5.ftir spectra and some optical properties of.pdf

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 Pergamon . I CUl22-3697/96 si5.00 + 0.00

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 150- 25,00Ocm-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 are good candidates for

many technological applications. These glasses have a

low melting point (800C), are not hygroscopic, have

low glass transformation temperature (5 4OOQ

high dielectric constant, high thermal expansion

coefficient, and high optical transmission in the infra-

red region to 5 pm [l, 21. Moreover, these glasses are

typically of high density, and high refractive index

N 2 2 [3].

As known, transitional 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 phos- phate 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

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

1224 I. SHALTOUT et al.

glass compositions and their characteristics are matrix are four-fold coordinated due to the summarized in Table 1. formation of W-0-Te bonds.

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

FTIR spectra and optical properties of glasses 1225

5 i WOJ < 30mol% is due to the symmetric vibrations of W04 tetrahedra [ 191.

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 WO f 3 I

150 250 350 450

WAVENUMBER f CM- 1

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

CRYST. W03

1 150

-_ 250 350 4:

WAtJE NUMBER 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 450C for 18 h.


Table 2. FIR characteristic frequencies of crystalline TeQ, W09 and the glasses { (100 - x)TeO, + xW0,) with 5 5 x < 50mol%

Sample Crystalline TeOz

(100 - x)TeOz + xWO3

Frequency (cm-) 189 220 262 328 405

Heat treated at Untreated glasses 450C for 18 h

x=5 347 188 219 266 343 x= 15 354 189 219 266 343 x = 20 343 193 227 265 341 x = 25 347 188 224 262 347 x = 27.5 347 188 219 266 347 x = 30 340 189 223 270 347 x = 31.5 347 189 223 352 x = 33 347 360 x= 35 347 232 360 x = 40 347 230 280 360 x = 50 347 230 282 360

Crystalline W03 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 4d5s25p4 and 5d46s2, respec- tively, the lower valence state of the Te atom means lower average coordination state possibilities. There- fore, 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


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 funda- mental 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 W03

s 8 I I I I I I

0.5 I.5 2.5 3.5

WAVENUMSER wul- )

(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.


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 - 2R)e-2ax] 10 1 _ R+*aX

(3)

I.X= 15 5-

4-

3-

2-

I-

I I I 0 5 7 9 II 13 15 17 19 21 23

2.x=20

5- 1

O 5 7 9 I1 I3 I5 I7 I9 21 23

3.X=25

5 I

4-

3-

2-

o 579 II I3 I5 I7 I9 21 23 Wavenumber (cm-)

(Thousands)

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)becomesRz 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. How- ever, 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}

4. x = 21.5

5

4

3

2 I 0 579 II 13 I5 I7 19 21 23

0 579 II I3 I5 17 I9 21 23

Wavenumber (cm-) (Thousands)

Fig. 3. Optical absorption spectra of the glasses { (100 - x)TeO, + xW03}. & are the band tail energies.


N(E) I

N(E)

(bl

N(E)

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 Cohen-

Fritxsche-Ovshinski (CFO), Ref. [20].

E - 0.18 - 0.17 - 0.16 - 0.1s -

::1: -

::I: - 0 0.10 - 0

t:: y 0

0.07 - 0.06 - 0

0.05 - 0.04 -

i:: - 0.01 - o~oo15 17 , 19 I 21 I I 25 I 27 I 29 I

3 5 7 911 13 I5 17 19 21 23

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.

2.93 -

:3: - 2.9015 17 , 19 I 21 I I 25 I 27 I 29 I f

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

Wavenumber (cm-) (Thousands)

3.09 3.08

T 3.07 u 3.06

2 :z E

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) of the glasses {(lOO-x)TeD2+xWOs}with15


3.0

::i - 2.7 -

z - :::,, Cl 0 9 2.2 - 21-

0

2.0 - 19- 1.8 - 1.7 - 1.6 - I.5 -

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 that such behavior is unknown.

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

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

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