Scientific Highlights

Phosphorous containing fertilizers are essential to feed the growing population on earth. Because phosphorus (P) is a scarce resource in the European Union, recovering P from wastewater and sewage sludge has become extremely important. However, the availability of P to the plant is limited in such recycling P fertilizers. To overcome this problem, co-fertilization with nitrogen (N) in the form of ammonium and nitrification inhibitors, is a promising pathway. By applying the novel N K-edge micro-XRF and micro-XANES methods at the PHOENIX beamline on the soils, we could verify that a nitrification inhibitor indeed promotes ammonium fixation in fertilized soils, and hence causing a slow-release of temporarily fixed ammonium. This deceases local pH, making P better available to plants.

High entropy alloys (HEA), medium entropy alloys and multi-phase compositionally complex alloys (CCA) have gained much attention in the last 20 years because of their outstanding mechanical properties. Such baseless alloys provide different open questions on local chemical ordering, lattice distortions, orbital hybridization and/or charge transfer which define the very nature of alloys’ mechanical properties. By combining EXAFS measurements in the rarely served tender x-ray range  (PHOENIX-SLS, Al K-edge) and at higher X-ray energies (BM08-ESRF, transition metal K-edges), local chemical ordering in a CCA, Al₈Cr₁₇Co₁₇Cu₈Fe₁₇Ni₃₃ was quantified showing preferred Al-Ni and Al-Cu pairs. In addition, slight structural distortions, much lower than the predicted ones of metallic radii, were found.

Being able to replace Lithium by the much more abundant sodium for new batteries would be an important asset for energy storage. For example, Na_xMn_yO₂ cathodes would offer a high initial specific charge and a relatively high working potential. Despite long, intensive research of the electrochemical properties of these materials, the open key question remains unresolved: Where does the sodium goes to in the charging /discharging process. Unfortunately, the (de)sodiation mechanism in those materials was not completely understood, especially in terms of types of phases in which Na stays during cycling, which in turn impeded the optimization of its performance. Using the unique tender energy range of the PHOENIX beamline, we used Na K-edge X-ray absorption spectra measurements to gain a better understanding about the Na atomic positions in phases appearing during cycling. Thanks to this unique method, we established that observed high capacity in Na_xMn_yO₂ is due to the high-voltage phase being an intergrowth structure between P2 and O2 type phases were Na ions stays both in tetrahedral and octahedral sites.

The new generation of cathode materials from the Li-rich NMC (nickel-manganese-cobalt) group are under constant investigation due to their extremely high energy densities resulting from redox reactions involving both transition metals and lattice oxygen. Although a lot of research has been done so far, the exact mechanism of lithium (de)insertion in those materials, especially the reactions involving redox reactions of lattice oxygen is still elusive. Due to the particular battery design the observed reactions starts at the surface of the electrode that contacts the electrolyte and, as the reaction continues, goes deeper into the bulk structure. In order to follow the reactions taking place in the Li-rich NMC materials we aimed to exactly distinguish and characterize the phase transitions taking place on the surface and within the bulk of the Li_1.2Mn_0.6Ni_0.1Co_0.1O₂ electrode. To do so we used comprehensive XAS measurements at the PHOENIX beamline, taking advantage of the unique options to perform in situ experiments in the soft energy range to study both the Oxygen K edge and the L edges of Ni, Co and Mn.

Taking up nutrients from the soil is key to plant growth. Understanding and potentially controlling this process is important when growing food but also when caring for natural habitats, which are the basis for life on Earth. Typically, nutrients are tightly chemically bound to the soil, and roots need to create a chemical environment to harvest nutrients.  Here we use the special capabilities X-ray microscopy with tender X-rays to study the chemical changes of sulfur, phosphorus, and iron in the vicinity of plant roots (rhizosphere). We can show that Fe is slightly reduced, S is increasingly transformed into sulfate (SO₄²⁻) and phosphorus (P) is increasingly adsorbed to humic substances in this enrichment zone around the root.

When bridges, dam walls and other structures made of concrete (cement and aggregates such as sand/gravel) are marked by map-like cracks after a few decades, the diagnosis is ASR (alkali-silica Reaction), in popular science terms also called “concrete disease or concrete cancer”.  The ASR-induced microscale crack initiation can hardly be modelled, mainly due to our limited knowledge of the structure and property of the ASR products. Using X-ray absorption micro-spectroscopy at the PHOENIX beamline of the Swiss Light Source (SLS) allowed a refined diagnosis of ASR products by providing new insights into the crystallinity and structure of ASR products with micro-scale resolution.

Magnesium rich calcites are important functional biominerals. For example, they can be found in protective shells or eye lenses. Natural organism provide a surprisingly high degree of control on the amount of magnesium incorporation into calcites by yet not well understood mechanisms. Understanding such control mechanism is important when designing bio inspired functional materials. Here we systematically explore the impact of thermodynamic parameters on the degree of magnesium incorporation into calcite. In particular, we identify the thermodynamic conditions, where very high magnesium rich calcites (50% Mg/50% Ca) forms under ambient conditions of temperature and pressure. This is an important finding for geochemistry: Very high magnesium rich calcite is believed to be the precursor for dolomite. Despite its frequent occurrence in nature, its unknown formation pathway remains one of the big mysteries in geochemistry.