XPS INVESTIGATION OF BISMUTH GALLATE GLASSES CONTAINING IRON
XPS INVESTIGATION OF BISMUTH GALLATE GLASSES
CONTAINING IRON
V. Simon1, Laura Barză1, S.G. Chiuzbăian2 M. Neumann2
1Babes-Bolyai University, 3400 Cluj-Napoca, Romania
2 University of Osnabrück, Physics Department, 49069 Osnabrück, Germany
Abstract
Atomic environment of xFe2O3∙(80-x)Bi2O3∙20Ga2O3 glasses (0 ≤ x ≤ 20 mol %) was investigated with respect to electronic structure of the samples. Data obtained from Bi 4f, Ga 2p, Fe 2p, and O1s core-level photoelectron spectra indicate changes in the local order on the account of partial substitution of bismuth atoms by iron atoms. The bismuth cations behave essentially as network formers while the iron and gallium ones acts as network modifiers. The number of nonbridging oxygens depends on Fe2O3 content introduced in samples.
1. Introduction
Current interests in functional glasses include gallium based systems, which have useful optical properties. Due to their high optical nonliniarity, high magneto-optic effect and extended IR transparency heavy metal gallate glasses are of technological importance for potential application in optical switching, optical isolators (Faraday rotators) used to avoid self-focusing in high power laser systems, IR windows and sensors [1-3]. Bismuth gallate glasses appear to be promising host materials for waveguide devices in the microwaves telecommunication windows, broad band amplifier and high power laser applications [4-7].
An attractive reason in studying these glasses also consists in the fact that they do not contain any conventional glass formers such as SiO2, B2O3, P2O5, GeO2, etc. The interesting properties of these glasses are mainly due to the high polarisability and the relatively low field strengths of heavy metal cations as compared to conventional glass formers.
The present study is focused on the atomic environment of an iron-bismuth-gallate glass system investigated by X-ray photoelectron spectroscopy (XPS).
2. Experimental
The starting material used to prepare xFe2O3∙(80-x)Bi2O3∙20Ga2O3 glass samples with x = 0, 5, 10 and 20 mol % were analytically pure reagents Fe2O3, Bi2O3 and Ga2O3. They were obtained by melting oxide mixtures of desired compositions in sintercorundum crucibles at 1200oC for 30 minutes in an electric furnace in air. The melts were quickly undercooled at room temperature by pouring onto stainless steel support and pressing in form of thin plates. The bismuth-gallate matrix is metallic yellow. By addition of iron to this matrix the glasses become reddish brown. All samples have been analysed by X-ray diffraction and no crystalline phase was evidenced.
XPS measurements were performed using a PHI 5600ci Multi Technique system with monochromatised Al Ka radiation from a 250 W X-ray source (hn = 1486,6 eV). During the measurements the pressure in the analysis chamber was in the 10-9 Torr range. Low energy electron beam was used to achieve charge neutrality at the sample surface. High resolution core level scans were acquired for the Fe 2p, Bi 4f, Ga 2p and O 1s photoelectron peaks. The position and full width at half maximum of photoelectron peaks were estimated using spectra simulation based on summation of lorentzian and gaussian functions.
3. Results and discussion
In the attempt to identify the local environment of the different elements in heavy metal glasses several techniques are employed. The X-ray photoelectron spectroscopy (XPS) is used to obtain information on binding energy of the glass component elements from their photoelectron peaks [8]. XPS survey spectra recorded from the fractured surface of investigated Fe2O3-Bi2O3-Ga2O3 glasses permit to determine the elemental chemical composition of samples (Table 1).
Table 1. Atomic percentage of the elements experimentally obtained from XPS analysis on the surface of the fractured samples (exper. on surf.) along with the nominal values expected for the bulk samples (nominal in bulk).
xmol % /
Fe (signal 2p)
/Bi (signal 4f)
/Ga (signal 2p)
/O (signal 1s)
nominal in bulk / exper. on surf / nominal in bulk / exper. on surf / nominal in bulk / exper. on surf / nominal in bulk / exper. on surfat %
0 / 0 / 0 / 32 / 36.73 / 8 / 1.98 / 60 / 61.29
5 / 2 / 0 / 30 / 30.62 / 8 / 4.59 / 60 / 64.79
10 / 4 / 1.43 / 28 / 29.90 / 8 / 4.33 / 60 / 64.34
20 / 8 / 4.16 / 24 / 25.30 / 8 / 4.16 / 60 / 66.38
One observes that the values obtained for iron and gallium are much lower compared to the nominal concentrations that could suggest a migration of these atoms from the surface into the inner of the samples. On the other hand this result points out the modifier behaviour for these cations, in contrast with the conclusion drawn by Man et al. [4] from Raman spectroscopy data. They consider both Bi3+ and Ga3+ cations as glass network formers.
The high resolution Fe 2p, Bi 4f and Ga 2p core level XPS experimental spectra are well fitted by the curves obtained from summation of Fe 2p1/2 with Fe 2p3/2, Bi 4f5/2 with Bi 4f7/2, respectively of Ga 2p1/2 with Ga 2p3/2 lines.
The positions of photoelectron peaks in Bi 4f high resolution spectra are shifted to some higher energies relative to pure Bi 4 f7/2 (157 eV) and Bi 4 f5/2 (162.31 eV) but the spin orbit splitting is very close to 5.3 eV. The binding energies are close to the values reported for other bismuth oxide compounds [9-11]. The full width at half maximum (FWHM) of photoelectron peaks increases with the iron content. The decrease of FWHM is believed to be a reduction in the site distribution or disorder of the glass [8]. In this case the composition dependence of FWHM in our samples indicates an increase of the disorder degree with the iron content.
The network of multicomponent bismuth - transition metal glasses is built up of both [BiO6] octahedral and [BiO3] tetrahedral units [12]. Similar to the glasses with conventional glass network formers in bismuth gallate glasses the structural units are connected by means of bridging oxygens. The O 1s spectra from all samples are presented in Figure 1. The peaks are not symmetric and denote the presence of bridging (BO) and non-bridging (NBO) oxygen atoms. The BO photoelectron peak occurs at higher binding energy and NBO photoelectron peak at lower binding energy.
The fraction of NBO relative to the total number of oxygens Ot was estimated from the areas corresponding to O 1s photoelectron peaks. One observed a decrease of the fraction NBO/Ot from 0.74 in 80Bi2O3∙20Ga2O3 bismuth-gallate matrix to 0.29 by addition of a low Fe2O3 content (5 mol %) and then NBO/Ot increases again by further addition of iron.
According to the recent approach of Dimitrov and Komatsu [13] the oxides could be simply classified based on the correlation between electronic polarisabilities of the entering ions and their binding energies determined by XPS. It was established that O 1s binding energy of different oxides varies in 528.0- to 533.5-eV range and its value corresponds to different degree of ionicity in the M-O bonds. The O 1s binding energy for the oxide glass samples Fe2O3-Bi2O3-Ga2O3 ranges between 529.57 and 532.97 eV range and according to Dimitrov model they are semicovalent compounds with a ionic component.
The polarisability of the samples is related to the bismuth ions and decreases with the substitution degree of bismuth by the iron atoms.
4. Conclusion
XPS investigation of heavy metal xFe2O3∙(80-x)Bi2O3∙20Ga2O3 glasses shows that the binding energy and full-width at half-maximum in core level spectra are modified by introducing iron, that denotes changes in the atomic environments and an increase of the disorder degree. In the investigated glass system Bi3+ cations are formers while Ga3+ and Fe3+ are modifiers of the glass network. By addition of a low iron content to Bi2O3-Ga2O3 host glass the number of nonbridging oxygens is diminished and it increases again by further addition of iron. According to the polarisability model these glasses are semicovalent compounds, partially ionic.
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