After showing the coexistence of Eu2+/Eu3+ ions in hydrated EuCl3 salt, which was then proved to be EuCl2,85 salt (see 2013 study published in J. Phys Chem. C) a new advance has been made on the TEMPO beamline. A team of researchers from IRAMIS (Saclay), ILM (Lyon) and the ICMR (Reims), in collaboration with researchers from the TEMPO beamline, has performed a full characterization of this ionic solid by combining core level and resonant photoemission spectroscopy techniques. Such a study had never been done on a europium-based ionic solid.
Lanthanide ions, such as europium, exhibit good photonic qualities, such as spectral purity. Applications using europium ions, especially Eu3+, are experiencing a new boom in photonics with organic LEDs (OLEDs), lasers, optical communications and as fluorescent markers in biological systems They are also interesting candidates for qubits.
Eu2+ and Eu3+ ions ions do not have the same optical and electronic properties. The very fine red atomic-line luminescence of Eu3+ ions is the reason why these are of interest in applications. The emitted light is generated mainly from the electronic transitions between levels of the partially filled 4f shells. On the other hand, Eu2+ shows a broad emission band in the UV and has low quantum efficiency.
Precise determination of the composition of the EuCl3 salt and a better understanding of the spatial distribution of both types of ion in this solid are likely to provide important information about the expected conductivity of this salt.
Until now europium salts have hardly been studied, with little data regarding potential interactions between Eu2+ ions, Eu3+ ions and ligands, interactions that often limit the performance of devices involving these ions. Yet, such information is vital when considering the development of such devices.
Due to its high chemical sensitivity, NEXAFS absorption spectroscopy is a preferred technique for the study of mixed valence compounds. However, it requires a spectroscopic reference for measuring the electronic properties and the chemical composition of the sample. Furthermore, with solid samples such as these salts, numerous surface effects can complicate the interpretation of the photoemission spectra.
Figure: (a) Near band edge X-ray absorption spectrum measured for the mixed valence EuCl2.85 salt deposited on gold surface at the Eu M4,5 absorption edge in total electron yield. (b) Resonant photoemission map measured at the M4,5 absorption edge. The right part corresponds to 3d3/2 -4f transitions. The left part is related to 3d5/2 -4f transitions, dash lines delimit the area of localized resonant states.
The researchers thus decided to use a combination of 2 photoemission spectroscopy techniques: HRPES and RPES, together with NEXAFS spectroscopy, in order to overcome these limitations. This allowed them to compare the chemical sensitivity (to different degrees of oxidation) of each technique, in order to accurately determine the concentrations of the different species.
They were thus able to determine both the composition and distribution of the different oxides on the surface of the salt, thus providing the first reference for the electronic structure of EuCl2.85.
It was found that the Eu2+ ions are homogeneously distributed in the crystal, leading to n-type doping of the material.
In addition, this combination of spectroscopic techniques is known to have a very high sensitivity to chemical species: up to 0.1% atomic concentration, which makes it an attractive alternative to conventional chemical analysis for detecting impurities at very low concentrations.