IOURNA L OF Journal of Non-Crystalline Solids 151 (1992)134-142 ~ l~] , i i~ i ~ North-Holland

Glass formation, properties and structure in the TeO2-ZnO system

H. Biirger, K. Kneipp, H. Hobert and W. Vogel Friedrich-Schiller University-Jena, Otto-Schott Institute, Jena 0-6900, Germany

V. Kozhukharov Sofia University of Technology, 1756 Sofia, Bulgaria

S. Neov Institute of Nuclear Research and Nuclear Energy, 1784 Sofia, Bulgaria

Received 20 March 1989 Revised manuscript received 14 May 1992

of glasses

Glass formation occurs in the zinc tellurite system in the region of the eutectic (21 mol% ZnO) on the TeO2-rich side of the phase diagram. Glasses are characterized by a high refractive index which increases with TeO 2 content. The glasses are transmitting from about 400 nm to about 6 ~zm with O-H absorption bands at 3.3 and 4.4 ~m. Short range order of the glasses was deduced with neutron diffraction, infrared and Raman spectroscopy by comparison with the spectra of the synthesized crystalline a-TeO2, Zn2Te30 8 and ZnTeO 3. Glasses consist of disordered TeO4, TeO3+ 1 and TeO 3 building units. The number of the TeO3+ 1 units is limited by ZnO addition. There is a relatively strong structural correlation between the glasses and the crystalline compound ZnzTe30 s in accord with the phase diagram.

I. Introduction

The glass-forming range in the ZnO-TeO 2 system has been previously reported [1-3]. De- tailed structure studies have employed phase equilibrium [4] and X-ray diffraction of the com- pounds ~x-TeO 2 [5,6], Zn2Te308 [7] and ZnTeO 3 [8]. It is known that TeO 2 in combination only with modifiers, including ZnO, forms stable glasses at cooling rates typical of glass prepara- tion (< 1 K/min). Pure a-TeO2 was obtained in the vitreous state only by extremely high cooling rates (> 2 X 102 K/s) [9,10].

Tellurite glasses are characterized by low glass

Correspondence to: Dr V. Kozhukharov, Private PO Box 2, 1756 Sofia, Bulgaria. Tel: + 359-2 62 441, ext. 408. Telefax: + 359-2 62 1042.

transition temperature, high refractive index and high transmittance from ultraviolet to near in- frared (NIR) [1,3,11-13]. There is limited knowl- edge of the structure of tellurite glasses. A struc- tural model was developed from X-ray data [5-8]. This should be possible for glasses of this system through comparison of spectroscopic data of glasses and crystalline compounds. We used neu- tron diffraction, infrared and Raman spec- troscopy of crystals, glasses and recrystallized glass samples for this purpose. The structure of the zinc tellurite glasses should be useful to evaluate the structure of other RnOm-TeO 2 systems. Analogous glass structure investigations have been performed in binary tellurite systems with V205 [14], VO 2 [15], Li20 [16], P2Os [17], B203 [18], WO 3 [19-21], MoO 3 [22] and the TeO 2- B203-K20 [23] ternary system.

0022-3093/92/$05.00 1992 - Elsevier Science Publishers B.V. All rights reserved

H. Biirger et aL / Glasses in the TeO2-ZnO system 135

2. Experimental

Glasses were obtained with 17.4-37.2 mol% ZnO prepared at cooling rates of 1 K /min and 10 K /s from batches of TeO 2 'pure for optical purpose' and ZnO 'pro analysis' of Chemapol, Prague, and at cooling rate > 103 K /s by roller techniques outside the glass-forming range [3]. Batches were melted in gold crucibles at temper- atures of 1133-1223 K with a melting time of 45 min. Glass transition temperature, density, re- fractive index and transmittance of the glasses were measured. We calculated the partial disper- sion, the anomalous relative partial dispersion and transmittance values A~i0.5 and A~i0.1 * of the samples. The transmittance of glasses was determined from 185 to 900 nm, using a Specord M40 (Carl Zeiss, Jena, Germany) spectropho- tometer with optical pathlengths of 11 and 1 mm and in the NIR with a sample thickness of 2 mm. The standard KBr pellets technique was used for IR measurements from 900 to 250 cm-] using a Perkin-Elmer, model 457 spectrophotometer.

The glasses, recrystallization products ob- tained by annealing at 673 K in inert atmosphere and compounds a-TeO2, Zn2Te30 8 and ZnTeO 3 synthesized by solid state reactions, were used for structural investigations. Crystalline phases were identified by X-ray diffraction using standard procedure.

Raman spectra of crystalline materials, glass powders and glass rods were obtained using an Ar laser (model ILA 120, Carl Zeiss, Jena, Germany) at A = 514.5 nm and of 700 mW excita- tion. The experimental apparatus is described in detail in ref. [23].

A glass of the composition 80 ml% TeO 2 and 20 mol% ZnO was melted at 1123 K in a Pt crucible and cooled with a rate of about ~ 10 K /s for the neutron diffraction measurements. Neutron diffraction curves of powdered glasses were obtained by a diffractometer installed at nuclear reactor type WVR-M (LINPH, USSR). Experimental data were taken at increments of

~'~ 900

W

~ 700 <

a. 500

w I-

~' \ \ \

{2 3O0

GFR a b

I I ~ l l l l l l l

t ...... ; ..... i ...... i . . . . . . L0 60 80 100

TeO 2 (mole%)

Fig. l. Glass-forming range (GFR) in the ZnO-TeO 2 system. (a) Limits at cooling rate ~ 1 K/min. (b) Limits at cooling

rate ~ 10 K/s.

20 = 0.1; angular range was 20 = 4.125 at a wavelength AN = 1.113 ,~. Radial distribution function (RDF) could be calculated from the diffraction experimental curve. Experimental de- tailes are reported in ref. [24].

3. Results

3.1. Glass formation

Glasses were obtained in the range of 20-30 mol% ZnO (100 g batches) at cooling rates of ~ 1 K/min. The glass-forming region of 17.2-37.6 mol% ZnO given in ref. [3] is valid for a cooling rate of about 10 K/s. Figure 1 shows the location of the glass-forming region in the phase equilib- rium diagram [4], determinated by differertt eo01- ing rates. The glass compositions prepared for structure measurements are given in table 1. The composition of the glasses obtained was analyzed (TeO 2 content by a method of Browning and Flint [25] and ZnO by complexometric titration). The analyzed composition of glasses corresponds to that of the batches within + 0.2 mol%.

3.2. Properties

* That is, wavelength values of inner transmittance at 50 and 10% spectral transmittance, respectively.

Refractive indices were obtained at wave- lengths given in table 2. The calculated partial

136 H. Biirger et al. / Glasses in

Table 1 G lass - forming tendency in the ZnO-TeO 2 system

Compos i t ion Cool ing rate

(mol%) rol ler copper graph i te

TeO 2 ZnO techn ique mould mould ( > 10 3 K /s ) ( ~ 10 K /s ) (1 K /min)

100 - 0 a )

85 15 o o - 80 20 o o o 75 25 o o o 70 30 o o o 65 35 o o -

a) O, stable glass.

the TeO2-ZnO system

dispersion, relative anomalous partial dispersion and Abbe number (V e = n e -- 1/n F, - nc,) are also presented in table 2.

Transmittance in the UV-VIS spectral region (table 2) is characterized by the )%i0.5 and Azi0.1 values evaluated from the transmittance curves shown in fig. 2(a). A selected NIR spectral trans- mittance curve of a TeO2-rich glass sample is presented in fig. 2(b). Density of the glasses as well as linear expansion coefficient, tg and t~ (softening point) transformation temperatures of glass samples near the eutectic composition are shown in table 2.

Table 2 Physical proper t ies and values

Sample no: 1 2 3 4 5 6 Er ror

Composition (tool%) ZnO: 17.4 19.9 24.6 29.6 33.2 36.4 + 0.1 TeO2: 82.6 80.1 75.4 70.4 66.8 63.6

Refractive index, n C' (643.8 nm): 2.1099 2.0996 2.08072 2.05951 2.0433 2.0297

d (587.6 nm): - - 2.09431 2.07245 - - + 0.00005 e (546.1 nm): 2,1395 2.1275 2.10768 2.08516 2.0682 2.0538 F ' (480.0 nm): 2.1721 2.1598 2.13867 2.11456 2.0966 2.0809 g (435.8 nm): 2.2071 2.1937 2.17094 2.14502 2.1266 2.1097

Dispersion n F ' - n C': 0.0622 0.0602 0.05795 0.05505 0.0533 0.0512 ng-nF , : 0.0350 0.0339 0.03227 0.03046 0.0300 0.0288 _+ 0.0001

Abbe number re: 18.3 18.7 19.1 19.7 20.0 20.6

Relative dispersion ng - rl F,

- - : - - 0.0577 0.0553 - - - n F, - n C,

Transmittance (nm) A,i0.1: - 384 378 372 368 - - A ~i0.5: - 399 393 385 380 -

Transformation temperature (C) tg: - - 315 320 - - + 5C ts: - - 328 335 - -

Linear thermal expansion ( x 10 - 76 - l) a: - - 174 170 - - -

Density (g / cm 3) p: 5.54 5.53 5.51 5.49 5.48 5.46 _+ 0.01

" 100 U Z < I =

5C

Z <

H. Biirger et al. / Glasses in the TeO2-ZnO system

WAVELENGTH C,um)

2.5 5 10

Q 1-

4.4

O-H stretch

, = t =

I,- 300 Z.O0 500 /. 3 2 Ix i000 WAVELENGTH(nrn} WAVENUMBER[ern-ll

Fig. 2. Spectral transmittance curves of zinc tellurite glasses. (a) UV cut-off net transmittance (sample thickness of 10 mm): curve 1, 70.4TEO 2 .29.6ZNO (mol%) glass; curve 2, 75.4TEO 2 24.6ZNO (mol%) glass; curve 3, 80.1TeO 2.19.9ZNO (mol%) glass. (b) N IR spectral transmittance curve of glass with composition 80.1TeO2-19.9ZNO (mol%) (sample thickness of 2 mm). Abscissa accuracy is better than +5 cm t; ordinate accuracy and %T repeatability is better than +0.2%T; typi- cally limited by noise level, pen recorder dead band < 0.5%.

3.3. Structural measurements

137

j

900 700 500 300 WAVENu 1'4 BER (cm-1)

Fig. 3. IR transmittance spectra: curve 1, ct-TeO2; curve 2, Zn2Te308; curve 3, for ZnTeO 3. ff (cm - ] ) accuracy is better than + 1 cm 1; run-to-run abscissa repeatability after warm- up of 0.005 cm- I ; ordinate accuracy and repeatability better

than 0.1%T.

For detailed IR structural study, glasses of the composition presented in table 1 were prepared at different cooling rates. Glass samples recrystal- lized at thermal treatment of 623 K for 20 h. These devitrificated samples as well as the crys- talline compounds a-TeO 2, Zn2Te30 8 and Zn- TeO 3 were also used for the structural measure- ments. The IR spectra of these compounds have been studied and results shown in fig. 3 and table 3 are in good accord with prior work [26]. The correlation of IR spectra of glasses prepared at different cooling rates and the same samples af- ter devitrification process are shown in fig. 4.

The Raman spectrum of a-TeO 2 has already been published in refs. [23,27]. The structure of oL-TeO 2 is characterized by Raman lines at 640 and 600 cm -] and a deformation vibration at about 400 cm -1 (fig. 5(a)). One can also find these three lines belonging to the TeO 4 building units in the spectrum of the Zn2Te30 8 shown in fig. 5(b) but their intensity is smaller. Raman vibrations at 790, 740 and 690 cm-] as well as deformation vibrations at 360 and 310 cm-t were observed. A wide accordance with the IR absorp- tion spectra exists (see table 3) besides the band at 790 cm -t. Well resolved Raman spectra of

Table 3 IR and Raman frequencies of crystalline zinc tellurites

Compound Frequencies (cm l)

Infrared spectroscopy a-TeO 2 IR - 775 710 660 ZnzTe30 s IR - 750 685 655 ZnTeO 3 IR - 765 695 670

Raman spectroscopy c~-TeO 2 - - 640 Zn 2Te 3 O 8 790 740 690 640 ZnTeO 3 790 - 690 -

620 600

600 600

. . . . 340 570 520 480 400 370 310

- 485 400 - -

400 - - 400 360 310 400 360 300

138 H. Biirger et aL / Glasses in the TeOe-ZnO system

t t l

u Z < I- I-

z

I-

I

~', f (mo e%)

:,~." ~' ~ ..... S ~ '~" ao : 70

:80 ~ .. . . . """

; ~ 1115:85

I I 1 900 700 500 300

WAVENU MBER (crn-1)

Fig. 4. Correlation dependence of IR spectra of glasses at different cooling rates: . . . . . . , cooling rate > 103 K /s (roller technique); - - , cooling rate ~ 10 K /s (copper mould); . . . . . . , recrystallized glass sample (KBr pellets of 1 g KBr, 10 mg glass powder). Errors of the measurements are given in

fig. 3 caption.

1 >-

I .--

.,o

z I.LI

I.---

z

Lt.I

I - -

Zn0 : Te0 2

30:70

25:75

.-3

.,0

I

20:80 (m01e%)

c / ~-

WAVENUMBER (cm-1) Fig. 6. Raman spectra of glasses cooled at ] K/min; data

errors are given in the fig. 5 caption.

z

I - - ;z

e~

N

d J

WAVE

=-]e 02

~ Z n 2Te TO 8

lj Zn Te 03 5

,/ZnO : Te02 ~ 30 : 70

(m01e%)

UMBER (cm -1 )

Fig. 5. Raman spectra of polycrystalline compounds (a-c) and (d) recrystallized sample containing 70 mol% TeO 2. ff (cm- i ) accuracy is better than +0.2 cm-X; ordinate accuracy and repeatability are better than 0.02% scattering intensity.

~z indicates the laser plasma line.

zinc tellurite glasses cooled at 1 K/min are shown in fig. 6.

Because of the similar, nearly identical scatter- ing amplitudes of 80, 3Zn and 52Te, the method of neutron diffraction is sensitive to light oxygen nuclei. Therefore this method is expected to sup- ply information about the short range order of the vitreous network. We calculated the Fourier transform curve, F(R), from the neutron interfer-

~9,10o /

z ~s~

l

298K

I

t 2 3 4 5 6' 7 8 9Q( l-I)

Fig. 7. Experimental neutron scattering curve of glass powder sample with composition 80TeO2.20ZnO (mol%); statistical average error is < 2% after fourfold measurement of the

experimental points registered.

H. Biirger et aL / Glasses in the TeO2-ZnO system 139

(4 ,65)

3F . .5 , ,2 .0 , 7

-0[ /l/l / ' ' ' P3 ' ' 'V ' ~

2 3 4 6 7 8 9 R

Fig. 8. Neutron pair scattering function, F(R) vs. interatomic distances, R. The values in parentheses represent the position of the peak (in ,~). The accuracy of determination of the position of the P1-P4 peaks is 4R ~< 0.05 .~ and for peaks

P5-P9 it is 0.10 ~,.

ence function, I(Q), shown in fig. 7 which is presented in fig. 8. Figure 7 shows the character- istic broad scattering maxima, typical for vitreous samples. The calculation of the RDF 4~rR2p(R) for the glass 20ZnO.80TeO 2 (mol%) is de- scribed in detail in ref. [24]. The pair distribution function, F(R), shows nine maxima, in the range

o

up to 10 A, which are attributed to the atom density as is illustrated in fig. 8. Figure 9 shows a

36 6.10

28

"7 24

2G

,0

g 'e 8

4

o

F ,t o , 2 3~ 4 s 6

R (~ )

Fig. 9. Radial distribution function 4~rR2p(R) of the glass investigated (curve 1), RDFcalc (curve 2) of a glass with com- position 80TeO2.20ZnO (mol%) and RDFc~lc (curve 3) of Zn2Te30 8 atomic density; quantitatively there was no regis- tration of sharp oscillation for R values < 2 ,~, i.e., the

normalization procedure is carried out correctly.

comparison of the model and experimental RDF. A well resolved coordination maximum appears in the RDFex p curve at R~ = 0.202 nm (P1), R 2 = 0.297 nm (P2) and two unresolved maxima at 0.385 nm (P3) and 0.475 nm (P4).

4. Discussion

4.1. Glass-forming and properties

Figure 1 illustrates that the glass-forming range in the TeOz-ZnO system is quite wide. Glass formation limits depend strongly on the cooling rate and melts size. The upper limit of the glass- forming range at a cooling rate of ~ 10 K /s correlates with the peritectic point of ZnTeO 3 and with the isopleth of the Zn2Te30 8 com- pound as well. Glasses more stable to devitrifica- tion glass are obtained in compositions situated around the eutectic and above the liquidus line for primary crystallization of Zn2Te30 8.

The refractive indices of the glasses increase while the ue-values decrease with increasing TeO 2 content. Density of the glasses decreases slowly up to 5.46 g /cm 3 with increasing ZnO.

Figure 2(a) indicates that the UV transmit- tance edge shifts to shorter wavelengths with increasing of ZnO content. The transmittance in visible spectral region of the zinc tellurite glasses is mainly influenced by the formation of non- bridging oxygens, and the increase of the TeO e content which leads to a shift of the UV cut-off to longer wavelengths. These two factors operate oppositely. The second parameter is dominant; thus, with increasing TeO 2 and decreasing non- bridging oxygens, a long-wavelength shift occurs.

Absorption bands appear in the NIR spectral region at 3.2 and 4.4 txm belonging to O-H stretching vibrations, as shown in fig. 2(b). The multiphonon absorption edge at about 6 ~m (1660 cm -1) is determined by overtones of the Te-O stretching vibrations at 600-800 cm- ].

4.2. Structural characteristics

It is known [5,6] that a-TeO 2 consists of TeO 4 units in the form of trigonal bipyramids. In each

140 H. Biirger et al. / Glasses in the TeOe-ZnO system

case, two equatorial and axial oxygen atoms are located at distances of 0.191 and 0.208 nm, re- spectively from the Te atom. Each polyhedron is bonded to four others. According to ref. [26], the IR absorption bands at 780, 714, 675 and 635

S cm-1 are assigned to the stretching vibration Veq, veaSq, Va xas and Va xs , respectively, corresponding to different Te-,O bonding lengths (see table 3 and fig. 3).

The monoclinic compound Zn2Te30 8 investi- gated by Hanke [7] is of chain-like structure with Te30 8 groups built up by one TeO 4 and two TeO3+ 1 units. In the compound, these groups are connected by a long Te-O bond to ~[Te30 8] chains [7]. A different distribution of the Te-O bonding length was found. The observed IR ab- sorption bands at 765, 685, 655 and 600 cm -~ may be assigned to Te -O stretching vibrations. In the orthorombic compound ZnTeO 3, isolated TeO 3 groups occur [8] belonging to different Te -O lengths of the bond at 0.186, 0.188 and 0.190 nm. Due to the different Te -O bond lengths, the degeneration is raised and the tel- lurium-oxygen stretching vibrations at 775, 700 and 660 cm-1 can be assigned to different oxy- gens, i.e., to vTe_ o [1-3]. Bond vibrations in the region of 300-500 cm-~ belong to deformation.

The spectra of glasses shown in fig. 4 show broad absorption bands between 500 and 700 cm-1 with a shoulder at 770-800 cm-n. IR spec- tra of fast-cooled glasses (roller technique) show an additional broadening, a small shift and a relative increase in intensity of the longer wave- length bands VT~_ o. The broadening could be explained by states frozen in at a higher tempera- ture with a.large disorder, where a broad distri- bution of bond lengths occurs.

A better structure view of the glasses can be obtained by taking into account the spectra of recrystallized glasses. Absorption bands occur in the spectra of the crystalline compounds Zn2Te30 8 and ZnTeO 3 between 750 and 760 cm-1, as well as some deformation vibrations at 400-600 cm-k These bands give an additional contribution to the glass spectra with increasing the ZnO content. This leads to an intensity distri- bution over a broad wavelength region in the glass spectra and the 780 cm -a band of the

a-TeO 2 occurs only as a shoulder of the broad 600-700 cm -1 band. The spectra of the glass containing 85 mol% TeO 2 can be described mainly by that of oL-TeO/. We find an additional small intensity in the spectrum of the devitrifi- cated sample in the region from 400 to 550 cm-1. The influence of Zn2Te30 8 dominates in the spectrum of the recrystallized glass with 35 mol% ZnO and 65 mol% TeO 2 whereas in the sample with composition 30 mol% ZnO and 70 mol% TeO 2 some influence of ot-TeO 2 is observed. It was established that the bands at 790, 720, 660 and 600 cm -~ belong to et-TeO 2, while those at about 750, 680, 525 and 480 cm -1 belong to Zn2Te30 8.

The Raman spectra of the crystalline samples of o~-TeO 2, Zn2Te30 8 and ZnTeO 3 represent the basis of the Raman spectroscopic discussion of the zinc tellurite glasses. As in Zn2Te308, the Raman lines at 790 and 690 cm- 1 as well as those at 400 (very weak), 360 and 310 cm-1 were also observed in the spectrum of ZnTeO 3. The rela- tive intensity of the Raman lines in the devitrified samples at 640 and 400 cm -1 increase signifi- cantly with increasing TeO: content. At the same time, the lines at 790 and 690 cm -1 exhibit a small shift to shorter wavelengths, i.e., the 690 cm-~band changes to the known 640 cm-~ band of the a-TeO 2 (see fig. 5(d)). According to refs. [26,27], the observed vibrations in the spectrum of cx-TeO z at 640 and 400 cm-1 are typical for TeO 4 building units. The lines in the ZnTeO 3 spectrum at 790, 690, 360 and 310 cm -1 refer to the exis- tence of TeO 3 units. The additional Raman line at 740 cm -~ occurring in the spectrum of ZnzTe30 8 can be related to the formation of TeO3+ ~ ** units. A corresponding shift of the band at 790 cm-~ towards 740 cm-~ is ascribed to coordination evolution from TeO 3 to TeO3+~ which can also be seen in fig. 5(c) and fig. 5(d).

Figure 6 shows the polarized Raman spectra of 70, 75 and 80 mol% TeO 2 glasses. Raman spec- tra show characteristic broadening compared with crystalline samples shown in fig. 5. In the crys-

** It is conditionally accepted that one Te-O bond length is > 2.2 A.

H. Biirger et al. / Glasses in the TeO2-ZnO system 141

talline sample, three bands in the region of the deformation vibration from 310 to 400 cm -1 su- perpose to one broad band in the glass. The bands at 640 and 740 cm-1 overlap and can also include parts of the vibrations appearing in crys- talline samples at 790 and 690 cm-1. The spectra of glasses suggest the presence of TeO4, TeO3+ t and TeO 3 building units, considering the Raman spectra of the crystals. The intensity of the band at 740 cm-1 decreases with increasing TeO 2 con- tent (see fig. 6(b) and fig. 6(c)) compared with the intensity of the band at 640 cm-l ; for 80 mol% TeO2, the 640 cm-~ band dominates. Simultane- ously, the broad band in the region of deforma- tion vibration shifts to slightly higher wave num- ber. This could be explained by a stronger influ- ence of the band at 400 cm-1, appearing in the spectra of ot-TeO 2 by contrast with that of 360 and 310 cm -1 in the Zn2Te30 8 compound.

Neutron diffraction was carried out to define short range order up to ~ 6 .& (0.6 nm) of glass investigated. The first peak, P1, can be inter- preted as distribution of Te -O and Zn-O inter- atomic distances, i.e., first coordination sphere of TeO 4 and ZnO 6 building units. Determination of the theoretical positions of P1 gives 0.203 nm in good accord with the experimental value of 0.202 nm (fig. 9, curve 1). One cannot recognize a greater change of the tellurium-oxygen coordina- tion from this peak. The TeO 4 polyhedra are easily deformed and exhibit strong dynamics of both Te-Oax bonds. An elongation of over 0.22 nm undoubtedly exists because of the lack of a centre of symmetry of the TeO 2 units. This is in accord with coordinations 3 + 1 for Te atoms at RTe_ o from 0.208 to 0.298 nm and 3 for RTe_ o > 0.298 rim. It is of interest to know how the Te-Oax bonds influence the RDF curve. The influence can be clearly seen in the RDF~x p curve as a broad distribution of the Te-O distances in the region between 0.22 to 0.3 nm. The value of the Te-O coordination number calculated from P1 at 0.202 nm is 3.35. This leads to the result that approximately 35% of the Te atoms build TeO 4 groups and 65% of them build TeO3+ 1 plus TeO 3 groups. The position of the first maxi- mum (P1) of the RDF~x p curve compared with the RDFcalc curves gives good conformity but a strong

discrepancy exists in the region of 0.25-0.45 nm. The peak at 0.297 nm (P2) belongs essentially to the O-O distances. These are found in the model curves RDF (e~-TeO 2) at 0.283 nm and RDF (ZnO) at 0.335 nm. Both parts superimpose in the glass and lead to a strong contribution in the region from 0.279 to 0.325 + 0.04 nm. Addition- ally, in the RDFex o curve, unresolved peaks ap- pear at R ~ 0.385 nm and R ~ 0.475 nm belong- ing to Te-Te, Te-Zn, O(secondsphere)--O , Zn-Zn and Te(secondsphere)-O pair distributions. Curve 3 of fig. 9 is compared with the RDF curve and indicates analogous structure groups. There should also be chain-like structures mainly with TeO3+ 1 building units in the glass with composi- tion ZnO 20, TeO 2 80 mol%, in accord with the chain-like structure ~[Te30 8] of Zn2Te30 8 which, according to Hanke [7,8], includes one TeO 4 and two TeO3.. 1 polyhedra. There is no significant difference between TeO3+ l and TeO 3 groups because they are relatively unstable configura- tions.

5. Conclusion

It has been shown that glass formation in the ZnO-TeO 2 system depends strongly on the cool- ing rate, especially in the TeOz-rich region. Opti- cal properties, density, linear thermal expansion, transmittance in the UV-VIS and NIR regions of the glasses show that the zinc tellurite glasses are of super dense flint type with good spectral trans- mittance. Structural information has been ob- tained especially about structural changes as a function of glass composition, by means of IR and Raman spectroscopy as well as neutron diffraction study of the ZnO-TeO 2 glasses. Char- acteristic vibration frequencies of the appropriate crystalline compounds were helpful for interpre- tation of glass IR and Raman spectra. TeOz-rich glasses are characterized by TeO 4 building units in analogy to ot-TeO 2. The influence of TeO3+ 1 structural groups increases with increasing ZnO. This leads to a characteristic band in the Raman spectra at 740 cm -1 caused by ZnzTe30 8 and to a typical deformation vibration in the region from 450 to 550 cm-1 in the IR spectra. This result is

142 H. Biirger et al. / Glasses in the TeO2-ZnO system

also supported by the RDF curves computed from neutron diffraction data, which show that the O-O pairs at distances greater than 0.22 nm are fixed, thus causing an increase of asymmetry in the basic building units. The glass structure becomes increasingly ZnzTe3Os-like with increas- ing ZnO and consists of a ~[Te30 8] chain-like structure with TeO3+ t and TeO 4 groups. The reason for the change of tellurium-oxygen coor- dination polyhedra is the lack of a centre of symmetry in the TeO 4 building units. This leads to a dynamic relation of the Te-Oax bonds. The glass network is built up mainly by distorted TeO3+ t and TeO 4 groups, the contribution of which is a function of the composition.

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