Doping induced spin state transition in LaCoO3: dynamical mean-field study

Fe-Doped Metal Complex (LaCo0.9Fe0.1O3) and g-C3N4 Formed a Nanoheterojunction for the Photocatalytic Decomposition of Water

Photocatalytic water decomposition provides an environmentally friendly method of hydrogen production similar to "photosynthesis", while current research aims to develop affordable yet efficient photocatalysts. Oxygen vacancy is one of the most significant defects in metal oxide semiconductors, including perovskite, which substantially influences the semiconductor material's efficiency. To enhance the oxygen vacancy in the perovskite, we worked on doping Fe. A perovskite oxide nanostructure of LaCo_xFe_1-xO₃ (x = 0.2, 0.4, 0.6, 0.8, and 0.9) was prepared by the sol-gel method, and a series of LaCo_xFe_1-xO₃ (x = 0.2, 0.4, 0.6, 0.8, and 0.9)/g-C₃N₄ nanoheterojunction photocatalysts were synthesized using mechanical mixing and solvothermal methods for LaCo_xFe_1-xO₃ (x = 0.2, 0.4, 0.6, 0.8, and 0.9). Fe was successfully doped into the perovskite (LaCoO₃), and the formation of an oxygen vacancy was verified by various detection methods. In our photocatalytic water decomposition experiments, we observed that LaCo_0.9Fe_0.1O₃ demonstrated a significant increase in its maximum hydrogen release rate, reaching 5249.21 μmol h^-1 g^-1, which was remarkably 17.60 times higher than that of LaCoO₃-undoped Fe. Similarly, we also explored the photocatalytic activity of the nanoheterojunction complex LaCo_0.9Fe_0.1O₃/g-C₃N₄, and it exhibited pronounced performance with an average hydrogen production of 7472.67 μmol h^-1 g^-1, which was 25.05 times that of LaCoO₃. We confirmed that the oxygen vacancy plays a crucial role in photocatalysis.

Rietveld Refinement and X-ray Absorption Study on the Bonding States of Lanthanum-Based Perovskite-Type Oxides La1−xCexCoO3

Metal-oxygen bonding of the Ce-doped LaCoO3 system remains largely unexplored despite extensive studies on its magnetic properties. Here, we investigate the structure and local structure of nanoscale La1−xCexCoO3, with x = 0, 0.2, and 0.4, using the Rietveld refinement and synchrotron X-ray absorption techniques, complemented by topological analysis of experimental electron density and electron energy distribution. The Rietveld refinement results show that LaCoO3 subject to Ce addition is best interpretable by a model of cubic symmetry in contrast to the pristine LaCoO3, conventionally described by either a monoclinic model or a rhombohedral model. Ce4+/Co2+ are more evidently compatible dopants than Ce3+ for insertion into the main lattice. X-ray absorption data evidence the partially filled La 5d-band of the pristine LaCoO3 in accordance with the presence of La–O bonds with the shared-type atomic interaction. With increasing x, the increased Ce spectroscopic valence and enhanced La–O ionic bonding are noticeable. Characterization of the local structures around Co species also provides evidence to support the findings of the Rietveld refinement analysis.

Thermoelectricity in correlated narrow-gap semiconductors

We review many-body effects, their microscopic origin, as well as their impact on thermoelectricity in correlated narrow-gap semiconductors. Members of this class-such as FeSi and FeSb₂-display an unusual temperature dependence in various observables: insulating with large thermopowers at low temperatures, they turn bad metals at temperatures much smaller than the size of their gaps. This insulator-to-metal crossover is accompanied by spectral weight-transfers over large energies in the optical conductivity and by a gradual transition from activated to Curie-Weiss-like behaviour in the magnetic susceptibility. We show a retrospective of the understanding of these phenomena, discuss the relation to heavy-fermion Kondo insulators-such as Ce₃Bi₄Pt₃ for which we present new results-and propose a general classification of paramagnetic insulators. From the latter, FeSi emerges as an orbital-selective Kondo insulator. Focussing on intermetallics such as silicides, antimonides, skutterudites, and Heusler compounds we showcase successes and challenges for the realistic simulation of transport properties in the presence of electronic correlations. Further, we explore new avenues in which electronic correlations may contribute to the improvement of thermoelectric performance.

Dimensionality control of d-orbital occupation in oxide superlattices

Manipulating the orbital state in a strongly correlated electron system is of fundamental and technological importance for exploring and developing novel electronic phases. Here, we report an unambiguous demonstration of orbital occupancy control between t_2g and e_g multiplets in quasi-two-dimensional transition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO₃ and a band insulator LaAlO₃. As the LaCoO₃ sublayer thickness approaches its fundamental limit (i.e. one unit-cell-thick), the electronic state of the SLs changed from a Mott insulator, in which both t_2g and e_g orbitals are partially filled, to a band insulator by completely filling (emptying) the t_2g (e_g) orbitals. We found the reduction of dimensionality has a profound effect on the electronic structure evolution, which is, whereas, insensitive to the epitaxial strain. The remarkable orbital controllability shown here offers a promising pathway for novel applications such as catalysis and photovoltaics, where the energy of d level is an essential parameter.

Dynamical mean field theory-based electronic structure calculations for correlated materials

We give an introduction to dynamical mean field approaches to correlated materials. Starting from the concept of electronic correlation, we explain why a theoretical description of correlations in spectroscopic properties needs to go beyond the single-particle picture of band theory.We discuss the main ideas of dynamical mean field theory and its use within realistic electronic structure calculations, illustrated by examples of transition metals, transition metal oxides, and rare-earth compounds. Finally, we summarise recent progress on the calculation of effective Hubbard interactions and the description of dynamical screening effects in solids.

Double exchange via t(2g) orbitals and the Jahn-Teller effect in ferromagnetic La0.7Sr0.3CoO3 probed by epitaxial strain

The magnetic exchange in hole-doped ferromagnetic cobaltates is investigated by studying the magnetic and electronic properties of La0.7Sr0.3CoO3 films as a function of epitaxial strain. We found a strong-coupling double exchange mechanism between Co3+ (4t(2g)   2e(g)) and Co4+ (3t(2g)   2e(g)) high-spin states mediated by t(2g) electrons--in contrast to the moderate coupling provided by the e(g) exchange in manganites. The strong sensitivity of the Curie temperature TC to the bulk compression can be explained by the small bandwidth of the t(2g)-derived states. A strain-induced Jahn-Teller effect is likewise observed. The experimental results clarify the magnetic exchange mechanism in the cobaltates.