Infrurrd P/I,L~. Vol. 29, No.
THEORETICAL AND EXPERIMENTAL BINARY RARE EARTH TELLURITE R. A. Physics
Copyright ‘c 1989 Pergamon Press plc
Printed in Great Britain. All rights reserved
IR SPECTRA GLASSES-l
EL-MALLAWANY of Science,
binary rare earth tellurite glasses (A,O,),+TeO,), ~,where x = 0.1 and A = Sm, Ce, La, were prepared by melting the oxides at 800 C for I h and quenched rapidly. The IR spectra has been measured in the frequency region 4OOB200 cm- ‘. The main absorption bands in these glasses related to the characteristics of TeO,. The detected shift in these bands are found to be sensitive to the glass structure. The A-O bond vibration in the glasses has been calculated. The results were interpreted on the basis of stretching force constant of each bond.
Tellurite glasses have been studied for over 150 yr, (‘Ibut it is only recently that TeO, glass of purities exceeding 98.5 mol% have been made earlier. (2)Tellurite glasses are of technical interest on account of their low melting points and absence of the hygroscopic properties which limit the application of phosphate and borate glasses. Also, they have high densities and low transformation temperature.(3.4) They have large refractive indices, and hence high dielectric constant, and are good IR transmitters for wavelengths up to 5 pm. The first measurements of the static dielectric constant have been measured previous1yc5’ for pure tellurite glass and both binary systems (Te02),,67-(W03)0,33 and (TeO,),,,-(ZnCl,),,, in the range of l-500 kHz. The static dielectric constant of vitreous TeO, at room temperature is 20.1, which is of the same order as the mean value for crystalline TeOz (paratellurite): the molar polarizabilities of the glass is 2.2 x 10e6 m3 and crystalline is 2.8 x 10m6m3 forms are very similar. But the pressure dependence of the static dielectric constant of pure tellurite and binary (TeO,),,,,-(WO,),,,, glass at elevated pressure &70 kbar and temperature 295-380 K were positive pressure [email protected]
’ This is in contrast with the behavior of crystalline insulators. Using complex admittance techniques, the first measurements of electrical conductivity of TeO, and (TeO,),,,-(WO,),,, glasses have been measured”’ throughout the temperature range 90430 K. The conductivities have been analyzed using small polaron theory to establish the thermal, disorder and activation energies for the carriers in both glasses. Also, a comparison between the elastic moduli of pure TeO, and the above two systems has been carried out previously.“’ A very little work has been done on the rare earth oxide glasses. Fournier et aLf9’ studied the broadening of optical spectra of Yb3+ in phosphate glass. Bahagat et ~1.“‘) interpreted some physical properties of rare earth ternary tellurite glasses like IR, Mossbauer parameters and electrical conductivity which were carried out by electrons in these new tellurite glasses. Bahagat et ~1.“‘) concluded that the fraction of the rare earth oxide (lanthanum, neodymium, samarium, europium and gadolinium) was incorporated in the network of these glasses and acts as a network intermediate. Marinov et al.(“) investigated the changes in optical absorption of amorphous thin films based on tellurium dioxide and rare earth oxides as a function of thermal treatment. They concluded that the absorption change in the thin layers was due to the thermally induced phase change from amorphous to a partial crystalline state. Burger et al.(I’) have proved that heavy ions influence the absorption ability of the glass systems TeO,-R,X, where X = F, Cl, Br and shift the IR cut-off towards longer wavelengths. Also, a discussion on the influence on atomic mass, interatomic distances and interatomic forces of a diatomic molecule of the main building units in the vitreous matrix has been made by Ref. (12) and Higazy et al.‘13)Dimitriev et ~1.“~)have shown that with IR spectra investigations that the short range order of certain tellurite glasses, obtained from monomineral crystalline phases, is similar 781
IR spectra of rare earth oxides: lanthanum oxide, cerium and tellurium oxide in the range 200-4000cm ‘.
to that of the crystals. The optical absorption edges the glass composition of the binary tungsten tellurite the author.u5’ Also, but out of the scope of this paper second-order elastic constant and stiffness of these theoretical interpretation based on the interatomic
Fig. 2. Experimental
and IR absorption spectra are functions of glasses which has been mentioned earlier by the author has measured experimentally the binary rare earth tellurite glasses”@ with a distances and interatomic force constant.“”
of the binary
glasses in the range
Theoretical and experimental IR spectra-l
Fig. 3. Experimental
bands of the binary rare earth tellurite glasses in the range 200-IOOOcm-I.
The glasses were prepared by melting the oxides at about 800°C for 1 h and quenched rapidly at room temperature then annealed at 300°C for 1 h. The details for the preparation can be found elsewhere.“@ The produced lanthanum tellurite glass was transparent pale lime green, cerium tellurite glass was opaque dark reddish and samarium tellurite was opaque yellow. The X-ray diffractometer studies of powdered samples confirmed the glassy state. The IR absorption spectra of the prepared glasses TeO,, (La,O,),,,-(TeO,),,,, (CeO,),,,-(TeO),,~ and (Sm,O,),.,-(TeO,),, and the starting oxides TeO,, La, 0,) CeO, and Sm, 0, in KBr matrix were recorded on a Perklin-Elmer spectrophotometer model 598 at room temperature. The produced glasses were thoroughly mixed with KBr in the ratio 1:40. The pellets were clear and uniform. While recording the spectra, the gain of the spectrometer was kept the same for all the samples.
The room temperature IR spectra in the region 200-4000 cm-’ of all the rare earth tellurite glasses are in Fig. 1, and for more details Fig. 2 represents the IR spectra in the range 200-lOOOcm_‘. For comparison, the spectra of the oxides TeO,, La,O,, CeO, and Sm,O, are shown in Fig. 3. The major absorption of all glasses are summarized in Table 1. The region below 300cm-’ 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 bands or -OH stretching modes are observed in the IR spectra of the crystalline TeO,. But the binary samples exhibited a water band at 3400 cm-’ and an -OH stretching at 2920 cm-‘. The main absorption bands are at frequencies around 635-640 cm-‘, a shoulder at 665-670 cm-’ and frequencies at 740 cm-’ possess deformed
(TeO,lou-CL+, )I,, (TeO,h ,dCeO:I,,, [email protected]
,dSm,O,I,,, Oxide T.20, La,O, ceo:
TeO, groups as mentioned by Ref. (14). A new shoulder at 575 cm ’ was attributed to Lasso. Sm -0 or Ce-0 stretching vibration as measured by Ref. (IO), By analogy with the crystalline tellurites “‘I it may be accepted that with the introduction of other oxides in tellurite glass, part of the TeO, groups are transformed into TeO,. Tellurites containing TeO, groups have four bands rzq, v:;, \I:: and ~1;~modes. The stretching frequencies in the following manner ( 19): r&(TeO,)
= 780 cm
\l:,‘(TeOz) = 714cm
\I,:: (TeO, ) = 675 cm
\‘;‘X(TeO,) = 635 cm
Dimitrive et u/.“~’ concluded that glasses containing symmetric TeO, groups equivalent to maximum at 670cm -’ and shoulder at 635 cm ‘, while glass containing deformed TeO, groups corresponded to a shoulder at 670 cm ’ and a maximum at 635cm ‘. but glasses containing symmetric TeO, groups corresponded to a maximum at 670-700cm ’ (shoulder at 635cm ’ decreases TeO, symmetric groups). While Mochida et u/.“” found with the aid of IR spectra and according to Yakhkind”” that the number of TeO l+, group is stimulated with the increase in the content of modified oxides. The quantitative justification of the above bands are as follows. that the wavenumber of vibrational modes in the IR spectra is determined by the mass of the atoms; the interatomic force within the groups of atoms comprising the glass network; and the cation sterchemical arrangement of the units in the matrix. The effect of heavier atoms on absorption bands can be seen from equation (I), which is:
and valid for the fundamental stretching vibration mode of a diatomic molecule: where i; is the wavenumber in cm-‘. c is the speed of light.f‘is the force constant of the bond. and 11is the reduced mass of the molecule R-O and is given by:
where mR and m, are the atomic weight in kg of the cation R and anion 0 respectively. Bugger et ul.“*’ considered the interatomic force separately from the cation sterchemical arrangement where these two factors are interdependent. But Higazy et u/.“~’ considered ,f as the bending or stretching force constant, and can be calculated from the empirical formula; ,f‘= 1.67N[X,X,ir’]‘“+0.3(mdii~‘). where N is the bond order, I’, and X, are electronegativities al.‘**’ estimated values off from the empirical relation:
(3) and r is the bond
f = 5.28 N[X,X,,jr’13 4 + 30 (Nm ‘). The band at 780 cm-’ may be used as an additional test to show whether the samples not, it is due to v:,(TeO?) vibrations of the TeO, groups or to \s’TeO, vibrations
are indeed in the vitreous of the TeO, groups.
Theoretical Table 2. The theoretical
IR band position
f by using the rather
cerium and samarium
Stretching force constant (Nm-‘)
0.19eq 0.208 ax 0.238 0.245 0.272 0.23 0.266 0.238 0.271
Bond length (nm)
of cation-o (10-*6kgU-‘)
But we will obtain
of the tellrium,
Rest mass of cation (IO-*‘kg U ‘)
248 189 126 II5 84 136 90 124 84
Theoretical wave number (cm-‘)
Experimental wave number (cm-‘)
544 474 386 369 315 434 349 407 335
720 660 415
17/r3 (md A-‘)
which suggested by Bridge et al. (23),by using the bond length of the cation-anion in the crystalline state. The bond length of Te-0, (24)LaaO, Ce-0 and Sm-O’ZS’ are tabulated in Table 2. Also, Table 2 summarizes the calculated reduced mass of the cation-anion stretching force constant and the theoretical wavenumber. From inspection of Table 2, we find that the theoretical values of v agree reasonably well with the experimental values for all the assumed stretching vibrations using equation (5). It is found that the experimental band wave number is higher than the theoretical one for TeeO,,, Te-O,, and all La-O bonds, suggesting that the attribution of the bond to this group may be a band combining a stretching motion with the harmonics of a bending motion. But in the case of Ce-0 and Sm-0 it is found that the experimental band wave number is less than the theoretical value obtained from equation (l), assuming stretching force constants, suggesting that the vibrations might involve mixed bending and stretching character. Ce-0 and Sm-0 have 98 and 91% stretching character, respectively, which are very close to those of CeO, and Sm,O, oxides.‘*@ Our broad objective now is to try to develop glasses in which the rare earth ions might be in the intermediate valence state. Such glasses should exhibit a range of unique properties which might have uses as high pressure sensors or as new laser hosts. A~knowlrdKPm~nr-The author thanks the group University, Egypt, for the laboratory facilities.
Studies & Research,
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. IO. Il. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
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