X-ray photoelectron spectroscopic studies of zinc–tellurite glasses

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Abstract

Zinc–tellurite glasses with chemical composition [(ZnO)x(TeO2)100−x], where x=25, 30 and 35 have been prepared and investigated by X-ray photoelectron spectroscopy (XPS). Zn 2p peaks shift by about 0.25 eV towards higher binding energy in the zinc–tellurite glasses in comparison to its value in ZnO powder, while the Te 3p, Te 3d, and O 1s core levels for the glasses remain essentially unchanged from those of TeO2 powder. Each O 1s spectrum is deconvoluted into two peaks, in which the higher energy one is due to oxygen atoms in the hydroxides covering the sample surface. Although both bridging oxygen (BO) and non-bridging oxygen (NBO) atoms should certainly exist in these tellurite glasses, the electrons are delocalised in the NBO–Te–BO bonds which equalize the electronic density of the valence shell between BO and NBO atoms and make it difficult to distinguish.

Introduction

Tellurite glasses are a relatively new non-crystalline material and are at present the subject of intensive investigations because of their technological and scientific importance. Tellurite glasses are candidates for new optical materials because of their superior properties, such as high refractive index, high dielectric constants, a wide band infrared transmittance and large third order non-linear optical susceptibility [1], [2]. Furthermore, their low melting temperatures and non-hygroscopic nature, which limit applications of phosphate and borate glasses, make them of much current interest. From various spectroscopic methods such as infrared [3], [4], [5], Raman [5], [6], [7], [8], [9], nuclear magnetic resonance [10] and X-ray absorption spectroscopy [11], [12], [13] as well as X-ray [13], [14] and neutron [15], [16] diffraction methods, it has been established that the basic structure of these tellurite glasses is a TeO4 trigonal bipyramid (tbp) with a lone pair of electrons in one of its equatorial sites. Tellurium oxide under normal conditions does not have the ability to form glass easily without a modifier like alkali, alkaline earth and transitional metal oxides or other glass formers [15], [17], [18]. There have been several opinions regarding the difficulty in the formation of pure TeO2 glass. Neov et al. [15], [17] assumed that TeO2 could not form glass by itself because the Te–O bond is too strongly covalent to permit the requisite amount of distortion. Another prevalent view [19], [20] is that the repulsive forces due to the lone pair electrons resist the free movement of the tbp in space during cooling of the melt and forming the glass. In a binary tellurite glass the effect of the lone pair electrons is limited by the introduction of new structural units compatible with TeO4 (tbp) and glass formation becomes easier. Most of the tellurium atoms of the TeO4 structure are connected at vertices by the Te–eqOax–Te bond, a pre-requisite for the glass formation. Here, O atom is common to the two units. The symbol ‘eqO’ refers to oxygen in an equatorial plane and ‘Oax’ refers to the oxygen in an axial position with respect to the Te atom [15], [17]. These structural peculiarities may be reflected in various physical properties of these glasses, making them interesting objects for study.

Structure of the TeO2–ZnO glasses was investigated by X-ray diffraction, Te K EXAFS and Zn K EXAFS [21] and it has been concluded that the coordination states of tellurium atoms change from TeO4 (tbp) to TeO3+1 polyhedron and TeO3 (tp) with an increase of ZnO contents. Zinc atoms are coordinated to oxygen atoms like that in Zn2Te3O8 or ZnTeO3 rather than in ZnO. In order to obtain further information related to the electronic structure of atoms in tellurite glasses, we have studied zinc–tellurite glasses by XPS as it has proven to be an effective technique for these types of investigations.

Section snippets

Glass preparation

The glasses were prepared by melting dry mixtures of reagent grade ZnO and TeO2 in pure alumina crucibles with the batch composition [(ZnO)x(TeO2)100−x], where x=25, 30 and 35. Since the oxidation and reduction reactions in a glass melt are known to depend on the size of the melt, on the sample geometry, on whether the melt is static or stirred, on thermal history and on quenching rate, all glass samples were prepared under similar conditions to minimize these factors. Approximately 30 g of

Results

Relatively low resolution X-ray photoelectron survey scans, in the binding energy region 0–1200 eV, were recorded for each glass sample and a typical wide-scan X-ray photoelectron spectrum for the sample [(ZnO)0.35(TeO2)0.65] is shown in Fig. 1. These low-resolution spectra were obtained in about one h using non-monochromatic Al Kα. Apart from the photoelectrons and Auger transitions of the glass constituents, the C 1s transition is evident. This feature at 284.6 eV associated with the

Discussion

As mentioned in the Section 3, a shoulder/asymmetry is observed in the O 1s spectra of the glasses. A careful examination of Fig. 2 indicates some differences in the O 1s spectra for glasses with different composition and in particular the variation in high energy component. The FWHM of the O 1s peak (Fig. 2) generally increases with Zn concentration, becomes maximum for x=0.3. As the O 1s peaks for the glasses may be composed of more than one component peak, each O 1s spectrum was deconvoluted

Conclusions

The XPS spectra for O 1s, Zn 2p, Te 3p and Te 3d core levels of the zinc–tellurite glass network have been studied. The binding energies of both Te 3p and Te 3d for these glasses are found to be the same in comparison to those for pure TeO2 powder. The shift to higher binding energy of Zn 2p for these glasses in comparison to pure ZnO powder probably arises as a result of changes in the chemical environment. The shoulder measured in the larger binding energy side of the O 1s peak (∼532 eV) is

Acknowledgments

The support of the KFUPM Physics Department and Research Committee is greatly acknowledged.

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