Eur. Phys. J. AP 19, 165172 (2002) DOI: 10.1051/epjap:2002062

THE EUROPEAN PHYSICAL JOURNALAPPLIED PHYSICS

UV-IR spectra of new tellurite glassesR. El-Mallawanya, A. Abdel-Kader, M. El-Hawary, and N. El-KhoshkhanyPhysics Dept., Faculty of Science, Menoa Univ., Egypt Received: 7 March 2002 / Received in nal form: 26 May 2002 / Accepted: 7 June 2002 Published online: 12 September 2002 c EDP Sciences Abstract. Binary tellurite glass systems of the forms TeO2 (100 x)-xAn Om where An Om = La2 O3 or V2 O5 and x = 5 to 20 for La2 O3 and 10 to 50 for V2 O5 were prepared. UV-spectra of these glasses were recorded in the range of 200700 nm at room temperature. The optical energy gaps Eoptical and Etail have been calculated from the optical absorption edge. UV cut-o and IR cut-o have been measured. Calculations of the oxide ion average polarizability (2 ) and optical basicity () have been done. 0 PACS. 61.43.Fs Glasses 74.25.Gz Optical properties

1 IntroductionTellurite glasses are of scientic and technological interest because of their high refractive indices, low melting temperatures, high dielectric constants as well as their good UV and IR transmissions and considered as promising materials for non-linear optical devices as explained. Also, tellurite glasses regarded as an optical materials for up-conversion lasers and nonlinear optical materials. For potential use of tellurite glasses materials in optics and communication systems, an enough knowledge of absorption edge is required. Polarizability is related to many macro and microscopic physical and chemical properties such as UV absorption, ionic refraction and dielectric properties. Some tellurite glasses are also reported to be suitable for setting up optical ber ampliers, optical disk for data storage, bonding material in magnetic heads as collected in the recent book Tellurite Glass handbook: Physical properties and data [1]. Also elastic and anelastic properties of tellurite glasses were collected [2,3].

4B) in the wavelength range 200600 nm at room temperature.

2.2 Infrared absorption spectra measurements (IR) The IR absorption spectra of the prepared glasses in KBr matrix were recorded on a Perkin-Elmer double beam spectrophotometer model 598 at room temperature over the spectral range of 4000200 cm1 . The produced glasses were thoroughly mixed with KBr in the ratio 1:40, after that the mixtures were pressed at 10 tons for 5 min. The pellets were clear and uniform. While recording the spectra, the gain of the spectrometer was kept the same for all the samples.

3 Results and discussion3.1 UV of tellurite glasses The ultraviolet light absorbance of the prepared tellurite glasses containing La2 O3 and V2 O5 as a function wavelength in the range 200600 nm and at room temperature are shown in Figure 1(a, b) respectively. The glass samples were (x)La2 O3 -(100 x)TeO2 where [x = 5, 7.5, 10, 12.5, 15, 17.5 and 20 mol.%] and (x)V2 O5 -(100 x)TeO2 where [x = 10, 20, 25, 30, 35, 40, 45 and 50 mol.%]. The absorption edge of each glass can be identied. There are no sharp absorption edges and this is characteristic of most glassy oxide materials. It is shown in Figure 1(a, b) that the absorption edge depends on the kind of the modier. The position of the fundamental absorption edge shifts to lower wavelength with increasing of La2 O3 content in binary La2 O3 -TeO2 glass or increasing of V2 O5 content in binary V2 O5 -TeO2 glass. The shifts of the absorption edge

2 Experimental procedureBinary tellurite glass systems of the forms TeO2 (100 x)xAn Om where An Om = La2 O3 or V2 O5 and x = 5 to 20 for La2 O3 and 10 to 50 for V2 O5 were prepared as described in Part 1 of this work [4]. 2.1 Ultraviolet absorption spectra measurements (UV) The UV absorption measurements for the bulk glass samples with the two parallel-polished faces were measured using a UV-spectrophotometer (Perkin-Elmer Lambadaa

e-mail: relmallawany@hotmail.com

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(a)

(b) Fig. 1. (a) and (b) Optical absorption spectra as a function of wavelength.

are most likely related to the structural rearrangement of the glass and the relative concentrations of the various fundamental units. The absorption edge of each glass depends on the type of network modier. The absorption coecient, (), can be determined near the absorption edge using the formula: = 1 d ln I0 I (1)

Urbach rule [5] has been applied. = 0 exp( /Et ) (2) where ( ) is the photon energy, 0 is a constant, is Plancks constant, is the angular frequency and Et is the Urbach energy which is interpreted as the width of the tails of the localized states in the band gap associated with the amorphous nature of the material. When the density of states is proportional to some power of energy ( )1/2 and the relation between absorption coecient and photon energy gap is considered [6,7], this relation can be re-written as follows: ( )1/2 = A( Eopt ) (3)

where d is the thickness of the sample, I, I0 are the intensities of the incident and transmitted beams respectively. For many oxide glasses and other amorphous materials

R. El-Mallawany et al.: UV-IR spectra of new tellurite glasses Table 1. Optical properties of binary TeO2 -La2 O3 and TeO2 V2 -O5 glasses for dierent percentage of modier in mol.%. Glass composition TeO2 -La2 O3 95-5 92.5-7.5 90-10 87.5-12.5 85-15 82.5-17.5 80-20 TeO2 -V2 O5 90-10 80-20 75-25 70-30 65-35 60-40 55-45 50-50 Cut-o wavelength (nm) 335.5 333 331 329 327 325 321 413 409 400 392.5 388 385 381 377 Eopt (eV) Constant A 1 (cm eV1 ) 5.38 4.9 4.825 4.815 4.71 4.785 4.797 3.039 3.07 2.87 2.84 3.03 3.22 3.21 3.38 Eta (eV)

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3.51 3.52 3.54 3.55 3.578 3.59 3.61 2.67 2.7 2.76 2.79 2.87 2.93 2.97 2.03

0.1006 0.0987 0.0932 0.927 0.0916 0.0871 0.0826 0.2085 0.2089 0.245 0.2489 0.2214 0.2192 0.1801 0.1619

1/2

(a)

where A is a constant and Eopt is the optical band gap determined by extra polating the linear parts of the curves to ( )1/2 = 0. The absorption coecient () can be determined near the absorption edge from Figure 1(a, b) and calculated according to the formula (1). The most satisfactory results were obtained by plotting the quantity ( )1/2 as a function of ( ) as suggested by Davis and Mott [7] as shown by equation (3). Absorption by indirect transitions applies to many oxide glasses, particularly at the higher values of absorption coecient and is the reduced Plancks constant and is the angular frequency of the incident radiation. Figure 2(a, b) show the plots of ( )1/2 against ( ) and the values of Eopt determined by extrapolating the linear parts of the curves to ( )1/2 = 0. The value of Eopt for each glass system are given in Table 1. It is clear from Table 1 that the value of Eopt increases from (3.51 to 3.61 eV) due to the modication of tellurite glasses by (5 to 20) mol.% La2 O3 . The values of the constant A in equation (3) can be determined from the slope of the linear part of the relation ( )1/2 and which is change from (5.38 to 4.8 cm1 eV1 ) due to the modication of tellurite glasses by (5 to 20) mol.% La2 O3 , while Eopt increase from (2.67 to 3.03 eV) due to the modication of tellurite glasses by (10 to 50) mol.% V2 O5 . The values of the constant A in equation (3) can be determined from the slope of the linear part of the relation ( )1/2 and which is change from (3.05 to 3.38 cm1 eV1 ) due to the modication of tellurite glasses by 10 to 50 mol.% V2 O5 . Those values are of the same order as reported by Davis and Mott [6]. The interpretation of the present

(b) Fig. 2. (a) Variation of (hw) with (hw) for lanthanum tellurite glasses. (b) Variation of (hw) with (hw) for vanadate tellurite glasses.

optical energy gap will be based on the assumption that tellurite glasses having a high concentration of TeO2 , the triangular bipyramid TeO4 is considered as the basic coordination polyhedron in which tellurium atoms are surrounded by four oxygen and each oxygen is connected with two tellurium atoms, accomplishing an axial-equatorial bonding at which the bonds are easily deformed because of the change in the Teax -Oeq-Te angle taken place along the c-axis due to the structural incorporation of the modier which creates defects, oxygen vacancies and micro holes, and also increases the concentration of non-bridging

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(a)

(b) Fig. 3. (a) Variation of ln() with hw for lanthanum tellurite glasses. (b) Variation of ln() with (hw) for vanadate tellurite glasses.

oxygen [8]. The doping with transition metal oxides decrease the Eopt of (TeO2 ) which was 3.79 eV to the values mentioned before and can be attributed to the increase of the non-bridging oxygen atoms. So, doping with La2 O3 gave a lower concentration of non-bridging oxygen atoms than V2 O5 . The estimated value of Eopt for La2 O3 tellurite glasses are close to the reported values in [9]. For many amorphous materials, Urbach [5] assumed that the absorption coecient () was an exponential function of

photon energy as shown by equation (2). Figure 3(a, b) shows the variation of ln() against for the present binary glassy systems. The values of Eta in equation (2) are calculated from the slopes of the linear regions of those curves. The values of Eta for each glasses system are given in Table 1. It is clear that the values of Eta decrease from (0.1006 to 0.0826 eV) due to the modication of tellurite glasses by (5 to 20 mol.% La2 O3 ) and from (0.2085 to 0.1619 eV) due to the modication of tellurite glasses by

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(10 to 50 mol.% V2 O5 ). These values are in the range of amorphous semiconductors between 0.046 and 0.66 (eV) were reported by Davis and Mott [6]. It is clear from these values that the widths of the band tails of the localized states are found to depend upon the kind of the modier. Now in the present study, we calculate the electronic polarizability and optical basicity values by using average electronegativity (2av ) for binary oxide glass V2 O5 -TeO2 by using equations (46) and the results are listed in Table 2. 2av = Xxf + Yxs X +Y 2 = 3.319 0.34222av 0 = 0.04375 + 0.30972av . (4) (5) (6)

cies in the following manner are:s ax eq (TeO2 ) = 780 cm1 , ax (TeO2 ) = 675 cm1 , ax eq (TeO2 ) = 714 cm1 , s ax (TeO2 ) = 635 cm1 .

Both calculated values of electronic polarizability and optical basicity values are collected in Table 2. Values electronic polarizability and optical basicity of the present V2 O5 -TeO2 are close to values of the electronic polarizability and optical basicity of V2 O5 -GeO2 [10]. While values electronic polarizability and optical basicity of the present La2 O3 -TeO2 are higher than values of the electronic polarizability of La2 O3 -B2 O3 [10]. The optical basicity of V2 O5 -TeO2 show large oxide ion polarizability between 23 3 . A

3.2 IR of tellurite glass The room temperature IR spectra in the region 200 4 000 cm1 of tellurite glasses containing (5 to 20 mol.% La2 O3 ) are shown in Figure 4 and for binary vanadium tellurite glasses are shown in Figure 5. The major absorption bands of glasses at dierent composition are summarized in Table 3. For the binary lanthanide tellurite glasses, the region below 300 cm1 for all samples did not show any sensitivity, possibly due to excessive noise and hence this region has been excluded from the discussion. No water bond or OH stretching modes are observed in the IR spectra of the crystalline TeO2 . But the binary samples exhibited a water band at 3 400 cm1 and an OH stretching at 2 920 cm1 . The main absorption bands are at frequencies around 610670 cm1 , a shoulder at 580630 cm1 , a small bands at 560610 cm1 and a shoulder at 750 780 cm1 posses deformed TeO4 groups as mentioned by one of the authors [11] and also by Dimitriev [12]. A new shoulder at 580630 cm1 was attributed to La-O stretching vibration as measured by Bahgat [13]. A small band is observed in the spectra at 560610 cm1 , which may be attributed to Te-O bond vibrations, where the oxygen anions are considered non-bridging (NBO) as measured by Shaisha [14]. By analogy with the crystalline tellurites [15,16] it may be accepted that with the introduction of other oxides in tellurite glasses, part of the TeO4 groups are transformed into TeO3 . Tellurites containing TeO4 groups have four s ax ax s band eq , eq , ax and ax modes. The stretching frequen-

Also, for the binary vanadium tellurite glasses the major absorption bands of glasses at dierent composition are summarized in Table 3. The region below 300 cm1 for all samples did not show any sensitivity, possibly due to excessive noise and hence this region has been excluded from the discussion. The data exhibited a water band at 3 400 cm1 . The infrared spectra consist of a major band in the 650670 cm1 , a shoulder at 770790 cm1 , a band at 9801 000 cm1 , a band at 600580 cm1 and a band at 480500 cm1 . The absorption band at 980 cm1 , attributed to V=O stretching, shifted to 1000 cm1 with increase V2 O5 content. This data are closed with those in reference [16]. The band around 480500 cm1 is due to Te-O-V stretching. Absorption at 650670 cm1 and shoulder at 770790 cm1 both attributed to TeO3 trigonal pyramid was also observed. On the other hand, absorption at 600 cm1 is due to the stretching vibration of Te-Oax bond in the deformed TeO4 units. From these results, we conclude that: infrared spectra of vanadate tellurite glasses indicate the transformation of the basic structural unit of TeO4 to TeO3 with the addition of modier. Table 4 summarizes the IR absorption band position for pure oxides TeO2 , V2 O5 and La2 O3 . By using the bond length of Te-O, La-O and V-O of each molecule as in reference [17], we can calculate the theoretical wave number that present in these glasses. Table 5 summarizes the calculated reduced mass of the cation-anion stretching force constant by using the next equation = (mR mO ) (mR + mO ) (7)

where is the reduced mass of the molecule R-O and mR , mO are the atomic weight in kg of cation R and anion O respectively. A (8) f = 17/r3 (md1 ) where f is the bending or stretching force constant and r bond length of the cation-anion. = 1 = 1 2c f (9)

where is a wave number in cm1 , c is the speed of light, f is the force constant of the bond, and is the reduced mass of the molecule R-O. Substituting by the reduced mass of dierent cations in equation (9), values of the theoretical wave number can be obtained as collected in Table 5. According to equation (9), the calculated wave number from Table 5 is in harmony with the values of the experimental values as in Table 4. Also, from equation (9) it si clear that the wave number is inversely proportional to the value of reduced mass. So quantitatively it is clear that, because La has a higher mass than vanadium, the binary lanthanum tellurite glass have a lower IR wave number than vanadium tellurite glasses.

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Table 2. Average electronegativity (2av ), optical basicity (2av ), oxide ion polarizability (2 )(2av ) values of binary oxide 0 glass V2 O5 -TeO2 and La2 O3 -TeO2 . Glass composition V2 O5 -TeO2 10-90 20-80 25-75 30-70 35-65 40-60 45-55 50-50 La2 O3 -TeO2 5-95 7.5-92.5 10-90 12.5-87.5 15-85 17.5-82.5 20-80 Average electronegativity (2av ) 2.9941 2.9852 2.9808 2.9763 2.9719 2.9674 2.963 2.9585 2.98 2.968 2.956 2.944 2.933 2.921 2.909 Electronic polarizability (2 ) (1024 cm3 ) 0 Optical basicity (2av )

2.2944 2.2975 2.299 2.3005 2.302 2.3036 2.3051 2.3066 2.299 2.303 2.307 2.312 2.315 2.319 2.324

0.971 0.9683 0.9669 0.9655 0.9641 0.9628 0.9614 0.96 0.967 0.963 0.959 0.956 0.952 0.848 0.945

Fig. 4. Experimental IR spectra of the binary lanthanum tellurite glasses.

4 ConclusionsThe value of Eopt of binary tellurite glasses increased from (3.51 to 3.61 eV) with increasing La2 O3 content from (5 to 20 mol.%) and increased from (2.67 to 3.03 eV) with increasing V2 O5 content from (10 to 50 mol.%). The doping with V2 O5 decreased the Eopt of pure TeO2 (Eopt of pure TeO2 = 3.79 eV) and can attributed to the increase of the NBO atoms, while doping with La2 O3 gave a lower concentration of NBO atoms than V2 O5 . The value of Eta decrease from (0.1006 to 0.0826 eV) with increasing

La2 O3 content from (5 to 20 mol.%) and from (0.2085 to 0.1619 eV) with increasing V2 O5 content from (10 to 50 mol.%). From the calculations of the oxide ion average polarizability (2 ) and optical basicity () for binary 0 (1 x)TeO2 -(x)An Om , where An Om a transition metal oxide or rare earth oxide V2 O5 or La2 O3 and x is the mol.% by using the average electronegativity (2av ) it has been concluded that: (2 ) are increased from (2.294 1024 cm3 to 0 2.307 1024 cm3 ) with increasing V2 O5 content from (10 to 50 mol.%) and from (2.299 1024 cm3

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Fig. 5. Experimental IR spectra of the binary vanadium tellurite glasses. Table 3. The experimental IR absorption band position for binary lanthanum tellurite and vanadium tellurite glasses. Glass composition TeO2 -La2 O3 95-5 92.5-7.5 90-10 87.5-12.5 85-15 82.5-17.5 80-20 TeO2 -V2 O5 90-10 80-20 75-25 70-30 65-35 60-40 55-45 50-50 q1 (cm1 ) q2 (cm1 ) q3 (cm1 ) q4 (cm1 )

560 560 580 580 590 610 480 485 480 480 485 495 500 500

580 590 600 610 620 630 600 600 600 595 590 585 580 580

610 630 640 650 650 655 670 650 660 655 660 660 670 665 670 770 780 785 760 780 790 790 790

740 750 760 770 770 780 785 980 980 980 980 985 990 995 1000

Table 4. The experimental IR absorption band position for pure oxides. Glass TeO2 Oxide TeO2 La2 O3 V2 O5 q1 (cm1 ) 340 340 590 500 q2 (cm1 ) q3 (cm1 ) 640 635 640 610 q4 (cm1 ) 740 780 820 1010 q5 (cm1 )

550

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The European Physical Journal Applied Physics Table 5. The theoretical IR band position of the tellurium, lanthanum and vanadium oxides. Atomic weight Rest mass of cation (1027 kg U1 ) 211.94 230.73 84.615 Reduced mass of catio-O (1026 kg U1 ) 2.361 2.383 2.022 Bond length ( ) A Ref. [17] 1.99 2.517 1.83 Stretching force constant (N m1 ) 215.7 106.6 277.4 Theoretical wave number (cm1 ) 507.08 354.83 621.39

Cation

Te La V

127.61 138.91 50.942

to 2.324 1024 cm3 ) with increasing La2 O3 content from (5 to 20 mol.%); while () is decreased from (0.971 to 0.96) with increasing V2 O5 content from (10 to 50 mol.%) and from (0.977 to 0.945) with increasing La2 O3 content from (10 to 50 mol.%). The IR spectra the ordinary band for pure TeO2 at 640 cm1 shifts from (610 to 670 cm1 ) with increasing La2 O3 content from (5 to 20 mol.%) and from (650 to 670 cm1 ) with increasing V2 O5 content from (10 to 50 mol.%). The previous qualitative description will be analyzed quantitatively according to each stretching force constant of bond F , and reduced mass of the cation-anion. A strong correlation between the obtained experimental values of wave number and calculated wave number has been found.

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