. Introduction2. Glass formation

2.1. Arsenic-based chalcohalide glasses2.2. Germanium-based chalcohalide glasses2.3. TeX glasses2.4. Other chalcohalide glasses

3. Properties of chalcohalide glasses3.1. Optical properties3.2. Physical and thermal properties3.3. Mechanical properties3.4. Chemical properties3.5. Electrical properties

4. Structure4.1.4.2.4.3.

5. Summary6. Acknowledgment7. References

1. INTRODUCTIONHalide and chalcogenide glasses have received a great deal ofinterest as candidate materials for infrared fiber optic technol-ogy.1-3 However, the relatively poor chemical durability andlow thermal stability of halide glasses pose serious problems forpractical applications.4 On the other hand, chalcogenide glassesare well known5 for their good chemical durability and infrared

Abstract. Chalcohalide glasses are mixtures of chalcogenides and halides.This paper reviews the properties and structure of various glass-formingchalcohalide systems. These materials possess high transmittance in theinfrared, which make them candidates for various applications in the areaof infrared fiber optics. In general, the packing density, glass transitiontemperature ( Tg), and refractive indices decrease with the addition of ahalogen component into binary chalcogenide glasses. It also seems to betheoretically possible that the attenuation loss of the glasses and fibers,especially at 10.6 m, decreases at the same time. The observed changesin the properties of glasses are in good agreement with the proposedstructural model, suggesting degradation of the network connectivity bythe addition of network-terminating halogen atoms.

transmittance above 12 jim. However, their relatively large ex-trinsic and intrinsic losses in the midinfrared region limit theirapplications as long-distance optical communication media. Thereis yet a third family of infrared transmitting glasses that hasheretofore received little attention. These glasses are preparedfrom mixtures of chalcogenides and halides and have been de-noted ' 'chalcohalide glasses .'

6Chalcohalide glasses are normally prepared using high-purity

starting materials in their elemental forms. The calculated amountof these materials is weighed in the silica ampoule under acontrolled atmosphere to reduce the effect of oxygen and hy-droxyl groups in the atmosphere. The ampoules containing thestarting materials are then evacuated and sealed before beingtransferred to the furnace for melting. To obtain homogeneousglasses, a forced homogenization of the melts is necessary, andthis has been routinely done by the vibration of the melts usingeither rocking or rotating furnaces.6 The heating schedule of thesynthesis is highly dependent on the melting temperature of eachconstituent as well as on the vapor pressure. In most cases,however, it is essential to use a slow heating rate, typically withless than 10°C/mm. Otherwise, the vapor pressure of the chal-cogen and/or halogen components will become so high that thesilica ampoule may break. In some cases, a stepwise meltingschedule has been used to avoid the difficulties associated withvapor pressure.6 The synthesis of glasses can be completed afterholding the molten mixtures for several hours at the meltingpoint of the most refractory component. At the final stage, thesilica ampoules containing the melts are usually removed fromthe furnace and cooled in air to form glasses.

The incorporation of halogen elements into chalcogenide glassesis expected to produce changes in the various properties of glasses.Optical properties are the most important ones to be considered.Traditionally, chalcogenide glasses (particularly selenides) areconsidered7 to be candidates for CO2 laser power delivery at10.6 pm. However, the multiphonon absorption of metal-selenium

Subject terms: chalcohalide; infrared; glasses; fibers; properties; structure.

Optical Engineering 30(4), 470-4 79 (April 1991).

Arsenic-based systemsGermanium-based systemsOther chalcohalide glasses

Invited paper received April 16, 1990; revised manuscript received September15, 1990; accepted for publication September 21 , 1990. This paper is a revisionof paper 970-13 , presented at the SPIE conference Properties and Characteristicsof Optical Glass, August 14—19, 1988, San Diego, California. The paper pre-sented there appears (unrefereed) in SPIE Proc. Vol. 970.

1991 Society of Photo-Optical Instrumentation Engineers.

470 / OPTICAL ENGINEERING / April 1991 / Vol. 30 No. 4

Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2013 Terms of Use: http://spiedl.org/terms

CHALCOHALIDE GLASSES FOR INFRARED FIBER OPTICS

Table 1. Known glass-forming systems in chalcohalides.

As-based systems Ge-based systems Other systems

As-S-I Ge-S-I Sb-S-I P-S-I

As-Se-I Ge-Se-I Sb-Se-I P-Se-I

As-Te-I Ge-Te-I Sb-S-Br Te-Br

As-S-Br Ge-S-Br Si-S-I Te-Ci

As-Se-Br Ge-Se-Br Si-S-Cl Te-Se-Br

As-Te-Br Ge-As-S-I Si-Se-I Te-S-Br

As-S-Cl Ge-S-Ag-I Al-Cs-S-Cl Te-Se-Cl

As-Se-In-I Ga-Cs-S-Cl Te-Se-Br

bonds starts to affect the optical loss of the materials seriouslyand thereby results in an undesirably high attenuation loss atthis wavelength.8 The addition of heavy halogen elements isexpected to reduce the number of metal-selenium bonds andaccordingly can possibly produce glasses with a lower atten-uation loss at the wavelength of practical interest. Obviously,one should also consider a compromise between the improve-ment of optical properties and the possible degradation of thethermal and chemical properties incurred by the formation ofweak metal-halogen bonds. Further, the development of molec-ular structure with the addition of halogen elements is also in-teresting from a fundamental viewpoint because chalcogenideglasses are predominantly covalent while halide glasses are highlyionic in character.

This paper presents a review of the current status of chal-cohalide glasses as potential candidates for infrared fiber opticapplications. The glass formation and the physical, chemical,mechanical, and optical properties are reviewed along with theproposed structural models for these glasses.

2. GLASS FORMATION2.1. Arsenic-based chalcohalide glassesGlass formation in the chalcohalide systems (Table 1) was firstobserved in the As-S-I system by Flaschen et al.9 in 1960.Glasses were prepared by melting high-purity elemental startingmaterials inside evacuated, sealed silica ampoules at approxi-mately 600°C. They observed an extensive area of glass for-mation (Fig. 1), and glasses containing up to 55 wt% (33 at.%)of iodine were successfully prepared. The glass-forming regionin this system was limited by liquid immiscibility at high sulfurconcentrations and by crystalline phase formation at the lowsulfur concentration side. Since then, a considerable amount ofwork has been published5"°3 on the preparation, properties,and structural aspects of As-S-I glasses. Generally speaking,glass samples tend to be colored from deep red to orange withincreasing iodine concentration while pale yellow colors havebeen reported for glasses containing excess amounts of 13

All glasses in this system are transparent in some region of thevisible part of the spectrum.5

Subsequently, glass formation in the As-S-Br s'stem wasinvestigated using similar experimental procedures. 4—17 Kou-delka et al'6 have reacted mixtures of the quasibinary As253-AsBr3system and up to 50 at. % bromine can be introduced for stableglass formation. The glass-forming region is bounded on thearsenic-rich side by crystalline materials and on the sulfur-richside by a region composed of amorphous materials, which pre-

At. %Fig. 2. The glass-forming region of the ternary (1) As-Se-Br and (2)As-Te-Br systems (from Ref. 4).

cipitate sulfur over a relatively short period of 17 The colorsof glasses in the As-S-Br system range from ruby red (As2S3glass) through orange to yellow with increasing bromine content.On the other hand, experimental work on the As-S-Cl system'8is limited possibly due to the toxic nature of chlorine gas andits associated handling difficulties.

The literature also reports on the glass formation in theAs-Se-halogen and As-Te-halogen systems (Fig. 2). For ex-ample, Turyanitsa et have successfully prepared glassescontaining up to 40 at. % bromine or iodine in the As-Se-Br andAs-Se-I systems, respectively. The glass-forming regions aresmaller than the sulfur-containing counterparts (Fig. 1) and ex-tend into the AsSeBr and AsSel direction. Borisova'9 also de-scribed the formation of laminating glasses when more than 15at. % iodine was added to compositions in the As-Se-I ternarysystem enriched with selenium. However, there is no informa-tion in the literature on the glass formation in the As-Se-Clsystem.

Glass formation in the As-Te-X (X = Cl, Br, I) systems israther restricted because no binary As-Te glasses are formedunder conventional melting and cooling conditions. However,glasses can be obtained'4" by the addition of a third componentsuch as bromine or iodine. For instance, glasses in the As-Te-Br

OPTICAL ENGINEERING / April 1 991 / 'lot. 30 No. 4 1 471

S

LIquidx immiscIbilIty

20 xx 80x. x

40 • •• 0 60Glass • so

formatlonS • •.60 • • 40A52S3 4.. • • 0S

80 s.•'•S 20%•Crystalline 0

0As t, '-K

20 40 60 80 AsI3wt.%

Fig. 1. The glass-forming region in the ternary As-S-I system (fromRef. 9).

As40 60 80


HEO, SANGHERA, MACKENZIE

[ATOM %]

S

Fig. 3. The glass-forming region and the compositions of glassesstudied in the Ge-S-I system (o: Ref. 21; •: Ref. 6; and El: Ref. 22).

system can be produced from the eutectic region of the binarysystem As2Te3-AsTeBr, presumab1' due to the liquidus tem-perature effect. Hruby and Stourac 0 also suggested the intro-duction of iodine to the composition of As50Te5o facilitated glassformation in the As-Te-I systems by preventing the attainmentof metallic As-Te bonds, which are highly susceptible to re-arrangement leading to crystallization. The colors of glasses inthe As-Te-I system range from black to silvery black with de-creasing iodine content.

2.2. Germanium-based chalcohalide glassesGlass formation in the germanium-based systems has been stud-ied less extensively compared to their arsenic counterparts, de-spite their good glass-forming ability over a wide compositionalrange. The Ge-S-I system was the first one investigated andshowed the largest glass-forming region of all the chalcohalidesystems studied (Fig. 3) including the As-S-I system.6'21'22 Glassescontaining a maximum of 70 at.% iodine were successfully pre-pared and, within a certain compositional range, a cooling rateof only 2°C/mm was required to obtain stable glasses.

Little data are available for the glass-forming regions in theGe-S-Br and Ge-S-Cl systems. However, Heo and Mackenzie23were able to obtain glasses with up to 40 at. % bromine in theGe-S-Br system. Typically, the glasses tend to be light yellow

colored in the Ge-S-Br and Ge-S-I systems, but in the lattersystem the increasing addition of iodine produces deep red col-ored glass samples. Glasses in the Ge-Se(Te)-I systems 4,25 werealso prepared by heating the elemental starting materials at around800°C within evacuated silica ampoules and subsequentlyquenching the ampoules in air. The Ge-Te system does not formbinary glasses but the addition of iodine facilitates glass for-mation similar to the As-Te-I case. The colors of glasses in theGe-Se-I system range from black (selenium-rich) to differentshades of red. No visible description is available for Ge-Te-Iglasses.

2.3. TeX glassesZhang et al.26'27 have prepared new families of chalcohalideglasses based on the binary TeX (X = Cl or Br) system. Forthe Te-Cl system, binary glasses exist between the compositionsof Te3Cl2 to TeC12 and in the Te-Br system between Te2Br andTeBr. However, there was no evidence of glass formation inthe Te-I system using the conventional melt-quenching tech-nique. The addition of the third element such as sulfur or Se-lenium further extends the glass-forming region.28 Typically,the glass samples tend to be black in color for all the systemsstudied.

2.4. Other chalcohalide glassesThis section deals with the chalcohalide glasses that have lessextensive glass-forming abilities when compared to the systemsdiscussed previously and, consequently, have received less at-tention. Turyanitsa and Koperles 9,30 have prepared numerousglasses in the Sb-S-Br and Sb-S-I systems. No binary glassesare formed within the Sb-S system and the glass-forming regionsin the ternary systems are relatively small and isolated (Fig. 4).The Sb-S-Br system has a wider glass-forming region comparedto its iodine counterpart and is located around the pseudobinaryjoin of Sb2S3-SbBr3. For the Sb-S-I system, the glass-formingregion is confined to the vicinity of the eutectic composition(Sb2S3)o.75(SbI3)o.25. Khiminets et also reported the glassformation in the Sb-Se-I system. All glasses have a metallicluster.

Very little work has been done on silicon-based chalcohalideglasses. Dembovskii and Popova24 successfully prepare glassesin the Si-Se-I systems. These glasses have a greater tendencyto crystallize than their arsenic or germanium counterparts and,

Br

Fig. 4. The glass-forming region in the ternary systems: (a) Sb-S-I (Ref. 30), (b) Sb-Se-I (Ref. 31), and (c) Sb-S-Br (Ref. 30). o: homogeneousglasses; A: glasses based on selenium; : unstable glasses; •: crystals.


20 40 60

(a) S (b) (C)

S

Sb 20 40

atomic %



Wavenumber(cm')Fig. 5. Infrared spectra of various glasses being investigated for in-frared fiber optic applications.

therefore, had to be quenched rapidly. Nevertheless, the glass-forming region extends well into the system from the selenium-rich corner up to the composition with 50 at.% iodine. Theglasses were highly unstable in the ambient atmosphere andfumed severely.

Glasses in the Cs-Al-S-Cl and Cs-Ga-S-Cl systems were dis-covered rather serendipitously by Berg and his coworkers.32'33After several days of melting at 600°C, the melt with the com-position of 2 M Al, 3 M S , 1 M CsA1C14, and 2 M CsCl becamevery viscous, colorless liquid, which, when cooled, produced atransparent glass. They further proceeded to the new Cs-Ga-S-Clsystem by replacing aluminum with gallium. The starting ma-terials were Ga:S:CsC1:GaC13 in the molar ratio of 2:3:3:1 andthe melt formed a stable, colorless glass.

3. PROPERTIES OF CHALCOHALIDE GLASSES

3.1. Optical propertiesOne of the most important properties of chalcohalide glasses,especially from a practical viewpoint, is their rather unique op-tical properties in the midinfrared wavelength region. Unlikefluorozirconate lasses, which only transmit up to about 7 imin the infrared, chalcohalide glasses transmit beyond 10 im(Fig. 5). For instance, transmittance15 of a 1-mm-thick glasswith a composition of 33 wt% arsenic, 30 wt% sulfur, and 37wt% bromine is more than 90% from approximately 0.55 to13 xm and is still around 70% at 16 jim. Heo et al. also ob-served that glasses with path lengths of 2 mm in the Ge-S-I andGe-S-Br systems are transparent up to above 10 pm.

The absorption coefficient a of the optical materials is afunction of the extinction coefficient of the individual con-stituent C1 by the following relation:

a = 2.3O3Cr . (1)

Assuming that the main contribution to the multiphonon ab-sorption of the chalcogenide glasses, for instance, GeS2 glass,arises from the vibration of Ge-S bonds, the absorption coeffi-cient of the glasses can be calculated using the concentration ofthe Ge atoms, [Ge]:

U = 2.3O3KEGe[Ge] , (2)

where K is the average number of sulfur atoms linked to each

atom and EGe 5 the extinction coefficient of germanium. Therelative variation of a with the changes in the composition by

40 the addition of iodine can be written as:

a/a = zXKIK + [Ge]/[Ge] + LEGe/EGe , (3)

where [Ge]/[Ge] denotes the variation in the germanium con-centration while AK/K shows the changes in the coordinationnumber of sulfur atoms around each germanium atom, which isapproximately the same as z[SI/[S]. Therefore, the overall changesin the absorption coefficient of the Ge52 glass with the additionof iodine can be expressed as:

&xla = z[Ge]I[Ge] + [S]I[S] . (4)

This relationship indicates that the addition of iodine into GeS2glass decreases the absorption coefficient of the glass at themidinfrared region by reducing the number of Ge-S bonds perunit volume through the formation of new Ge-I bonds withsmaller reduced mass as indicated by Raman spectroscopy.6 Infact, this phenomenon has been confirmed by various researchworks on the chlorine-containing fluoride glasses35 as well ason the chalcohalide glasses 36

Another interesting aspect of the optical properties of chal-cohalide glasses lies in their refractive index and its variationwith the halogen content. For example, the refractive index17(at 632.8 nm) of As2S3 glass is approximately 2.66 but that ofAs18S41Br41 is 1 .94. The general trend is that the refractive indexof arsenic-based chalcohalide glasses decreases as the haloencontent increases. A similar effect was found in Ge-S-I glasses ,23where the refractive index appears to decrease with the iodinecontent.

Most of the information on the optical properties of fibers isrelated to the TeX glasses.2628'37 Fibers with a diameter ofapproximately 300 pm were obtained from a glass with a Te3SeiJ3composition using a crucible method and a drawing rate of 2mlmin without applying a cladding and coating. The fiberattenuation was measured over the wavelength region of 6 toabout 13 im with heated Zr02 as broadband light source. Aminimum loss of 4 dB/m was obtained at the wavelength ofabout 10 m and the number increases to 4.8 dB/m at 10.6 pm(Fig. 6). There is no report on the estimated values of theattenuation loss on this particular composition of the glass;Chiaruttini et al. 28 however, suggested the lowest values to be2000 dB/km for Te6SelBr3 and 0.5 dB/km for TeSe6Br6 glasses(both located at — 10 rim), respectively, by extrapolating thebandgap and multiphonon absorption curves to construct aV-shaped curve. They further explained that the bandgap absorption

11

9

7

5

36 7 8 9 10 11 12 13

Fig. 6. Attenuation loss spectrum of Te3Se4l3 fiber obtained usingthe crucible method (From Ref. 37).

OPTICAL ENGINEERING / April 1991 / Vol. 30 No. 4 / 473

Wavelength().tm)

cJSS

ESSI-

50

1600 1400 1200 1000 800 600 400

E

.5S0

CS

TeX glass fiber

Te3Se 413

Wavelength (pm)



rather than multiphonon vibration plays an important role on thetransparency in the midinfrared. This suggests that thecompositional optimization is extremely important in order toobtain ultratransparent chalcohalide glasses . Theyalso claimed37that approximately 40 kW/cm2 input power density from a cwCo2 laser was focused into 3OO-pm-diam fiber for at least 2mm without damaging the fiber.

Generally speaking, data are sparse on the optical propertiesof these materials mainly because research activity in this areahas not reached its maturity. The refractive index cat 632.8 nm)of some glasses in the Ge-S-I system were reported6'2 as mentionedpreviously. There are also some ongoing efforts to understandthe effect of the iodine element on the multiphonon absorptionof the various Ge-As-Se-I glasses .36 A large amount of research,however, still needs to be done to gain a comprehensiveunderstanding of the optical properties of various chalcohalideglasses and their relative merits for the future applications.

3.2. Physical and thermal propertiesThe most remarkable characteristics of chalcohalide glasses ingeneral are the range of softening temperatures exhibited by thevarious compositions. The values of softening and glass tran-sition (Tg) temperatures decrease with increasing halogen con-tent. For instance, a glass of As3oS,oBr1o (at.%) compositionhas a Tg value of 1 20°C while increasing the bromine concen-tration to As33S33Br33 results'4 in a significant decrease to ap-proximately 5°C. The softening temperatures of glasses in theAs-S-I system also decrease with increasing iodine content9 withthe most striking effects being for the iodine range 42 to 52 wt%(Fig. 7). The physical properties of chlorine-containing As-Sglasses as reported by Deeg'8 are shown in Table 2. A glasscontaining five components , namely , 51 S34C15Br5I5 (at. %),was also prepared for comparison. The chlorine-containing glasseshave properties similar to As-S-I and As-S-Br glasses. The sameeffect has also been observed for the glasses in the As-Se-X(where X = Cl, Br, I)' and As-Te-X (where X = Br, I)systems.

17

The decrease in the Tg values for glasses becomes less seriousin the germanium-based glasses because the Tg values of thegermanium-based chalcogenide glasses are normally about 200°Chigher than their arsenic counterpart. Therefore, a relativelylarge amount of halogen elements can be incorporated while the

Glass As 40 30 51

Composition S 50 60 34

(at.%) Cl 10 10 5

Br ---- ---- 5

I ---- ---- 5

Tg(°C) 145 122 136

Linear Coefficientof ThermalExpansion{10 O(,1J

467 490 473

Density (g/cm3) 2.62 2.46 2.88

Vickers Hardness 71 40 48

Refractive Index (nD) 2.31 2.20 2.56

glasses can still exhibit a reasonably high viscosity (> 1014 poise)at room temperature. For instance, the melts tend to be viscousliquids at room temperature for iodine concentrations greaterthan 60 at.% (e.g. , Ge2oS2oI,o) while in the case of the As-S-Isystem, approximately half as much iodine concentration pro-duced a similar effect.9 Heo et al.6 and Heo and Mackenzie23have observed that glasses with up to 30 at.% of iodine (Ge23S47I30)still show Tg values above 200°C. They further showed that itis possible to prepare chalcohalide glasses with Tg values corn-parable to those of ri'2 and chalcogenide lasses38 butstill much higher than those of chloride,' bromide, and iodideglasses.4° The glass transition temperatures of Ge-Se(Te)-I alsoshow a similar behavior.24'25

The densities of chalcohalide glasses largely depend on thetype of chalcogen and halogen element comprising the glassesand range from about 3 g/cm3 for the As-S-Br system'4 to a-proximately 5 g/cm3 for the As-S-I system.5 Turyanitsa et al.measured the densities of some glasses in the As-S-Br, As-Se-Br,and As-Te-Br systems. In general, densities decreased with in-creasing bromine content probably due to the loosening of theAs-S (or Se, Te) structure with the addition of bromine. Thehighest value reported was 5 .5 g/cm3 for a glass in the As5oTe5o-Isystem.4'

Heo et al.6 and Heo and Mackenzie23 also studied the changesin the densities of Ge-S-I, Ge-S-Br, and Ge-Se-I glasses withincreasing halogen concentration. The densities increased withthe addition of halogen elements in the case of the Ge-S-I andGe-S-Br systems but decreased in the Ge-Se-I system. The ob-served increase in the densities of the sulfur-containing glassesseemed rather anomalous since the addition of halogen opensup the network structure and normally results in the large de-crease in densities. They claimed that the large difference inatomic mass between sulfur and iodine (or bromine) is mainlyresponsible for the observed density increase even though thenetwork structure becomes more open as manifested by the de-crease in the packing densities of glasses with the halogen ad-dition.6

474 / OPTICAL ENGINEERING / April 1 991 / Vol. 30 No. 4

Table 2. Physical properties of As-S-Cl glasses (from Ref. 18).

S

2(

40

As

20

wt.%80

Fig. 7. The softening temperatures of the glasses in the ternary As-S-Isystem (from Ref. 9).



The densities of glasses in the Sb-S-I system also showed abehavior31 similar to that of the Ge-S-I and Ge-S-Br glasses.Further, an increase in the antimony content gave rise to a hiherdensity. For example, the density increased from 4.45 g/cm for5b55e9015 glass to 4.80 g/cm3 for Sb2oSeoI2o glass. This is anal-ogous to the previous cases because the atomic masses of an-timony (121 .75 g/mol) and iodine (126.90) are much larger thanthat of selenium (78.96) even though the authors did not provideany information on the atomic packing densities of the glasses.Generally, a simple comparison of density values among dif-ferent types of chalcohalide glasses can be misleading unlesssome other parameters such as molar volume or packing densityare taken into consideration. However, up to the present time,no systematic study has been done on the analysis of the densitiesin relation to the other properties and structure of chalcohalideglasses.

The viscosity of the 1asses with composition of AsSI andAsSel was also reported4 over the temperature range from roomtemperature up to 1 10°C. The value of Tg obtained from theviscosity-temperature curve for AsSI was 35°C. According toVinogradova and Dembovskii,43 the difference in the value ofTg between the DTA (70°C) and viscosity measurement must bedue to the presence of severe microheterogeneity. The activationenergy (E1) and entropy (Si) for viscous flow in the vicinityof log = 13 for the AsSI glass are 17 kcal/mol and 200 eu,respectively while the values for the AsSel glasses are 18 kcallmoland 220 eu, respectively. According to Nemilov," the activationenergy and entropy for viscous flow indicate that both AsSI andAsSel glasses have a chain-like structure. Other physical prop-erties of chalcohalide glass systems can be found elsewhere. 1945

3.3. Mechanical propertiesThe mechanical properties of chalcohalide glasses are extremelyimportant from a practical viewpoint since optical fibers madefrom these materials should not break during handling. Despiteits importance, no work has been published in the literature.However the elastic moduli of three compositions have beenreported' and if we use the relationship that the theoreticalstrength is equivalent to one-tenth the elastic modulus,46 then itis possible to give an estimate of the theoretical strength asshown'8'475° in Table 3. Although the chalcohalide glasses re-ported here possess potentially lower strengths than silica andfluorozirconate glasses, the values are adequate for practicalpurposes. However, it does not seem unreasonable that chal-cohalide glasses, based on, for example, GeSe2, may possesshigher strengths for improved performance.

3.4. Chemical propertiesAside from the favorable optical properties and potentially fa-vorable mechanical properties, the chemical durability, espe-cially with respect to attack by moisture, is a most importantparameter if chalcohalide glass optical fibers are to perform underpotentially hostile environments . Generally speaking ,chalco-halide glasses are resistant to attack from moisture. For example,the Ge-S-I glasses show no sign of hydrolysis when subjectedto water at 70°C for three days.6 Typically, the arsenic-basedand germanium-based chalcohalide glasses are resistant to attackfrom most dilute acids (including HF) and weak bases. However,these glasses are dissolved by strong alkalis (pH > 10) andstrong oxidizing agents. For instance, a glass of compositionAs30S60I,0 has a weight loss of 21 mg/cm after exposure to asolution of 0.2 M NaOH (pH = 12) at room temperature.5' The

Table 3. The elastglasses.

ic modulus and predicted theoretical strength of

composition Elastic modulus(GPa)

Predicted theoreticalstrength (GPa)

reference

As40S50C110 5.2 0.5 18

As30S60C110 4.6 0.5 18

As51S34C15Br5I5 7.4 0.7 18

As2S3 16.0 1.6 45

As2Se3 18.0 1.8 45

GeSe2 19.0 1.9 45

GeS2 15.7 1.6 46

Si02 74.0 7.4 47

ZBLAN 55.9 5.6 48

dissolution rate increases slightly with high halogen content suchthat a glass of composition As30S4oI30 has a weight loss of30 mg/cm2 under the same conditions. The dissolution rates arecomparable5' to As2S3 glass (24 mg/cm2). The As-S-Br glassesexhibit appreciable dissolution in methylene iodide'5 while theglasses are completely soluble in dilute nitric Perhapsnitric acid can be used to etch the optical and electronic As-Te-Iglasses in a preferred manner for potential device applications.

3.5. Electrical propertiesThe As-S- and As-Se-based chalcohalide glasses possess highresistivities (1016 to 10' ' flcm) at room temperature althoughthe resistivity decreases slightly with increasing halogen content.For example, the glasses As30S60Br10 and As33S33Br33 haveroom temperature resistivities of l0' and 1012 cm respec-

14 The melts of these lasses exhibit lower resistivities,typically in the range of 10 to i0 1cm. The lack of anypolarization effects suggests the absence of any ionic contri-bution to the conductivity and it is therefore inferred that themechanism for conduction is electronic in nature.

The most interesting electrical properties of chalcohalide glassesare those exhibited by the As-Te-I and As-Te-Br glassTypical resistivity values lie in the range of 108 to l0 1cm butunlike the other arsenic-based chalcohalide glasses the resistivitydoes not decrease with increasing halogen content. Instead, theconductivity increases with the tellurium content, which is notsurprising since tellurium possesses metallic character. The ac-tivation energies for electrical conduction lie in the range of 0.7to 1 .0 eV. However, an unusual property of As-Te-I glasses(and presumably As-Te-Br glasses, although their electronicproperties have not been measured to date) lies in their memoryswitching ability'7 where simple pulsing (l0 to l0 V/cm) re-turns the system from a low to a high resistance state. Themechanism for this behavior is not known for this glass system,but it is believed that the highly conducting state arises from thepresence of microcrystals. Electrical pulsing melts the crystalsand returns the system to its original high-resistance glassy state.

The electrical properties of silver-containing chalcohalideglasses52 (up to 50 mol% AgI added to GeS2, As2S3, and P2O5)reveal that these glasses exhibit cationic conductivity with ac-tivation energies for conduction approachin 0.25 eV and con-ductivities of approximately 10 (flcm)

OPTICAL ENGINEERING / April 1 991 / Vol. 30 No. 4 / 475



4. STRUCTURE4.1. Arsenic-based systemsThe earliest structural work on chalcohalide glasses was by Hop-kins et on the As-S-I system using x-ray diffraction. Theyexamined the radial distribution function (RDF) of As2531165and As2531065 glasses prepared by the addition of elementaliodine to powdered As253. The addition of iodine had the effectof shifting the peak at 2. 3 A, associated with the As-S nearestneighbor, to 2.45 A and was attributed to the breaking up ofsome As-S bonds and the formation of As-I bonds (Fig. 8). Thepresence of S-I bonds and molecular iodine was discounted be-cause of their low stability and the absence of a maximum at2.66 A, respectively. They further suggested that the monova-lent sulfur produced by the As-I bond formation may not surviveas a radical due to the high mobility in the melts, and will insteadform the -5-5- linkages. However, they failed to separate thepeak from the -5-5- linkages probably because of the smallestatomic mass of sulfur among the constituents. They proposed astructural model of a twisted chain (Fig. 9) by assuming that noarsenic atom is bonded to more than one iodine and S-As-S andAs-S-As bond angles are 100 deg in analogy to the crystal struc-ture of orpiment (As253). They also indicated that a few -As12groups may be present at the end of chains and the iodine atomswere located equidistant from two sulfurs.

Khiminets and coworkers' ' investigated the Raman spectraof various glasses in the As-S-I system along the pseudobinaryjoin As253-As13 . The band at 344 cm , which occurs in allglasses studied, was ascribed to the symmetrical stretching vi-bration of the AsS312 pyramidal units. The intensity of this peakdecreased as the amount of As13 in the glass increased. Theformation of a new band at 210 cm ' in the spectra of ternaryglasses was assigned to the stretching vibration of Sj AsI groups.Based on their results, they suggested the formation of IAsS2/2-S212AsI type of bonds by the introduction of iodines.

Koudelka and Pisarcik'2 investigated glasses in the systemAs4o —x S60'x with x < 20. From their reduced Raman spectrain Fig. 10, they assigned the peak observed at 208 cm 1 to thesymmetrical stretching vibration of discrete AsI3 pyramidal mol-

Fig. 8. Radial distribution function curves of As-S and As-S-I glasses(from Ref. 10).

Fig. 9. The proposed twisted chain structural model of AS2S3I1.65glass based on the x-ray diffraction study (from Ref. 10).

Fig. 10. Reduced Raman spectra of As,,o-,Ssolx glasses (from Ref.12).

ecules. They claimed that the formation of discrete AsI3 mol-ecules is more favorable than the mixed S212AsI or SAsI2 struc-tural groups due to the high chemical affinity of iodine to arsenicand a lower total energy of the system with symmetric groupsof AsS3 and AsI3 in comparison to the mixed halide-chalcogenidegroups. They further observed the formation of 58 rings forglasses containing an excess amount of sulfur over the stoichi-ometric composition of As2S3. Based on the assignment of thepeaks observed, they assumed that A540—xS6OIx glasses werecomposed of AsS3,2 pyramidal units connected via single ordouble sulfur bridges mixed with dissolved species of AsI3 and58. A more recent study'3 on the As-S-I glasses covering thewhole glass-forming region virtually proposed the same type ofstructure. Infrared transmission spectra of these glasses alsosuggested the presence of As-I bonds based on the peak for-mation at 2 10 cm . Gerasimenko et also proposed a struc-tural model consisting of the AsS3/2 pyramids mixed with AsS2,2Istructural units and 58 rings when the amount of AsI3 exceeded40 mol% in the pseudobinary As2S3-AsI3 join.

The molecular structure of the AsSI compound in three dif-ferent states, namely, crystalline, glassy, and molten states, wasinvestigated using x-ray diffraction.5 The RDF curves for aglass and a crystal were similar to each other but different fromthat of the melt. It was concluded that, in the molten state, theAs-S bonds in the AsSI compound were partially dissociated


CD

Ca,

0C

100 300 500

Raman shift (cm1)

C0

C

C0CD

0C-)

80

40

080

40

4r (A)



such that there was a statistical distribution of the bonds and theprobabilities of forming As-As and S-S bonds were the same asthat of As-S bond formation.

Several publications also exist on the structure of glasses inthe As-S-Br system using Raman spectroscopy. Slivka et al.55studied the structural changes along the As2S3-AsBr3 binary join.The addition of AsBr3 into As2S3 glass led to the formation ofnew peak at 250 cm in the Raman spectra. This peak wasassigned to the vibrational mode of S2i2AsBr groups. Based onthis interpretation, they proposed a structural model for the As-S-Brglasses as a mixture of AsS312 pyramidal units with groups ofS2i2AsBr. In more recent studies, Koudelka et 16 and Kou-delka and Pisarcik56 recorded the Raman spectra covering thewhole glass-forming region of the As-S-Br system. They as-signed the same peak to the symmetrical stretching vibration ofthe discrete AsBr3 molecules. This was based on the fact thatthe peak due to the As5312 unit is still highly intense even in theglasses with an (As2S3)o.5(AsBr3)o.5 composition where it shouldbe completely diminished according to the model of Slivka etal.55 They finally concluded that the structure of these glassesis a solid solution of AsBr3 molecules in a network matrix ofAs2S3 . From the glasses of compositions with excess amountsof sulfur, the formation of 58 rings was also suggested.

4.2. Germanium-based systemsThe germanium-based chalcohalide glasses have received lessattention compared to their arsenic counterparts, especially withrespect to the structural aspects . Koudelka et ' ,58 were thefirst to report the spectra of glasses in the Ge-S-Br system. Intheir first study on GeS3Br and GeS325Bro75 glasses,57 theyobserved a new peak at 287 cm with a bromine addition andsuggested that the structure consisted of GeBr2 molecules withina glass network made up of GeS4 tetrahedra. However, a morerecent study58 revealed the formation of some new peaks as largeamounts of bromine were added. They assined the peaks at235 cm to GeBr4 molecules, at 256 cm to SGeBr3 units,and at 287 cm to both S212GeBr2 units and GeBr2 molecules.In fact, they agreed on the formation of mixed GeBrS4 —struc-tural groups in this system. This is somewhat contradictory tothe model they had proposed'2 for the other glasses such asAs-S-I and As-S-Br. In conclusion, they suggested that the addedbromine formed GeBrS4, GeBr2, and GeBr4 structural unitsmixed with Ge54 tetrahedra.

Recently, Heo and Mackenzie59 published a comprehensivestudy on the structure of Ge-S-Br glasses based on Raman andinfrared spectroscopy. Polarized Raman spectra were recordedfor glasses containing up to 30 at.% bromine while the S/Geratio was kept to two or three. They observed the formation ofseveral new peaks between 230 and 300 cm ' with the additionof successively increasing amounts of bromine (Fig . 1 1). Table4 shows the position and vibrational modes responsible for theRaman bands in Ge-S-Br glasses. The most pronounced changethey observed with the addition of bromine atoms was the for-mation of Ge-Br bonds substituting for Ge-S bonds in the GeS4tetrahedra. As a result, there are three types of the mixedGeBrS4 —x groups (x = 1 , 2, or 3) as well as GeBr4 moleculeswith the large bromine addition. Sulfur atoms isolated from GeS4tetrahedra due to the formation of Ge-Br bonds recombined toform S8 rings or chains. Based on this observation, they de-scribed the structure of Ge-S-Br glasses, in general, as a solidsolution of S8 rings with the network structure formed byGeBrS4_ (x 0, 1, 2, 3, or 4) tetrahedra (Fig. 12).

600

RAMAN SHFT CI/cr

Fig. 11. Raman spectrum of Ge23S47Br30(S/Ge = 2) glass (from Ref.59).

Table 4. Frequency and assignment of the Raman bands in Ge-S-Brglasses (from Ref. 56).

Frequency (cnf1) Assignments

130 SD Ge-Br3 (in GeBr3S)

152 S8 (E2)

220 S8(A1)233 SS GeBr4

254 SS Ge-Br (in GeBr2S2 & GeBr3S)

264 ss Ge-Br (in GeBrS3)

288 AS Ge-Br (in GeBr2S2)

300 AS Ge-Br (in GeBr3S)

342 SS GeS4

375 GeS4 edge-shared

475 S8(A1) and SSD : symmetrical deformationSS : symmetrical stretchingAS : asymmetrical stretching

As the Ge-Br bonds are formed by the addition of bromine,they effectively replace bridging sulfurs and subsequently ter-minate the network structure at those points . In other words , assoon as Ge-Br bonds are formed, this part of the structure willbe disconnected from the rest ofthe network structure. Increasingthe amount of bromine will further disturb the network structure,which will eventually lead to the formation of chain-like struc-tures.59 This type of structural development can be applied tovirtually all types of chalcohalide glasses.6° As discussed in theprevious section, observed changes in the properties with the

OPTICAL ENGINEERING / April 1991 / Vol. 30 No. 4/ 477

RAMAN Ge23S47Br3O

U

5

45

4

3.5

3

I

0 200 400



addition of halogen seem to support the proposed role of thehalogen atoms.

4.3. Other chalcohalide glassesA number of Raman spectra have also been recorded on ratheruncommon lasses and melts such as Cs-Al-Cl-S and Cs-Ga-Cl-Ssystems.32' Berg et al.32 suggested that, from the stoichiometryof the components, the homogeneous glass must have the com-position Cs:Al:S:Cl equal to 1 : 1 : 1 :2. A prominent peak at 325cm in their Raman spectra was assigned to the A1C12-A1C12unit of [AlS_ lCl2n+21n species (n = 3, 4 . . .). Based onthe high viscosity of the melt and the Raman spectra, they con-cluded that sulfur atoms are dissolved in basic tetrachloroalu-minate units and form chain-like species of [AlS —1Cl2+ 2InThe Cs ions are present in the structure as charge compen-sators. In a more recent study, Berg and Bjerrum3 also pos-tulated the same polymeric structure in the Cs-Ga-S-Cl systemwhere aluminum was merely replaced by gallium.

5. SUMMARYThe research efforts on chalcohalide glasses were initiated inorder to prepare glasses with lower attenuation losses in the 8to 14 pm region compared to the existing chalcogenide glasses.Works up to the present time mainly focus on the incorporationof heavy halogen elements to reduce the number of metal-chalcogenbonds, which seriously limits the transmission capability of glassesespecially at CO2 laser wavelength (—10.6 rim). The observedrefractive index changes with the addition of halogen atomsprovide the good possibility of fabricating core and claddingoptical fibers through careful control of the compositions. How-ever, one cannot neglect the fact that the excessive addition ofhalogen elements will also result in considerable degradation invarious properties due to the undesirable structural developmentin the glasses. Further, as the amount of halogen elements ex-ceeds approximately 30 at. %, it is likely that phase separationwill occur in the glass. Therefore, it is suggested that the carefulselection of the compositions has to be made following the spe-cific application requirements. Further research and developmentis highly recommended in order to understand fully the effect

of halogen addition on the various properties of the chalcohalideglasses.

6. ACKNOWLEDGMENTThe authors gratefully acknowledge the financial support of theAir Force Office of Scientific Research, Directorate of Chemicaland Atmospheric Sciences.

REFERENCES1 . C. M. Baldwin, R. M. Almeida, and J. D. Mackenzie, "Halide glasses,"

J. Non-Cryst. Solids 43, 309—344 (1981).2. M. Poulain, "Halide glasses," J. Non-Cryst. Solids 56, 1—14 (1983).3. M. Drexhage, "Heavy metal fluoride glasses,' ' in Treatise on Materials

Science and Technology, 26, Glass IV, M. Tomozawa and R. H. Doremus,eds. , pp. 151—243.

4. C. J. Simmons, M. Sutter, J. M. Simmons, and D. C. Tran, ' 'Aqueouscorrosion studies of a fluorozirconate glass,' ' Mat. Res. Bull. 17, 1203—1210 (1982).

5 . A. D. Pearson, ' 'Sulfide, selenide and tellunde glasses,' ' inModern Aspectsof the Vitreous State, Vol. 3, J. D. Mackenzie, ed. , pp. 29—58. Butter-worth, London (1964).

6. J. Heo, H. Nasu, and J. D. Mackenzie, ''Infrared transmitting chalcohalideglasses,' ' in Infrared and Optical Transmitting Materials, R. W. Schwarz,ed. , Proc SPIE 628, 85—91 (1986).

7. A. R. Hilton, D. J. Hayes, and M. D. Rechtin, "Infrared absorption ofsome high-purity chalcogenide glasses," J. Non-Cryst. Solids 17, 3 19—338(1975).

8. D. S. Ma, P. 5. Danielson, and C. T. Moynihan, "Multiphonon absorptionin xAs2S3_(l )GeS2 glasses," J. Non-Cryst. Solids 81, 61—70 (1986).

9. 5. 5. Flaschen, A. D. Pearson, and W. R. Northover, "Low melting sulfide-halogen inorganic glasses," J. Appl. Phys. 31 , 219—220 (1960).

10. T. E. Hopkins, R. A. Pasternak, E. S. Gould, and J. R. Herndon, "X-raydiffraction study of arsenic trisulfide-iodine glasses," J. Phys. Chem. 66,733—736 (1962).

11. 0. V. Khiminets, P. P. Puga, V. V. Khiminets, I. I. Rosola, and G. D.Puga, ' 'Raman spectra of the As-S-I system of glasses,' ' Zh. Prik. Specktr.28, 700—703 (1978).

12. L. Koudelka and M. Pisarcik, ' 'Raman spectra and structure of Aso—xSoIxglasses," Solid State Commun. 41 , 1 15—1 17 (1982).

13. L. Koudelka and M. Pisarcik, "Raman spectra and structure of As-S-Isystem glasses," J. Non-Cryst. Solids 64, 87—94 (1984).

14. I. D. Turyanitsa, V. V. Khiminets, and 0. V. Khiminets, "Interaction andglass forming in As-S(Se,Te)-Br ternary systems," Fiz. Khim. Stekla 1,190—192 (1975).

15. A. G. Fisher and A. S. Mason, "Properties of an Ge-S-Br glass," J. Opt.Soc. Am. 52, 721—722 (1962).

16. L. Koudelka, J. Horak, M. Pisarcik, and L. Sakal, "Structural interpretationof Raman spectra of (As2S3)i _(AsBr3)X system glasses,' ' J. Non-Cryst.Solids 31, 339—345 (1979).

17. A. D. Pearson, W. R. Northver, J. F. Dewald, and W. F. Peck, "Chem-ical, physical and electrical properties of some unusual inorganic glasses,"in Advances in Glass Technology, Vol. 2, Plenum Press, New York pp.357—365 (1962).

18 . E. W. Deeg, ' 'Physical properties of glasses in the system arsenic-sulfur-halogen,' ' in Advances in Glass Technology, Vol. 2, Plenum Press, NewYork pp. 348—355 (1962).

19. Z. U. Borisova, Glassy Semiconductors, translated by J. George Adashko,Plenum Press, New York (1977).

20. A. Hruby and L. Stourac, Czech. J. Phys. , 24, 1 132 (1974).21 . S. A. Dembovskii, V. V. Kirilenko, and Yu. A. Buslaev, "Vitrification

in the system Ge-S-I," Izv. Akad. Nauk SSSR, Neorg. Mat. 7, 328—329(1971).

22. S. Maneglier-Lacordaire, J. Rivet, and J. Flahaut, "The ternary Ge-S-Isystem, construction of the phase diagram and glass forming region,' ' Ann.Chim. (in French) 10, 291—299 (1975).

23. J. Heo and J. D. Mackenzie, "Chalcohalide glasses, 1. Synthesis and prop-erties of Ge-S-Br and Ge-S-I glasses,' ' J. Non-Cryst. Solids 1 1 1 ,29—35

(1989).24. 5. A. Dembovskii and N. P. Popova, "Devitrification in the Ge-Se-I and

Si-Se-I systems," Izv. Akad. Nauk SSSR, Neorg. Mat. 6, 138—140(1970).25. A. Feltz, H. J. Buttner, F. J. Lippmann, and W. Maul,"About the vitreous

systems GeTel and GeTeSi and the influence of microphase separation onthe semiconductor behavior of Ge-Te glasses,' ' J. Non-Cryst. Solids, 8—10, 64—71 (1972).

26. X. H. Zhang, G. Fonteneau and J. Lucas, ' 'Tellurium halide glasses, newmaterials for transmission in the 8—12 p.m range,' ' J. Non-Cryst. Solids104, 38—44 (1988).

27. X. H. Zhang, G. Fonteneau, and J. Lucas, ' 'The tellurium bromide glasses:new JR transmitting materials," Mater. Res. Bull. 23, 59—64 (1988).

28. I. Chiaruttini, G. Fonteneau, X. H. Zhang, and J. Lucas, "Characteristics

oftellurium-bromide-based glasses for JR fiber optics," J. Non-Cryst. Solids111, 77—81 (1989).


Fig. 12. Proposed structural model for Ge23S47Br3o(SIGe = 2) glass(from Ref. 59).


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Chalcohalide glasses for infrared fiber optics

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Transcript of Chalcohalide glasses for infrared fiber optics