arXiv subjects

Kazuki Morita

Publications and source records attributed to Kazuki Morita.

7 recordsLinked to original sources

arXiv2026

How unconventional oxidation state Au$^{2+}$ is stabilized in halide perovskite Cs$_4$Au$_3$Cl$_{12}$: a first-principles study of its polaron crystal nature

Gold in crystalline compounds is typically only stable in oxidation states Au1+ and Au3+. Even compounds with nominal Au2+ usually disproportionate into Au1+ and Au3+. Recently, Cs4Au3Cl12 was synthesized, where gold took the 2+ state in the bulk. Here, we investigate this compound using first-principles calculations and show that stabilization of the Au2+ ion is through the formation of a polaron crystal. The electronic and phononic structure suggest that the bonding network can be interpreted as a collection of [Au2+Cl4]2- and [Au3+Cl4]1- square planar motifs, and the crystal lacks a smooth pathway for Au2+ to disproportionate into Au1+ and Au3+ without creating dangling bonds. The electronic states of Au are contained within each AuCl4 motif, which allows for the Au2+ state to be localized and isolated electronically. The Au2+-sites form an ordered structure, which is driven by a strong repulsive interaction between [Au2+Cl4]2- motifs due to their lattice distortion. By considering redox reaction, we show that Cs4Au3Cl12 has the maximal density of Au2+, and further reduction will induce a delocalized state. Cs4Au3Cl12 has distinctive electronic structure, with a narrow gap, isolated HOMO and LUMO bands strongly localized at the Au-sites, and magnetization at the Au2+-sites making Cs4Au3Cl12 unique among quantum materials. Cs4Au3Cl12 can be a testbed to explore novel gold chemistry, opening opportunities to control oxidation state through engineering of lattice distortions.

cond-mat.mtrl-sci↗

arXiv2026

Direct spatiotemporal imaging of carriers reveals a long-lived bulk photovoltaic mechanism

The bulk photovoltaic effect (BPVE), a manifestation of broken centrosymmetry, has attracted interest as a probe of the symmetry and quantum geometry of materials, and for use in photovoltaic and optoelectronic devices. However, so far the effect has not been captured directly in space and time. Here, we use contactless pump-probe microscopy to visualize the spatiotemporal evolution of photoexcited carriers in single-crystal, mono-domain $BiFeO_{3}$, a prototypical ferroelectric material. We observe asymmetric carrier transport along the polar axis, which confirms the intrinsic bulk- and symmetry-driven nature of the BPVE. Remarkably, this asymmetric transport persists for several nanoseconds after photoexcitation, which cannot be explained by conventional short-lived shift or phonon ballistic current BPVE mechanisms. Our Monte Carlo simulations show that asymmetric momentum scattering by defects, such as oxygen vacancies, leads to long-lived asymmetric carrier transport, as observed experimentally. Beyond fundamental insights, this paves the way towards controlling symmetry- and defect-driven photoresponses.

cond-mat.mtrl-sci↗

arXiv2026

Phenothiazine-Based Self-Assembled Monolayer with Thiophene Head Groups Minimizes Buried Interface Losses in Tin Perovskite Solar Cells

Self-assembled monolayers (SAMs) have revolutionized the fabrication of lead-based perovskite solar cells, but they remain underexplored in tin perovskite systems. PEDOT is the material of choice for hole-selective layers in tin perovskite solar cells (TPSCs), but presents challenges for both performance and stability. MeO-2PACz, the only SAM reported for Sn perovskites, enables device fabrication but consistently underperforms when compared to PEDOT. In this work, we identify that MeO-2PACz's limitations arise from excessively strong interactions with perovskite surface and poor lattice matching, leading to poor interface quality. To overcome these issues, we design, synthesize, and characterize a novel SAM-forming molecule called Th-2EPT. Th-2EPT optimizes coordination strength and improves lattice compatibility, contributing to the creation of a high-quality buried interface and dramatically suppressing non-radiative recombination. We used Density Functional Theory (DFT) to evaluate coordination strength and lattice compatibility, complemented by nanosecond-resolution optical characterization techniques to confirm significantly reduced interfacial recombination and enhanced carrier lifetimes in Th-2EPT-Perovskite films. With Th-2EPT, we demonstrated the first SAM-based tin perovskite solar cells to outperform PEDOT-based devices, delivering a record power conversion efficiency (PCE) of 8.2% with a DMSO-free solvent system.

cond-mat.mtrl-sci↗

arXiv2025

Boron Clusters for Metal-Free Water Splitting

Electron-deficient boron clusters are identified as a fundamentally new class of oxygen evolution reaction (OER) catalysts, entirely free of transition metals. Selective sodium extraction from NaAlB14 and Na2B29 via high-pressure diffusion control introduces hole doping into B12 icosahedral frameworks, resulting in OER activity exceeding that of Co3O4 by more than an order of magnitude, and exceptional durability under alkaline conditions. B12 clusters are known for their superchaotropic character, which destabilizes hydrogen bonding in water. In this system, H2O, instead of OH-, preferentially adsorbs on the catalyst surface, suggesting a distinct OER pathway mediated by molecular water. This adsorption behavior contrasts with conventional transition-metal oxides and reflects the unique interfacial properties of the boron clusters. Density functional theory reveals unoccupied p orbitals and unique local electric fields at the cluster surface, both of which could promote the water activation. These findings suggest a paradigm shift in OER catalysis, in which the unique interaction between B12 clusters and water drives the reaction, replacing the conventional role of redox-active metals. Hole-doped boron clusters thus offer a promising platform for designing high-performance and durable water-splitting catalysts, opening new avenues for OER design beyond conventional transition-metal chemistry.

cond-mat.mtrl-sci↗

arXiv2024

Inverted band gap trend through octahedral ordering in Cs$_2$Au$_2$X$_6$ (X=Cl, Br, I)

Double perovskites Cs$_2$Au$_2$X$_6$ (X=Cl, Br, I) are prototypical materials that exhibit charge disproportionation of gold into 1+ and 3+ states. It is known that the disproportionation is resolved under high pressures, and this has stimulated many studies into the pressurization of these materials. At present, the phase changes in these materials are still strongly contested. Here, we use density functional theory to study the pressure-dependent behavior of Cs$_2$Au$_2$X$_6$. We find that a tetragonal--cubic transition occurs directly from the ground state $I4/mmm$ structure. Even so, we also found an intermediate tetragonal $P4/mmm$ structure to be very close in energy, suggesting it to be observable. We also find several other competing metastable phases, which explains some of the controversies in the literature. Focusing on one of the metastable phases, we suggest that Cs$_2$Au$_2$X$_6$ can be prepared in a $P4_2/mnm$ structure, analogous to that of KCuF$_3$. The band gap in the $P4_2/mnm$ structure widened as atomic number of the halide was increased, which is the inverse trend compared to the ground state structure. We explain this by the different octahedral distortion ordering in the two structural phases. Furthermore, we show that the conduction band in $P4_2/mnm$ is three dimensionally connected, which is favorable for opto-electronic applications. We submit that this work demonstrates that octahedral distortion ordering is a promising avenue for developing new double perovskites and suggests it to be particular effective in tuning the electronic structure properties.

cond-mat.mtrl-sci↗

arXiv2022

Switchable Electric Dipole from Polaron Localization in Dielectric Crystals

Ferroelectricity in crystals is associated with the displacement of ions or rotations of polar units. Here we consider the dipole created by donor doping ($D^+$) and the corresponding bound polaron ($e^-$).A dipole of 6.15 Debye is predicted, from Berry phase analysis, in the Ruddlesden-Popper phase of ${\rm Sr_3Ti_2O_7}$. A characteristic double-well potential is formed, which persists for high doping densities. The effective Hubbard $U$ interaction can vary the defect state from metallic, a two-dimensional polaron, through to a zero-dimensional polaron. The ferroelectric-like behavior reported here is localized and distinct from conventional spontaneous lattice polarization.

cond-mat.mtrl-sci↗

arXiv2020

Modelling the dielectric constants of crystals using machine learning

The relative permittivity of a crystal is a fundamental property that links microscopic chemical bonding to macroscopic electromagnetic response. Multiple models, including analytical, numerical and statistical descriptions, have been made to understand and predict dielectric behaviour. Analytical models are often limited to a particular type of compounds, whereas machine learning (ML) models often lack interpretability. Here, we combine supervised ML, density functional perturbation theory, and analysis based on game theory to predict and explain the physical trends in optical dielectric constants of crystals. Two ML models, support vector regression and deep neural networks, were trained on a dataset of 1,364 dielectric constants. Shapley additive explanations (SHAP) analysis of the ML models reveals that they recover correlations described by textbook Clausius-Mossotti and Penn models, which gives confidence in their ability to describe physical behavior, while providing superior predictive power.

cond-mat.mtrl-sci↗