arXiv subjects

Rikuya Ishikawa

Publications and source records attributed to Rikuya Ishikawa.

5 recordsLinked to original sources

arXiv2026

Liquid-state structural asymmetry governs species-selective crystallization in multicomponent systems

Multicomponent crystals are often assumed to form nearly random solid solutions when thermodynamically stable. However, crystal growth proceeds from structurally heterogeneous liquids, raising the possibility that the liquid state may influence which species are incorporated into the growing crystal. Here we demonstrate that liquid-state structural asymmetry can induce species-selective crystallization in multicomponent systems. Using molecular dynamics simulations of a multivalent rocksalt-type model (AgPbBiTe$_3$), we find that cations with higher valence readily form locally crystal-compatible coordination environments in the liquid and are efficiently incorporated into the growing lattice, whereas lower-valence cations exhibit more disordered liquid coordination and attach less efficiently at the crystal-liquid interface. This asymmetry leads to species-selective incorporation and slower crystal growth. Depth-resolved photoelectron spectroscopy measurements on AgPbBiTe$_3$ further reveal enhanced Ag concentration near grain-boundary and surface regions, consistent with the selective incorporation predicted by the simulations. These results demonstrate that structural compatibility between liquid-state structure and the target crystal motif governs selective incorporation during crystallization, providing a general kinetic mechanism by which compositional heterogeneity can emerge during growth of multicomponent crystals.

cond-mat.soft↗

arXiv2025

Cooperative Ion Conduction Enabled by Site Percolation in Random Substitutional Crystals

Efficient and safe energy storage technologies are essential for realizing a sustainable and electrified society. Among the key challenges, the design of superionic conductors for all-solid-state batteries often faces a fundamental trade-off between stability and ionic conductivity. Random substitutional crystals, where atomic species are randomly distributed throughout a crystal lattice, present a promising route to overcome this trade-off. Although the importance of cooperative motion in ion conduction has been pointed out, there is a lack of understanding of the relationship between mesoscale structural organization and macroscopic conductivity, limiting the rational design of optimal compositions. Here, we systematically investigate the ionic conductivity of rock salt random substitutional ionic crystals Li$_x$Pb$_{1-2x}$Bi$_x$Te as a function of Li concentration $x$ using molecular dynamics simulations. We find that ionic conductivity increases sharply once the $x$ exceeds a critical threshold, without disrupting the underlying crystal structure. Strikingly, this threshold aligns with the site-percolation threshold predicted by percolation theory. Our findings establish ion percolation as a universal design principle that reconciles the trade-off between conductivity and stability, offering a simple and broadly applicable strategy for the development of robust, high-performance solid electrolytes.

cond-mat.mtrl-sci↗

arXiv2024

Spontaneous formation of Frenkel defects in high-entropy-alloys-type compound

High-entropy alloys (HEAs) are attracting attention due to their exceptional properties, such as enhanced mechanical toughness, superconducting robustness, and thermoelectric performance. Numerous HEAs have been developed for diverse applications, ranging from self-healing in fusion reactors to addressing environmental concerns with thermoelectric materials. Understanding atomic diffusion within HEA crystals is crucial for these applications. Here, this study investigates diffusion mechanisms in PbTe-based HEAs, focusing on the role of indium (In). Molecular dynamics simulations reveal that In inclusion prompts spontaneous Frenkel defect formation, notably enhancing diffusion not only of In$^+$ but also other cations. Frenkel defect formation, closely linked to alloy properties, is predominantly influenced by charge rather than cation size. This insight not only enhances comprehension of HEA diffusion mechanisms but also develops HEAs with properties such as self-healing from damage and high ion permeability, advancing the field of material science.

cond-mat.mtrl-sci↗

arXiv2023

Initial perturbations dependence of non-equilibrium continuous and discontinuous pattern transition

A phase separation in a spatially heterogeneous environment is closely related to intracellular science and material science. For the phase separation, initial heterogeneous perturbations play an important role in pattern formations. In this study, a pattern transition from a lamellar pattern to a columnar pattern is investigated in the presence of a slit pattern as the initial perturbations. Here it is found that the transition behavior depends on the initial slit width. When the initial slit width is close to the width of the columnar pattern at the steady state, the pattern transition is the second-order-like (continuous) transition. Meanwhile, the pattern transition becomes the first-order-like (discontinuous) transition if the width of the initial slit is much larger than that at the steady state. Then those transition behaviors can be explained by the dynamical path during the pattern formation. This finding will advance understanding of the initial perturbation dependence of nonequilibrium phenomena.

cond-mat.stat-mech↗

arXiv2022

Selective 3-dimensional patterning during phase separation of a continuously laminated layer

Control over the physical properties of materials is ubiquitously required in many fields. One means by which this can be achieved is controlling the internal structure of multi-component materials with an eye to enhancing mechanical properties. Here, we focus on self-organized pattern formation in phase separating materials, where microscopic patterns with a smooth, continuous connection may be realized. We propose a feasible method to control pattern formation using phase separation combined with continuous ``lamination'' of material, when material is continuously and homogeneously layered on top of a base. We find that a random droplet pattern, a lamellar pattern, and a cylindrical pattern are formed depending on the lamination rate $V$. We clarify the dynamics of pattern formation, focusing on the mechanism. This study may lead to the creation of new functional materials through artificial pattern control.

cond-mat.soft↗