Hiroshi Takatsu

Discovering novel magnetic materials with unique spin-lattices for exotic quantum phenomena is a significant challenge. Here, we successfully synthesized a new antiferromagnet, Cu₃O₂(HSeO₃)(HSO₄)(H₂O), using mixed-anions of trigonal-pyramidal SeO₃^2- and tetrahedral SO₄^2-. Cu₃O₂(HSeO₃)(HSO₄)(H₂O) consists of corrugated layers forming a rare T26-type non-Archimedean (2-uniform) lattice (Figure 1f), where diamond chains are interconnected through their dimers to form hexagons between spin chains. Despite a large Weiss-temperature θ of -355(1) K, Cu₃O₂(HSeO₃)(HSO₄)(H₂O) exhibits antiferromagnetic order at 2.3 K, indicating strong quantum spin fluctuations. Interestingly, high-field magnetization measurement shows a robust 1/3-magnetization plateau that remains stable up to 54 T. This phenomenon was not theoretically predicted for regular T26-type non-Archimedean lattices. Notably, Cu₃O₂(HSeO₃)(HSO₄)(H₂O) represents the first example of all reported non-Archimedean lattices featuring a robust 1/3-magnetization plateau. Our DFT calculation results reveal that the robust 1/3-magnetization plateau arises from the fact that the distorted T26-type non-Archimedean lattice of Cu₃O₂(HSeO₃)(HSO₄)(H₂O) can be further simplified as a trimer with strong intratrimer interaction of -500 K, requiring a much higher magnetic field to overcome for full saturation. Our work demonstrates that utilizing mixed anions as bridging pathways to regulate magnetic couplings is a promising approach for discovering novel spin lattices with quantum phenomena.

Polar metals, characterized by the nontrivial coexistence of metallicity and polar structural order, define an emerging frontier in quantum materials research. However, the interplay between their structural phase transitions and fluctuation dynamics remains poorly understood. Here, we reveal distinct diffusive dynamics in metallic lithium rhenium trioxide (LiReO ₃ ) associated with its polar-to-nonpolar transition. Unlike isostructural lithium niobate (LiOsO ₃ ) and related systems, LiReO ₃ exhibits pronounced phase fluctuations both above and below T _s . Thermoelectric, Raman, and ultrasound measurements demonstrate a probe-dependent thermal hysteresis, while ultrasound data further show lattice softening and persistent resonant absorption at low temperatures across a broad timescale (1 to 100 microseconds). These observations indicate a multiscale spatiotemporal dynamics governed by a shallow anharmonic potential stabilized by itinerant electrons, as supported by finite-temperature first-principles calculations. By mapping the fluctuation landscape shaped by itinerant electrons, this work offers a previously unexplored perspective for exploiting fluctuation-driven phenomena in polar metals.

The controversial claim of near-ambient superconductivity in anion-excess fluorite-type nitrogen-doped lutetium hydride (LuH_3-δN_ε) has renewed interest in mixed-anion hydrides as platforms for emergent electronic properties, while highlighting the need for systems with well-defined structural and compositional control. Here, we report the high-pressure synthesis of lanthanum nitride-hydride La₂H_3-xN at 5 GPa. The compound crystallizes in a layered anion-ordered Ba₂H₃Cl-type (P-3m1) structure derived from anion-excess fluorite-type LaH₃ via a 3:1 substitution of H^- by N^3-, accompanied by a reconstruction of the close-packed La sublattice. This pressure-stabilized phase can be accessed only under hydrogen-poor conditions, resulting in intrinsic hydrogen vacancies (∼23%), corresponding to La₂H_2.3N. These hydride vacancies can be further filled by a subsequent topochemical H^- intercalation under H₂ gas pressure after releasing the mechanical pressure, thereby driving a metal-semiconductor transition. This combination of high-pressure synthesis and post-synthetic topochemical intercalation provides a powerful yet underexplored route to access pressure-stabilized mixed-anion frameworks with precisely tunable anion stoichiometry. In addition, Nd and Gd analogues were synthesized at 8 GPa, and density functional theory calculations indicate that chemical pressure induced by lanthanide substitution systematically tunes the band structures from semiconducting to semimetallic regimes.

Valence transitions in europium compounds, such as EuO and EuNi₂(Si_1-xGe_x)₂, usually induce substantial structural distortions because the ionic radius difference between Eu²⁺ (1.25 Å) and Eu³⁺ (1.06 Å) exceeds the Hume-Rothery tolerance. Here, we conducted valence-specific structural analysis of strained EuVO₂H thin films using X-ray fluorescence holography (XFH), where intersite charge transfer between Eu²⁺ and V³⁺ converts ∼30% of Eu²⁺ into Eu³⁺ (and V³⁺ into V²⁺). XFH measurements at the Eu L-edge revealed that a 15 nm single-crystalline film maintains remarkable structural coherence, with Eu²⁺ and Eu³⁺ randomly distributed and showing minimal local distortion despite the >15% size mismatch. This unusual stability is attributed to the size flexibility of H^- anions in the EuH layers, which act as structural buffers. The ability to accommodate mixed-valence states without introducing structural disorder likely underlies the enhanced magnetic anisotropy in EuVO₂H.

Topochemical reactions in transition metal oxides, typically involving oxygen removal or anion exchange, provide a versatile platform for creating metastable phases with diverse functionalities. While these reactions often modify the valence and coordination environment of transition metals, the underlying metal frameworks are usually preserved, maintaining a 1:1 structural correspondence. A representative example is SrFeO₃ → SrFeO₂, in which each octahedral layer transforms into a single square-planar layer. In this study, we report an unprecedented topochemical transformation in Mo₂Ta₂O₁₁, composed of alternating MoO₄ tetrahedral bilayers and TaO₆ octahedral bilayers. Ammonolysis at 500 °C in the presence of Mo(CO)₆ collapses the MoO₄ tetrahedral bilayer into a single MoO₆ octahedral layer, thereby breaking the conventional 1:1 structural correspondence. This collapse leads to an 18% contraction along the c axis, substantially exceeding the 10% contraction seen in SrFeO₂. The resulting compound, Mo₃Ta₂O₁₀N (space group R-3m), has an Mo-based kagomé lattice with finite d-electrons (Mo^4.33+), in sharp contrast to the d⁰ configuration of the precursor (Mo⁶⁺₂Ta⁵⁺₂O₁₁). Magnetic susceptibility and NMR measurements suggest that Mo₃Ta₂O₁₀N is an itinerant kagomé system. This study demonstrates that topochemical reactions can induce dynamic and extensive structural reorganizations, pushing the boundaries of what was previously considered accessible by such low-temperature routes.

Layered perovskite oxides with the Ruddlesden-Popper (RP) structure exhibit diverse functionalities, yet the synthesis of cobalt-based RP oxides is hindered by competing phase formation. In this study, we successfully synthesized Sr₄Co₃O₉, a triple-layered cobalt oxide with a layered brownmillerite structure, as an epitaxial thin film on a LaAlO₃ substrate. Soft X-ray absorption spectra confirmed oxygen nonstoichiometry and a mixed-valence state of Co^3.3+ while scanning transmission electron microscopy revealed the presence of a tetrahedral layer in the central perovskite block. The structural comparison indicates that unlike conventional brownmillerite A₂B₂O₅, the triple-layered A₄B₃O₉ phase features axially elongated octahedra with significant B-site off-centering, driven by Coulomb repulsion from the tetrahedral B cations. Density functional calculations suggest a charge-ordered antiferromagnetic state. The thermal power of Sr₄Co₃O₉ is lower compared to other layered cobalt oxides, such as Na_xCoO₂. Our results demonstrate the impact of structural modulation in layered brownmillerite oxides, offering a perspective for designing functional materials.