FTIR spectra and some optical properties of tungstate-tellurite glasses

FTIR spectra and some optical properties of tungstate-tellurite glasses

Pergamon 0022-369705w . J. Phys. Chem Solids Vol57, No. 9. pp. 1.223-1230, 1996 Copyright 0 1996 Ekwier Sumce Ltd Pnntcd in Great Britain. All rieht...

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Pergamon

0022-369705w .

J. Phys. Chem Solids Vol57, No. 9. pp. 1.223-1230, 1996 Copyright 0 1996 Ekwier Sumce Ltd Pnntcd in Great Britain. All riehts metvcd CUl22-3697/96 0.00

I

si5.00+

FTIR SPECTRA AND SOME OPTICAL PROPERTIES OF TUNGSTATE-TELLURITE GLASSES I. SHALTOUT?,

YI TANG?,

R. BRAUNSTEINT

and E. E. SHAISHAS

tDepartment of Physics, University of California, Los Angeles, CA 90024, U.S.A. IDepartment of Physics, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt (Received 26 July 1995; accepted 18 September

1995)

Abstract-The optical properties of the binary glass system ((100 - x)Te02 + xWOs} with 5 5 x 5 50mol% was studied using Fourier transform infrared spectroscopy in the spectral range 15025,00Ocm-i. The color of these glasses changes from yellow to light green, to dark green as WOj concentration increases. These glasses are disordered versions of tetragonal TeOz of D: symmetry where the Te atom is 4-fold coordinated. The W ion coordination states change from 4 to 6 when WO, increases beyond 3 1.5 mol %. The band tail energies are found to be between 0.103 and 0.112 eV, however these values do not show a monotonic behavior as WOs concentration increases. The optical band gap (&,,) was found to decrease from 3 to 2.93eV as WOs increases from 15 to 30mol% while the refractive index (N) as a function of WO, was found to change from 2.27 to 2.36 as WOs concentration increases from 15 to 30mol. Keywords: A. glass, C. infrared spectroscopy, D. optical properties.

1. INTRODUCTION Tellurite

glasses

in general

many technological

are good candidates

applications.

low melting point (800°C), are not hygroscopic, low high

glass

transformation

dielectric

coefficient,

constant,

temperature high

thermal

and high optical transmission

red region to 5 pm [l, 21. Moreover, typically

of high density,

for

These glasses have a have

(5 4OO”Q expansion in the infra-

these glasses are

and high refractive

doped with Ho+~ at room temperature [lo]. These glasses are expected to become important upconversion laser materials. In the present work, some optical properties of a wide composition range of the binary glass system {(loo-x)Te02+xW03} with 5
index

N 2 2 [3].

As known, transitional

2. GLASS PREPARATION AND F’TIR SPECTRA MEASUREMENTS

metal ions (TMI) in oxide

glasses result in very important electronic properties because of their presence in multivalence states. Many publications have been reported on the technological importance of tellurite glasses containing TM1 in uses as elements in memory switching devices and cathode materials for batteries [4, 51. Te02 based glasses are known to show electrical conductivity several orders of magnitude higher than silicate, borate, and phosphate glasses containing the same amount and type of the modifier [6]. DC and AC electrical conductivity studies, infrared spectra and the Mossbauer effect studies on a range of TeOz-based glasses containing different types of modifiers have been reported previously [7, 81. As for spectroscopical applications, some tellurite glasses were reported to be promising materials for use in non-linear optical devices [9]. Of great importance, an upconversion fluorescence has been reported very recently for the first time in several tellurite glasses

Reagent grade oxides (Alfa Johnson Matthey Electronics 99.995% purity Te02, and Alfa inorganic Ventron WOs) were mixed and milled in an agate mortar and then melted in a platinum crucible for half an hour to ensure complete homogeneity in a preheated furnace between 800 and 1000°C according to the composition. To minimize the volatilization of Te02 at high temperatures, the crucible was covered with a platinum plate during melting. After complete fusion, the melt was poured as quickly as possible on a stainless steel plate at room temperature. Bulk samples of about 2 cm diameter and 0.5 cm thickness were obtained. The color of these glasses changes from yellow to light green, to dark green as WOs concentration increases. The samples were not subjected to any annealing processes and were used as casting. The glassy state of all the samples was confirmed using X-ray diffraction techniques. Different 1223

I. SHALTOUT et al.

1224

glass compositions and summarized in Table 1.

their

characteristics

are

3. RESULTS AND DISCUSSIONS

3.1. Far infrared (FIR) spectra Infrared spectra (FIR) of crystalline TeOz, WO3 and the untreated and heat treated glasses {(lOO-x)Te02+xW03} with 5
matrix are four-fold coordinated due to the formation of W-0-Te bonds. As shown in Fig. 1, the band at 328 cm-’ of crystalline TeO, has been broadened and shifted to 343 or 360 cm-‘, in the spectra of all heat treated glasses. This broadening and shifting to a higher frequency as WO3 increases could be thought of as a convolution of the TeO,, and WO, vibrational modes. With the increase of W03 to 35mol% or more (spectra 9-l l), the spectra of the samples have become very similar to the spectrum of crystalline WO,, and this reflects the change of the coordination state of W ions from 4 to 6 in the glass samples containing 35 5 WO3 5 50mol%. 3.2. Near infrared (NIR) spectra Figure 2 shows the near infrared (NIR) spectra of the constituent oxides and the glass samples containing 5 5 WO3 5 50 mol%. Measurements were carried out on identical (KBr) pellets containing the same weight of the glass powder to enable us to roughly compare the relative intensities of the bands. As shown in Fig. 2, crystalline TeOZ is characterized by two bands at 660 and 78Ocm-‘ . These bands correspond respectively to the symmetric axial v~(A,) = (vTeOS),, = 635cm-‘, and the symmetric equatorial VI(Al) = (VTeO& = 78Ocm-’ vibrational modes of the Te04 tetrahedral units [18]. The spectra of the glass samples containing 5 5 W03 I 30mol% have the same characteristic bands of crystalline TeOz, and this suggests that (TeOJ tetrahedra are the basic structural units in these glasses. The spectra of the glasses containing 40 5 WO3 I 50mol% (spectra 6, 7) are broadened and appear as poorly resolved envelopes which represents the vibrational density of states of the constituent oxides. AS is known, the width of the peaks are due to the distribution of bond-angles and lengths and the fluctuation of the local electronic and atomic environments in the amorphous state. The weak band around 935crn-’ in the spectra of the glasses containing

Table 1. Characteristics of tungstate-tellurite gldsses W03 content

Melting

(MOW)

temperature (C)

5

800

15 20 25 21.5 30 31.5 33 35 40 50

800 800 800 800 800 800 1000 1000 1050 1050

*.

Melting time (min) 30 30 30 30 30 30 30 30 30 45 45

Preparation All samples quenched on stainless steel plate at room temperature.

Color Yellowish Yellowish Yellow Yellow Light green . Green Green Dark green Dark green Very dark green Very dark green

FTIR spectra and optical properties of glasses 5 i WOJ < 30mol% is due to the vibrations of W04 tetrahedra [ 191.

symmetric

3.3. Optical absorption and reflectivity spectra (4000-24,000 cm-‘) 3.3.1. Defect states and band rails. Optical absorption spectra In(cr) versus wavenumber of the glass samples containing 15 I W03 < 30mol% in the frequency range 3000-24,000 cm-’ are compared in

E

CRYST f WO 3

Fig. 3. The samples containing 5,40, and 50 mol% of WOs could not be measured because they are fragile and therefore could not be polished. The spectra were collected on bulk samples 0.3 mm thickness polished using extra fine alumina powder of grain sizes 0.05pm. Using these thick samples, low absorption coefficients of the order of 2OOcm-’ and optical transitions within the optical gap around 1.5 eV were observed. Such low energy transitions may not be

I

1 150

250

WAVENUMBER

350

150

450

f CM-’

1

1225

CRYST.

W03

250

350

WAtJE NUMBER

-_

4:

t CM-’ )

Fig. I. (a) FIR spectra of the glasses {( 100 - x)Te02 + xWOj} 5 5 x 5 27.5 mol.%: 1. x = 5; 2. x = 15; 3. x = 20; 4. x = 25; 5. x = 27.5. (b) FIR spectra of the glasses {( 100 - x)TeO, + xW09}: 6. x = 30; 7. x = 31.5; 8. x = 33; 9. x = 35; 10. x = 40; 11. x = 50. - - -Untreated glasses; heat treated at 450°C for 18 h.

1226

I. SHALTOUT et al. Table 2. FIR characteristic frequencies of crystalline TeQ, W09 and the glasses { (100 - x)TeO, + xW0,) with 5 5 x < 50mol% Frequency (cm-‘) 189 220 262 328 405

Sample Crystalline TeOz

(100 - x)TeOz + xWO3

Untreated glasses

x=5 x= 15 x = 20 x = 25 x = 27.5 x = 30 x = 31.5 x = 33 x= 35 x = 40 x = 50 Crystalline W03

347 354 343 347 347 340 347 347 347 347 347

Heat treated at 450°C for 18 h 188 219 189 219 193 227 188 224 188 219 189 223 189 223

266 266 265 262 266 270

343 343 341 347 347 347 352 360 232 360 230 280 360 230 282 360

169 227 285 325 374

detectable for thin film samples, where only high absorption coefficients (cr 2 lo4 cm-i) are usually observed. The low energy transitions around 1.5 eV are due to defects in the amorphous matrix related to dangling or non-bridging atoms, or due to the differences of the ionic radii and the electronic polarizability of the two metal cations (W and Te). These defects create deep localized states in the gap and transitions from one of these localized states to extended states or vice versa can occur. As shown in Fig. 3, the defect density of states increases as WO3 increases and finally overlaps with the band tails near the optical band edge as WOs reaches 27.5 mol% or more. Proposed models of defect states in amorphous solids are shown in Fig. 4, which shows the density of states in the gap of a non-crystalline semiconductor [20]. In Fig. 4a it is supposed that the Fermi energy EF is pinned near the mid-gap by some sort of defect which results in deep donors or acceptors. Figure 4b suggest that the energy of the defect states can vary from one defect site to another and this results in overlapping between the defect states and pinning of the Fermi level near the middle of the gap. In Fig. 4c the Cohen-Fritzsche-Ovshinsky (CFO) model [21] supposes that the tails of the valence band and the conduction band could be deep enough to overlap and consequently EF: is near the middle of the gap. A kind of correspondence between these proposed models and the defect states observed in Fig. 3 as a function of WOs may be noticed. That is, the defect density of states in the gap increases and finally overlap with the band tails near the optical band gap as W03 increases up to 30mol%. Also, the average measured energy of the defect states is about 1.5 eV, which is approximately half the optical energy gap (Eopt) (Table 3) [22] and this may indicate that the Fermi energy is near the middle of the gap.

The band tail energies (Eo) are shown as a function of WOs content in Fig. 5. These energies (Table 3) were calculated using the equation: (CX)= A exp(hv/Es)

(1)

where A is a constant and hu is the incident photon energy. As is known, deeper band states are expected to extend into the gap as the degree of disorder increases. However, as seen in Fig. 5, in spite of the small increase of the least square values of (EO) as WOs increases, the experimental values of E,, are not monotonic as W03 increases. 3.4. The Urbach edge behavior Transmission spectra of the glass samples in the range 3000-24,OOOcm-’ are shown in Fig. 6. Although the spectra were collected on quite thick samples (0.3mm), the optical transmission in this range is about 80% for these glasses. Therefore, in the light of their electrical, chemical and mechanical stabilities, and their non-hygroscopic properties, these glasses could be used as optical windows in this frequency range. As shown in Fig. 6 the glass samples show the Urbach edge behavior usually observed in amorphous solids. It should be noticed in Fig. 6 that the absorption edge broadened as WO, reaches 27.5mol% or more. This broadening as a function of WOs could be thought of as follows: Te and W atoms have the electron configuration 4d”5s25p4 and 5d46s2, respectively, the lower valence state of the Te atom means lower average coordination state possibilities. Therefore, it is more likely that the TeOz-rich glasses with 15 5 WOs 5 25 mol%, may have a more open structure and consequently fewer defects (dangling or broken bonds, non-bridging oxygen, etc.) and less distribution of bond angles and lengths which results

1227

FTIR spectra and optical properties of glasses

in a steeper band edge. This is in good agreement with our discussions about the defect states concentrations in Fig. 3. With W03 content higher than 25 mol%, the more compact structure which is associated with the higher coordination possibilities of W ions may result in higher defect concentrations and consequently broadening of the edge. 3.5. The optical gap (E,,,) The optical band gap (Eopt) of glasses is usually

obtained through the extrapolation of the relation (oh&$‘2 versus photon energy ti w for measurements on thin film samples to energies beyond the fundamental edge. With our experimental facilities we could not measure absorption coefficients for energies hv 1 24,0OOcm-‘. However, the interception of the transmittance spectra with the wavenumber axis in Fig. 6 simply results in values of Eopt quite close to those found in the literature for Tellurite glasses [22]. As seen in Fig. 7, the optical band gap decreases as

CRYSTALLINE

1002

6

)

CRYSTALLINE

s

8 I

0.5

W03

I

I

I

2.5

I.5 WAVENUMSER

wul-’

I

I 3.5

)

(THOUSANDS) Fig. 2. IR transmission spectra of the glasses { (100- x)TeO* + xOW~}: 1. x = 5; 2. x = 15; 3. x = 20; 4. x = 25; 5. x = 30; 6. x = 40; 7. x = 50.

1228

I. SHALTOUT et al.

WOs increases and this is due to the increase of the disorder and consequently the more extension of the localized states within the gap according to Mott and Davis theory [20]. 3.6. Refractive

index of glasses

Refractive indices of the glasses were calculated using the absorption and reflectivity spectra in the frequency range 3000-24,000 cm’. The equations used for the transmission (T) and reflectance R are:

(2)

Rt

=

R[l+ (1 -

& =

1_

10

I.X=

2R)e-2ax]

(3)

R+*aX

where IO = energy incident on the sample, Z = transmitted energy, I, = reflected energy, R = reflectivity = [(n - l)* + kz]/[(n + l)* + kz], a = absorption coefftcient = 4mk/X, and x = sample thickness.Ifax l,eqn(4)becomesR’z R. Refractive index N as a function of W03 is shown in Fig. 8. The refractive indices obtained in the present work are between 2.27 and 2.36, which are in good agreement with some previously reported values 2.17-2.28 of [Te02 - W03] glasses [24-261. However, as seen in Fig. 8, N shows a kind of anomalous behavior. The significance of this anomalous behavior is that the minimum of (N) is at WOj = 27Smol%. That is because a minimum of (N) at about the same content of the modifier (30 mol%) has been reported recently by Komatsu et al. [9] for {Te02 - LiNB03}

15

5432I0

5

7

9

II

13

I 15

I 17

I 19

21

4. x = 21.5

23

5 4

2.x=20 5-

1

3 2

O

5

7

9

I1

I3

I5

I7

I9

21

I

0

579

0

579

II

13 I5

I7

19 21

23

II

I3

17

I9

23

23

3.X=25 5

I

432-

I5

21

Wavenumber (cm-‘) (Thousands) o

579

II

I3

I5

I7

I9

21

23

Wavenumber (cm-‘) (Thousands)

Fig. 3. Opticalabsorption spectra of the glasses {(100- x)TeO, + xW03}. &

are

the band tail energies.

FTIR spectra and optical properties of glasses

N(E)

1229

I

N(E)

(bl 3

5

7

911

13

I5

17

19

21

23

Wavenumber (cm-‘) (Thousands)

N(E)

Fig. 6. Optical transmission spectra of the glasses ((100 - x)TeDr + xWOs}, with 15 5 x 5 30mol%: 1. x = 15; 2. x = 20; 3. x = 25; 4. x = 27.5; 5. x = 30.

E Fig. 4. Density of states in the gap of a noncrystalline semiconductor. (a) Compensated donors. (b) Centers acting as donors and acceptors with overlap. (c) Model of CohenFritxsche-Ovshinski (CFO), Ref. [20].

T u

3.09 3.08 3.07 3.06

2

:z

E

E 0.18 0.17 0.16 0.1s ::1: ::I: 0.10 t:: y

2.93 -

0

0

:3: 2.9015

0

0.07 0.06 -

19 I

21 I

I

25 I

27 I

29 I

f

0

0.05 0.04 i:: 0.01 o~oo15

17 ,

17 ,

19 I

21 I

I

25 I

27 I

Fig. 7. Optical band gap (&& as a function of W03 concentration (mol%); the sobd line is a least square fit.

29 I

Fig. 5. Band tail energy as a function of W03 concentration (mol%); the solid line is a least square fit. Table 3. Refractive index (N), optical band gap (J&t) and band tails (4) {(lOO-x)TeD2+xWOs}with15
15 20 25 27.5 30

Refractive index Thickness Oun) 475 350 320 330 425

&P) 2.25 2.35 2.41 2.13 2.44

of the glasses

Optical gap & t(ev) (%P)

Band tails E&v) (Exp)

3.0 3.0 2.97 2.95 2.93

0.103 0.064 0.101 0.087 0.112

1230

I. SHALTOUT et al.

function of W03 was found to change from 2.27 to 2.36 as W03 concentration increases from 15 to 30 mol% .

3.0 ::i 2.7 z :::,,

0

9

Cl

2.2 212.0 191.8 1.7 1.6 I.5 -

0

REFERENCES

I:: I::"15

17 ,

19 I

21 I

23 I

25 I

27 I

29 I_

Mel(%)

Fig. 8. Refractive index as a function of WOs concentration (mol%); the solid line is a least square fit.

glasses; however the authors mentioned behavior is unknown.

that such

4. CONCLUSIONS

These glasses are disordered versions of tetragonal TeOz of Di symmetry where the Te atom is four-fold coordinated. The W ion coordination states change from 4 to 6 when WO, increases beyond 3 1.5 mol%. Transmission spectra of bulk glass samples show defect states optical transitions at energy 1.5 eV. The density of these state which are suggested to be related to dangling or non-bridging atoms, increases as WO3 increases. Such defect states have not been detected previously for measurements on thin film samples of Tellurite glasses. Tungstate-tellurite glasses of the present work are highly transparent in the range of 4000-24,00Ocm-‘ . Therefore, these glasses can be used as optical windows in this range. The band tail energies are found to be between 0.103 and 0.112 eV, however these values do not show a monotonic behavior as WO, concentration increases. Urbach edge behavior is observed in these glasses and the slope of the edge was found to increase as W03 increases. Optical band gap (I&J are found to decrease from 3 to 2.93 eV as W03 increases from 15 to 30mol% while the refractive index (N) as a

1. Lambson E. F., Saunders G. A., Bridge B. and ElMallawany R. A., J. Non-Crys. Solids 69,117 (1984). 2. Nishida T., Yamada M., I&ii T. and Yakashima Y., Jpn. J. Appl. Phys. 30,768 (1991). 3. Hart S., j: Mat: Sci 18, 1264 (1983). 4. Ghosh A.. J. ADDI. Phvs. 64.2652 (1988). 5. Sakurai Y. and kamaki J., i. Elecwo. them. Sot. 132, 512 (1985). 6. Tanaka K., Yoko T., Nakano M., Nakamura M. and Kamiya K., J. Non-Cryst. Solids 125,264 (1990). I. Shaisha E. E., Baghat A. A. and Sabty A. I., J. Mat. Sci. Lerr. 5,687 (1986. 8. Bahaat A. A.. Shalthout I. I. and Abu-Elazm A. M.. J. Nokryst. S&a% 150,179 ( 1992). 9. Komatsu T., Tawarayama H., Mohri H. and Matusita K., J. Non-Cryst. Solids 135, 105 (1991). 10. Hirao K., Koshimoto S., Tanaka K., Tanabe S. and Soga N., J. Non-Cryst. Solids 139, 151 (1992). 11. Nelson B. N. and Examos G. J., J. Chem. Phys. 71,2739 (1979). 12. Shaltout I., Tang Y., Braunstein R. and Abu-Elazm A. M., J. Phys. Chem. Solids 56,141 (1995). 13. Peercy P. S. and Fritz, I. J., Phys. Rev. Left. 32, 466 (1974).

14. Herzberg G., Molecular spectra and molecular structure VII, Foreign Literature Press, Moscow (1949) 2nd ed, Van Nostrand, Princeton, New Jersey (1950). 15. Neov S., Gerassimova I. and Sydzhimov B., Phys. Sraf. Sofidi A 76,297 (1983). 16. Grivelder L. and Phillips W. A., J. Non-Cyst. Solids 109, 280 (1989). 17. Kozhukharov V., Neov S., Gerasimova 1. and Mikula R., J. Mat. Sci. 21, 1707 (1986). 18. Dimitriev Y., Dimitrov V. and Ardunov M., J. Mat. Sci. 18, 1353 (1983). 19. Dimitrov V., Ardunov M. and Demitriev Y., Manatshefe

Chem. 115,987 (1984).

20. Mott N. F. and Davis E. A., Electronic Process in NonCrystalline Materials 2nd edn. Clarendon Press, Oxford (1979). 21. Cohen M. H., Fritzsche H. and Ovshinsky S. R., Phys. Rev. L&t. 22, 1065 (1969). 22. Hogarth C. A. and Asadzadeh-Kashani E., J. Mar. Sci. 18, 1255 (1983). 23. Stanworth J. E., J. Sot. Glass. Tech. 36 217 (1952). 24. Yakhkind A. K., J. Amer. Cer. Sot. 49,670 (1966). 25. Al-Ani S. K. J. and Hogart C. A., J. Marh. Sci. Left 6, 519 (1987).