X-ray photoelectron spectroscopy (XPS) and magnetization studies of iron–sodium borate glasses

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Abstract

Iron–sodium borate glasses with the chemical composition [(B2O3)0.70−x(Na2O)0.3(Fe2O3)x], where 0.00⩽x⩽0.15, have been prepared and investigated by X-ray photoelectron spectroscopy (XPS) and magnetization measurements. The core-level binding energies of O 1s, B 1s, and Fe 2p have been measured with both O 1s and B 1s peaks shifting by about 2 eV towards smaller binding energies in the Fe-containing borate glasses while the Fe 2p3/2 and 2p1/2 core levels for the glasses remain essentially unchanged from those of Fe2O3 powder. The O 1s spectrum is deconvoluted into two peaks and the variation in the ratio of the peak areas is discussed in terms of the local iron structure. We suggest that both X-ray photoelectron spectroscopy and magnetization measurements show that the Fe ions remain essentially in one oxidation state, probably Fe3+, for the Fe borate glasses. In addition, the appearance of a large hysteresis between the zero field-cooled and field-cooled magnetization data indicate that the Fe moments are clustered and that the predominant interaction is antiferromagnetic.

Introduction

Studies of oxide glasses containing transition-metal (TM) oxides continue to be of technological interest because of the semiconducting properties that arise from electron hopping between two TM ions having different valence states in these glasses 1, 2, 3, 4. Borate glasses are also of interest as B2O3 is found in nearly all commercially important glasses. Furthermore, the occurrence of a boron anomaly [5]is of particular academic interest while barium and sodium borate glasses are used as a non-volatile flux in the crystal growth of garnets and ferrites [6]. With the addition of a TM (e.g., iron) to the borate glasses, mixed-valence compounds containing both Fe oxidation states (Fe2+ and Fe3+) are formed and hence the electrical (semiconducting) and magnetic (superparamagnetic) properties [7]are of particular technological importance.

Several electron paramagnetic resonance studies on oxide glasses containing varying amounts of iron have been reported 8, 9, 10. The spectra in general are found to exhibit resonances at g  2 and 4.28. Some investigators attributed these resonances to distorted octahedra and distorted tetrahedra of oxygens around the Fe3+ ions, respectively, while others have suggested that both tetrahedra and octahedra give rise to the g  4.28 resonance and that the g  2 resonance is due to clusters of paramagnetic Fe3+ ions coupled by exchange interaction 8, 9, 10. To provide more support for this interpretation, X-ray photoelectron spectroscopy (XPS) and magnetization studies have been carried out on the sodium borate glasses containing iron.

XPS can provide information about the chemical state of a given atom by the chemical shift associated with an appropriate core level for the atoms in different bonding configurations 11, 12. This chemical shift is due to a change of the binding energy of core electrons with changes in the chemical environment. Hence, XPS can be a useful quantitative probe of the short-range structure of borate glasses. In particular, resolving a O 1s spectrum into two peaks can be used to determine contributions from bridging oxygens and non-bridging oxygens even though both Fe2+ and Fe3+ contribute to the non-bridging oxygen signal. Thus, to account for the effect of these valence states on the structure and the properties of these glasses, it is important to measure the ratio of the ion concentration in the different valence states for these Fe ions. In the present work we have used magnetization measurements combined with inductively coupled plasma spectroscopy (ICP) and XPS to find the ratio of different valence states in these glasses. Another factor affecting the relative proportion of different valence states is that different vaporization rates of the constituents during the melting process and reactions with the alumina crucible can result in glasses being formed with compositions essentially different from that of the batch composition. This has been verified by ICP measurements performed in the present work as well.

Section snippets

Glass preparation

The glasses were prepared by melting dry mixtures of reagent grade B2O3, Na2CO3 and Fe2O3 in alumina crucibles with the batch composition [(B2O3)0.70−x(Na2CO3)0.30(Fe2O3)x] where x=0.00, 0.025, 0.050, 0.075, 0.100, and 0.150. Since the oxidation and reduction reactions in a glass melt are known [13]to depend on the size of the melt, on the sample geometry, on whether the liquid is static or stirred, on thermal history and on quenching rate, all glass samples were prepared under similar

Results

A wide-scan X-ray photoelectron spectrum for the sample [(B2O3)0.625(Na2CO3)0.30(Fe2O3)0.075] is shown in Fig. 1 with similar spectra being obtained for the other glass compositions. These low-resolution spectra were obtained in about 1 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 is the usual peak associated with the hydrocarbon contamination, is almost always present on

Discussion

If the O 1s peaks for the pure B2O3 powder and the glasses are composed of more than one component peak, they may correspond to the oxygen atoms in some or all of the following structural units:BOB, BONa, BOFe, BOAl.Although both bridging oxygen (B–O–B) and non-bridging oxygen (B–O–Na, B–O–Fe, B–O–Al, etc.) atoms certainly exist in these glasses, the `apparent' single peak in the O 1s photoelectron spectrum makes it difficult to resolve non-bridging oxygen atoms from bridging oxygen

Conclusions

From the XPS spectra, the binding energies of both O 1s and B 1s for these iron–sodium borate samples are found to decrease by ∼2 eV relative to those for B2O3 powder. This decrease is the result of changes in the next-nearest neighbor environment at the O and B sites. In comparison to Fe2O3, the binding energies of the Fe 2p3/2 and 2p1/2 core levels for the glasses remain unchanged indicating that Fe3+ ions are more prevalent and that the Fe3+/Fe2+ ratios remain approximately independent of

Acknowledgements

The support of the KFUPM Physics Department and Research Committee (Grant PH/PHYSPROP/43) and the WSU Institute for Manufacturing Research is greatly acknowledged. The assistance of M.A. Khan for experimental work is appreciated.

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