Europium oxidation state and local structure in silicate glasses

Abstract

Europium LIII-edge XAS spectra were recorded for silicate glasses of different compositions, quenched from melts equilibrated at different oxygen fugacity (fO2). The Eu XANES spectra vary systematically with glass composition and with fO2 (–log fO2 ~0 to ~11.9) indicating changes in the Eu oxidation state. The intensity of the main peaks on the absorption edges were quantified and used to determine the Eu2+/(Eu2++Eu3+) ratio. All the Eu-bearing glasses synthesized in air show the prevalent presence of Eu3+ but also, unexpectedly, the presence of a small amount of Eu2+ in the basaltic glasses and up to 20% of Eu2+ in the haplogranitic sample. Moreover, XANES analyses of the samples synthesized at reducing conditions (from FMQ to IW-2) show that europium in haplogranitic glasses is always more reduced than in basaltic glasses. No relationship has been found between Eu valence and alkali content in the studied glasses. The structural environment of Eu in the glasses was determined by EXAFS analyses, demonstrating the different Eu behavior as function of the fO2. In fact, in air, Eu3+ both for basaltic and haplogranitic compositions, is bonded to six O atoms in a regular octahedron (CN = [6 ± 0.5]) with similar <Eu-O> distances of about 2.30 ± 0.02 Å. On the other hand, the almost purely divalent samples have Eu2+ in a higher coordination (CN = [9 ± 1]) and longer <Eu-O> distances (2.68 ± 0.02 Å). This work clearly demonstrates that, in addition to oxygen fugacity, melt composition also plays a strong role in affecting Eu oxidation state. Moreover, for the first time, experimentally derived structural data of Eu2+ in silicate glasses of geological interest are presented.\ud Keywords: Europium, oxidation state, silicate glasses structure, XA

Chemical tunability of europium emission in phosphate glasses

“…Introduction of nitrogen into the glass matrix after the nitridation probably changes the local environment of the optical centers causing a decrease of lifetime, but it is important to note that still just one distribution band over the lifetimes is observed (see Table 2), and there are not further evidences of various Eu 3+ sites in the studied glasses. These results are in agreement with previous studies on simplified and multicomponent silicate glasses [7,29], where it was observed that the bulk chemistry does not produce strong modifications on the Eu 3+ local environment.…”

“…Previous studies on borophosphate, boroaluminate, and boroaluminosilicate glasses [3][4][5] shown that the optical basicity of the glasses affects the Eu oxidation state (higher the optical basicity, higher is the amount of oxidized species). In turn, it is also possible to correlate the NBO/T parameter (non-bridging oxygen over tetrahedra) to the Eu redox ratio, as described for simplified and multicomponent silicate glasses [6][7][8].…”

Control of Oxidation State of Eu Ions in Na2O–Al2O3–SiO2 Glasses

“…However, it is apparent that the glass structure, in particular, the Al 3+ ions play an important role to the Eu–O chemical bonds. XAFS measurements were used to verify the structures of Eu 3+ ions . After carrying out extraction of the EXAFS signals from experimental absorption spectra, the radial distribution functions were obtained from Fourier transformation of the k 3 ‐weighted EXAFS oscillation curves, k 3 χ(k), in the k ‐space region from about 2 to 12 Å −1 , where k is the magnitude of the wave vector of photoelectron; the calculated distribution curves around the Eu atoms are shown in Fig.…”

Competition between two redox states in silicate melts: An in-situ experiment at the Fe K-edge and Eu L3-edge

“…The Eu threshold energy was taken as the first maximum of the first derivative of the spectra, whereas peak positions were obtained by calculating the second derivative of the spectra. Eu data analysis was carried out by following the procedure described in Cicconi et al (2012). In brief, the peaks relative to Eu 2+ and Eu 3+ can be fitted by a combination of pseudo-Voigt (pV) and arctangent (atg) functions to simulate the XANES spectra (i.e., Takahashi et al 2005;Cicconi et al 2012).…”

“…Eu data analysis was carried out by following the procedure described in Cicconi et al (2012). In brief, the peaks relative to Eu 2+ and Eu 3+ can be fitted by a combination of pseudo-Voigt (pV) and arctangent (atg) functions to simulate the XANES spectra (i.e., Takahashi et al 2005;Cicconi et al 2012). The pV and atg functions simulate the resonance peak and the absorption edge step, respectively.…”