Yusuke Nanba

The band center of d orbitals ( d ‐band center) has been widely used as an effective descriptor for analyzing material properties. However, in high‐entropy nanoalloys, the diverse atomic environments present challenges in systematically exploring all possible combinations. Due to computational resource limitations, generating a sufficient number of samples is infeasible. Consequently, the d ‐band center should be treated as a response variable in machine‐learning models. We calculated the d ‐band center for individual atoms and applied supervised learning techniques to identify key factors influencing its behavior. While several factors were identified, their physical significance in predicting d ‐band centers remained unclear. To address this issue, we incorporated various interatomic distance terms as descriptors, along with element‐based coordination numbers (ECN). The resulting model closely resembled the overlap integral of the Slater‐type orbital, and the regression coefficients of the ECN exhibited sensitivity to the effective principal quantum number and nuclear charge. Understanding the physical significance of these descriptors is crucial for improving property predictions and facilitating data collection on novel materials.

To understand the electronic-structure change of LiCoO ₂ during charge–discharge, we performed ex situ resonant soft X-ray emission spectroscopy of the Co L ₃ edge in combination with charge-transfer multiplet calculations.

Technetium (Tc), atomic number 43, is an element that humans cannot freely use even in the 21st century because Tc is radioactive and has no stable isotope. In this report, we present molybdenum-ruthenium-carbon solid-solution alloy (MoxRu1-xCy) nanoparticles (NPs) that are expected to have an electronic structure similar to that of technetium carbide (TcCy). MoxRu1-xCy NPs were synthesized by annealing under a helium/hydrogen atmosphere following thermal decomposition of metal precursors. The obtained NPs had a solid-solution structure in the whole composition range. MoxRu1-xCy with a cubic structure (down to 30 atom % Mo in the metal ratio) showed a superconducting state, and the transition temperature (T-c) increased with increasing Mo composition. The continuous change in T-c across that of TcCy indicates the continuous control of the electronic structure by solid-solution alloying, leading to pseudo-TcCy. Density functional theory calculations indicated that the synthesized Mo0.53Ru0.47C0.41 has a similar electronic structure to TcC0.41.

With an increase in an element, the configurational entropy effect stabilizes the multi-element materials. However, the expansion of configuration space and heterogeneous surfaces such as nanoparticles preclude the analytical evaluation of configurational entropy. Then, we implemented the Wang-Landau algorithm, which is one of the Monte-Carlo algorithms, for evaluating the configurational entropy and probing the thermodynamic stable configuration of multi-element materials. The regression equation obtained by density functional theory calculation and multiple regression analysis is used in the energy estimation in the sampling. This method was applied to binary alloys in the bulk and ternary alloy nanoparticles and the obtained features of the stable configuration were discussed.

Composition, size, and structure dependences of stable configuration of Pd–Ru alloy nanoparticles under finite temperature were theoretically investigated by using density functional theory calculation, multiple regression, and Wang–Landau sampling.

The Mn 3d electronic-structure change of the LiMn ₂ O ₄ cathode during Li-ion extraction/insertion in an aqueous electrolyte solution was studied by operando resonant soft X-ray emission spectroscopy (RXES).