Charge ordering in cationic mixed-valence compounds is of crucial importance for materials science. The prototypic example for a transition from a charge-disordered to a charge-ordered state has been magnetite, Fe3O4, where Evert Verwey observed a sudden jump in resistivity near -150°C. In the journal Science Advances now a research team of scientists from Germany and Slovenia reports a Verwey-type charge-ordering transition in a different class of mixed-valence compounds which is composed of negatively charged dioxygen molecules.
The crystal structure and chemical bonding scenario of the complex intermetallic compound Be21Pt5 was solved in a joint collaboration spearheaded by scientists from the Max-Planck Institute for Chemical Physics of Solids. Be21Pt5 shows the rare case of superconductivity in a complex intermetallic compound with over 400 atoms per unit cell. Real space analysis of the calculated electron density and the electron localizability indicator (ELI) reveal structural units with collective intra-cluster interactions, termed cluster bonding, which are linked in the structure by strongly polar three-center interactions.
SmB6 is under focus because it may be the first topological insulator that is also strongly correlated. It is intermediate valent and a gap opens only at low temperatures due to an intricate interplay between correlation effects and hybridization, yet the surface is always conducting. The topological properties of the surface are determined by the bulk so that it is surprising that the bulk crystal-field ground state of Sm3+ was unknown until now despite many attempts by e.g. neutron scattering. Recently, scientists from the Max-Planck Institute for Chemical Physics of Solids and the University of Cologne clarified this issue by applying a new technique, namely core-level non-resonant inelastic x-ray scattering, see Sundermann et al., Phys. Rev. Letter 120, 016402 (2018). The finding that the ground state is the Γ8 quartet and not the Γ7 doublet, contradicts all existing band structure calculations and illustrates in a sobering manner the difficulties in making reliable predictions for the properties of correlated systems.
The rare earth nickelates RNiO3 with the high Ni3+ oxidation state have continued to attract enormous interest due to the famous metal-insulator transition and unusual charge and spin-order phenomena together with the prediction for multi-ferroicity and even superconductivity in thin film hetero-structures. One of the long standing puzzles is why LaNiO3 seems to be the only RNiO3 that stays metallic and paramagnetic down to lowest temperatures. Recently, the team around Alexander Komarek was able to grow centimeter-sized and impurity-free single crystals of LaNiO3 and then to unveil its true electrical and magnetic properties. The result is that the phase diagram of the RNiO3 system has to be redrawn and that theoretical concepts have to be reconsidered about how to explain the properties and the electronic structure of these high oxidation state materials.