Bo Brummerstedt Iversen

Electron densities provide valuable information, such as atomic charges and static structure factors, but all-electron densities can be computationally expensive to obtain. This work presents a method to approximate all-electron densities from pseudopotential electronic structure calculations, called the Valence Electrons Plus Independent Cores (VEPIC) model. For each pseudopotential in the calculation, a pseudocore is defined, and this is added to the ab initio pseudovalence density. Electron densities obtained this way yield accurate static structure factors, as well as atomic charges and dipoles from the Hirshfeld and Minimal Basis Iterative Stockholder methods, compared with those from all-electron calculations. While the total density of this model matches the all-electron density in the valence region, the model lacks core polarization, which is present in the Projector Augmented Wave (PAW) method, making the VEPIC model an intermediate between the Independent Atom Model (IAM) and the PAW model in terms of accuracy. It is also intermediate between the two in terms of computational complexity, as it only requires the pseudovalence density, unlike PAW, which additionally requires processing every pseudovalence wave function in the system.

DanMAX is a hard X-ray materials science beamline at the ultra-low emittance 3 GeV storage ring at the MAX IV laboratory. It is designed for performing in situ and in operando studies using powder X-ray diffraction and full-field imaging. The beamline is equipped with an in-vacuum undulator source providing photons in the energy range 15–35 keV. The beam can be focused using compound refractive lenses, thus providing a variable beam size range from approximately 15 µm to 1.2 mm (FWHM) that can be changed in seconds. This paper presents the X-ray optics, discusses its performance, and shows the first endstation instrument for powder X-ray diffraction, available sample environments, and the surrounding infrastructure. The powder diffraction instrument is equipped with a CdTe hybrid pixel detector for powder X-ray diffraction and total scattering, as well as an energy-resolved silicon drift detector for X-ray fluorescence experiments. The instrument and infrastructure are designed to be highly flexible to accommodate various sample environments for in situ and in operando experiments.

Machine learning and artificial intelligence approaches are expected to accelerate the development of new materials, for example through automated synthesis. Here, we investigate another approach to accelerate materials discovery, namely in situ synchrotron Powder x‐ray diffraction (PXRD), which enables efficient exploration of a large range of synthesis parameters to identify optimal synthesis conditions. We study the solvothermal formation of Pd _x M _y (M═Si, Ge, Sn, Pb) nanoparticles with varying temperature, solvent, and metal precursor. Intermetallic phases are obtained for all systems except for Pd _x Si _y , and specific conditions are identified for preparing Pd ₂₅ Ge ₉ , Pd ₂ Ge, SnPd ₂ , Sn ₁₃ Pd ₂₀ , SnPd, Pb ₃ Pd ₅ , Pb ₉ Pd ₁₃ , PbPd, and Pb ₂ Pd intermetallic phases. A general formation mechanism is suggested, where Pd nanoparticles are formed initially, followed by adsorption and diffusion of the secondary metal to form intermetallic phases. Based on the learnings from the in situ experiments, SnPd ₂ , Sn ₁₃ Pd ₂₀ , and SnPd nanoparticles are synthesized ex situ in a simple solvothermal process. Scanning transmission electron microscopy—energy‐dispersive x‐ray spectroscopy (STEM‐EDS) establishes a gradient of Sn across the nanoparticles with a Pd‐rich core and increasing Sn content towards the edges, reflecting the diffusion of Sn into the initial Pd nanoparticles and thereby corroborating the formation mechanism observed from the in situ data.

The magnetic properties of single molecule magnets (SMMs) relate to details of the metal–ligand chemical bonding. Halide ligands are typical structural entities in SMMs and are often interchanged assuming chemical transferability. However, the chemical bonding of the halides is different, and it is of high interest to understand how these differences affect the properties of SMMs. Here, two isostructural pentacoordinate cobalt complexes, [Co(L)Cl2] (1) and [Co(L)Br2] (2) with L = 2,6-bis(1-dodecyl-1H-benzimidazol-2-yl)-pyridine, are investigated through 25 K single crystal synchrotron X-ray diffraction (XRD) measurements with subsequent multipole modeling to derive experimental electron densities. Topological analysis of the electron densities reveal closed-shell interactions between Co and the halides and the nitrogen donors of L. The Co–Cl bonding resembles the Co–N bonds, with very similar electron density and Laplacian in the bond critical points, while the Co–Br bonding is weaker and more ionic. Estimated experimental d-orbital populations reveal an ordering of (dxy,dxz) < dz2 < dyz < dx2–y2 for 1 and dxy < dxz < dx2–y2 < dyz < dz2 for 2. The reversal of dz2 and dx2–y2 between 1 and 2 reflects a stronger interaction from Co to the equatorial Cl compared to the axial Cl in 1, but vice versa in 2.

Gd-doped ceria (GDC) is a key functional oxide for applications in catalysis and solid oxide fuel cells, where control over phase purity, dopant incorporation, and defect chemistry at the nanoscale is critical. Here, we use in situ synchrotron powder X-ray diffraction to identify synthesis conditions to obtain GdxCe1–xO2–x/2 nanoparticles in a one-step process. GDC nanoparticles with 0.0 ≤ x ≤ 0.3 are subsequently synthesized by a continuous-flow hydrothermal method using simple cerium and gadolinium nitrates and urea as precursors. Systematic variation of synthesis temperature and pH demonstrates that supercritical conditions (400 °C) under basic pH are required for the formation of a homogeneous fluorite solid solution under hydrothermal conditions. Ex situ synchrotron PXRD with Rietveld refinement reveals a monotonic lattice expansion from 5.41370(2) to 5.42701(7) Å with increasing Gd content, accompanied by increased microstrain and a reduction of crystallite size from 7 to 4 nm. Pair distribution function analysis and STEM-EDX mapping indicate limited local disorder, with no evidence of defect ordering or clustering. A statistically distributed arrangement of Gd ions and oxygen vacancy defects is favorable for obtaining high oxygen mobility at relatively low temperatures. This contrasts with the correlated defects known in oxygen conductors such as yttria-stabilized hafnia.

High-temperature polymorphs of hafnia (HfO ₂ ) are of significant interest in electronics and fuel-cell applications, and stabilization at ambient conditions can be achieved by aliovalent substitution and nanosize effects. Y ³⁺ stabilization of hafnia (YSH) introduces local cation disorder around charge-compensating oxygen-ion vacancies, and here we establish both the average and local structure of YSH nanoparticles using synchrotron powder X-ray diffraction (PXRD) and pair distribution function (PDF) analysis. A range of phase-pure crystalline nanoparticles of Hf _{1– x} Y _x O _{2– x /2} were prepared via continuous flow solvothermal synthesis and subsequent high-temperature annealing, and full stabilization of the cubic phase is achieved already at 13 at% Y ³⁺ . The average structure conforms to the cubic fluorite phase of HfO ₂ , but local displacive disorder caused by electrostatic attraction of neighbouring oxygen ions and repulsion of neighbouring metal ions by the net-positive oxygen-ion vacancies is established. The well-known Zr ₃ Y ₄ O ₁₂ structure, which incorporates such relaxation motifs, provides a good proxy description of YSH. In situ X-ray total scattering experiments provide insight into the formation mechanism of the YSH nanoparticles and initial precipitation of an atomically mixed amorphous phase is followed by crystallization over several minutes. The crystallization rate increases with higher reaction temperature, whereas an increased doping level results in slower crystallization.

Hybrid organic–inorganic crystals are attractive due to their large structural diversity and easy fabrication.

The oxygen evolution reaction (OER) can be fundamentally considered as a process in which an electrocatalyst converts electrical energy to chemical energy. In nanoparticle-based OER devices, current flows through complex substrate-particle and interparticle junctions before delivering the energy at the catalyst–electrolyte interface. Performance optimization requires the right combination of the metallic substrate and semiconducting catalyst. We demonstrate that the performance of Fe₃O₄ nanoparticles is strongly influenced by the principle of energy matching at the substrate–catalyst interface. Using substrates with distinct work functions, we show that the Ohmic junctions, formed between Fe₃O₄ and low work function metals (Cu, Ni), exhibit significantly lower overpotentials and Tafel slopes than Schottky junctions formed with high work function materials (glassy carbon (GC), Pt, and Au). Furthermore, we tune the GC and Fe₃O₄ nanoparticle interface from Schottky to Ohmic by introducing a Ni₂P buffer layer. Our findings highlight the importance of interfacial engineering for designing efficient water electrolysis devices.