X-ray photoelectron spectroscopy studies of MgB2 for valence state of Mg

Abstract

Core level X-ray Photoelectron Spectroscopy (XPS) studies have been carried out on polycrystalline MgB 2 pellets over the whole binding energy range with a view to having an idea of the charge state of Magnesium (Mg). We observe 3 distinct peaks in Mg 2p spectra at 49.3 eV (trace), 51.3 eV (major) and 54.0 eV (trace), corresponding to metallic Mg, MgB 2 and MgCO 3 or, divalent Mg species respectively. Similar trend has been noticed in Mg 2s spectra. The binding energy of Mg in MgB 2 is lower than that corresponding to Mg(2+), indicative of the fact that the charge state of Mg in MgB 2 is less than (2+). Lowering of the formal charge of Mg promotes the σ→π electron transfer in Boron (B) giving rise to holes on the top of the σ-band which are involved in coupling with B E 2g phonons for superconductivity. Through this charge transfer, Mg plays a positive role in hole superconductivity. B 1s spectra consist of 3 peaks corresponding to MgB 2 , boron and B 2 O 3 . There is also evidence of MgO due to surface oxidation as seen from O 1s spectra. PACS No. 74.70.Ad

Study of Magnesium Boride Films Obtained From Mg(BH4)2 by CVD

“…The B1s spectrum before sputtering (t = 0) is composed of two peaks of similar intensity (BE = 192.4 and 187.8 eV) that are assigned to oxide and boride, respectively. Obtained BE values of Mg2p and B1s peaks are in good agreement with most of the literature data, [9][10][11] however they are noticeably higher than the values indicated in the paper by Vasquez et al [7] No theoretically predicted charge-transfer satellites [19] have been observed in Mg2p…”

“…[9][10][11] The MgKLL spectrum, registered at t = 0, also indicates the presence of two chemical species; magnesium oxide at KE = 1179.9 eV, and a low amount of magnesium boride at KE = 1182.5 eV. value of oxide is comparable with the reference sample and literature data, [18] while the KE values of boride differ from the reference samples, suggesting that the compound could be non-stoichiometric.…”

“…[7,9,10] The XPS signals of boron oxide (BE = 193.3 eV) and boride (BE = 188.4 eV) can be easily separated by the peakfitting of the B1s spectrum, while the MgO and boride peaks are overlapped in the Mg2p line (∼51.0 eV). It is rather difficult, therefore, to determine the sample stoichiometry from the XPS measurements alone.…”

“…Even the surface of crystalline MgB 2 films, cleaned by non-aqueous chemical etching, contained impurities of magnesium hydroxide and carbonate. [7] Moreover, the binding energy (BE) values of Mg2p and B1s peaks, measured for crystalline MgB 2 films, [7,8] are controversial, with these values determined for polycrystalline MgB 2 samples [9][10][11] and other transition metal diborides. [12,13] Therefore, the question of whether it is possible to obtain experimentally pure and stoichiometric MgB 2 , remained open till now.…”

Chemical composition of magnesium boride films obtained by CVD

“…As it can be seen from the B 1s and Mg KLL spectra, the sample remained partially oxidized even after such a long time of intense sputtering, which can be explained [9,11,12] by the granular structure of the pressed powder. The signals of oxide [11,12] (BE = 193.3 eV) and boride [11 -13] (BE = 188.4 eV) can be easily separated by the peak fitting of the B 1s spectrum, while MgO and boride peaks are overlapped in Mg 2p line (∼51.0 eV). Therefore, it is rather difficult to determine the sample stoichiometry only from the XPS measurements.…”