Crystal structure and properties of perovskite-type rubidium niobate, a high-pressure phase of RbNbO3

We synthesized a perovskite-type RbNbO3 at 1173 K and 4 GPa from non-perovskite RbNbO3 and investigated its crystal structure and properties towards ferroelectric material design. Single-crystal X-ray diffraction analysis revealed an orthorhombic cell in the perovskite-type structure (space group Amm2, no. 38) with a = 3.9937(2) Å, b = 5.8217(3) Å, and c = 5.8647(2) Å. This non-centrosymmetric space group is the same as the ferroelectric BaTiO3 and KNbO3 but with enhanced distortion. Structural transition from orthorhombic to two successive tetragonal phases (Tetra1 at 493 K, Tetra2 at 573 K) was observed, maintaining the perovskite framework before reverting to the triclinic ambient phase at 693 K, with no structural changes between 4 and 300 K. The first transition is similar to that of KNbO3, whereas the second to Tetra2, marked by c-axis elongation and a significant cp/ap ratio jump (from 1.07 to 1.43), is unique. This distortion suggests a transition similar to that of PbVO3, where an octahedron's oxygen separates along the c-axis, forming a pyramid. Ab initio calculations simulating negative pressure like thermal expansion predicted this phase transition (cp/ap = 1.47 at -1.2 GPa), aligning with experimental findings. Thermal analysis revealed two endothermic peaks, with the second transition entailing a greater enthalpy change and volume alteration. Strong second harmonic generation signals were observed across Ortho, Tetra1, and Tetra2 phases, similar to BaTiO3 and KNbO3. Permittivity increased during the first transition, although the second transition's effects were limited by thermal expansion-induced bulk sample collapse. Perovskite-type RbNbO3 emerges as a promising ferroelectric material.

J_{eff}=1/2 Hyperoctagon Lattice in Cobalt Oxalate Metal-Organic Framework

We report the magnetic properties of a cobalt oxalate metal-organic framework featuring the hyperoctagon lattice. Our thermodynamic measurements reveal the J_{eff}=1/2 state of the high-spin Co^{2+} (3d^{7}) ion and the two successive magnetic transitions at zero field with two-stage entropy release. ^{13}C-NMR measurements reveal the absence of an internal magnetic field in the intermediate temperature phase. Multiple field-induced phases are observed before full saturation at around 40 T. We argue the unique cobalt oxalate network gives rise to the Kitaev interaction and/or a bond frustration effect, providing an unconventional platform for frustrated magnetism on the hyperoctagon lattice.

Nanostructure-induced L10-ordering of twinned single-crystals in CoPt ferromagnetic nanowires

L1₀-ordered ferromagnetic nanowires with large coercivity are essential for realizing next-generation spintronic devices. Ferromagnetic nanowires have been commonly fabricated by first L1₀-ordering of initially disordered ferromagnetic films by annealing and then etching them into nanowire structures using lithography. If the L1₀-ordered nanowires can be fabricated using only lithography and subsequent annealing, the etching process can be omitted, which leads to an improvement in the fabrication process for spintronic devices. However, when nanowires are subjected to annealing, they easily transform into droplets, which is well-known as Plateau-Rayleigh instability. Here, we propose a concept of "nanostructure-induced L1₀-ordering" of twinned single-crystals in CoPt ferromagnetic nanowires with a 30 nm scale ultrafine linewidth on Si/SiO₂ substrates. The driving forces for nanostructure-induced L1₀-ordering during annealing are atomic surface diffusion and extremely large internal stress at ultrasmall 10 nm scale curvature radii of the nanowires. (Co/Pt)₆ multilayer nanowires are fabricated by a lift-off process combining electron-beam lithography and electron-beam evaporation, followed by annealing. Cross-sectional scanning transmission electron microscope images and nano-beam electron diffraction patterns clearly indicate nanostructure-induced L1₀-ordering of twinned single-crystals in the CoPt ferromagnetic nanowires, which exhibit a large coercivity of 10 kOe for perpendicular, longitudinal, and transversal directions of the nanowires. Two-dimensional grazing incidence X-ray diffraction shows superlattice peaks with Debye-Scherrer ring shapes, which also supports the nanostructure-induced L1₀-ordering. The fabrication method for nanostructure-induced L1₀-ordered CoPt ferromagnetic nanowires with twinned single-crystals on Si/SiO₂ substrates would be significant for future silicon-technology-compatible spintronic applications.

Electronic and crystal structures ofLnFeAsO1-xHx(Ln= La, Sm) studied by x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction: II pressure dependence

We examine electronic and crystal structures of iron-based superconductorsLnFeAsO_1-xH_x(Ln= La, Sm) under pressure by means of x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES), and x-ray diffraction. In LaFeAsO the pre-edge peak on high-resolution XAS at the Fe-Kabsorption edge gains in intensity on the application of pressure up to 5.7 GPa and it saturates in the higher pressure region. We found integrated-absolute difference values on XES forLn= La, corresponding to a spin state, decline on the application of pressure, and then it is minimized when theT_capproaches the maximum at around 5 GPa. In contrast, such the optimum value was not detected forLn= Sm. We reveal that the superconductivity is closely related to the lower spin state forLn= La unlike Sm case. We observed that As height from the Fe basal plane and As-Fe-As angle on the FeAs₄tetrahedron forLn= La deviate from the optimum values of the regular tetrahedron in superconducting (SC) phase, which has been widely accepted structural guide to SC thus far. In contrast, the structural parameters were held near the optimum values up to ∼15 GPa forLn= Sm.

Electronic and crystal structures ofLnFeAsO1-xHx(Ln= La, Sm) studied by x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction (part I: carrier-doping dependence)

A carrier doping by a hydrogen substitution in LaFeAsO_1-xH_xis known to cause two superconducting (SC) domes with the magnetic order at both end sides of the doping. In contrast, SmFeAsO_1-xH_xhas a similar phase diagram but shows single SC dome. Here, we investigated the electronic and crystal structures for iron oxynitrideLnFeAsO_1-xH_x(Ln= La, Sm) with the range ofx= 0-0.5 by using x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction. For both compounds, we observed that the pre-edge peaks of x-ray absorption spectra near the Fe-Kedge were reduced in intensity on doping. The character arises from the weaker As-Fe hybridization with the longer As-Fe distance in the higher doped region. We can reproduce the spectra near the Fe-Kedge according to the Anderson impurity model with realistic valence structures using the local-density approximation (LDA) plus dynamical mean-field theory (DMFT). ForLn= Sm, the integrated-absolute difference (IAD) analysis from x-ray Fe-Kβemission spectra increases significantly. This is attributed to the enhancement of magnetic moment of Fe 3delectrons stemming from the localized picture in the higher doped region. A theoretical simulation implementing the self-consistent vertex-correction method reveals that the single dome superconducting phase forLn= Sm arises from a better nesting condition in comparison withLn= La.

Reversible 3D-2D structural phase transition and giant electronic modulation in nonequilibrium alloy semiconductor, lead-tin-selenide

Material properties depend largely on the dimensionality of the crystal structures and the associated electronic structures. If the crystal-structure dimensionality can be switched reversibly in the same material, then a drastic property change may be controllable. Here, we propose a design route for a direct three-dimensional (3D) to 2D structural phase transition, demonstrating an example in (Pb_1-x Sn _x )Se alloy system, where Pb²⁺ and Sn²⁺ have similar ns² pseudo-closed shell configurations, but the former stabilizes the 3D rock-salt-type structure while the latter a 2D layered structure. However, this system has no direct phase boundary between these crystal structures under thermal equilibrium. We succeeded in inducing the direct 3D-2D structural phase transition in (Pb_1-x Sn _x )Se alloy epitaxial films by using a nonequilibrium growth technique. Reversible giant electronic property change was attained at x ~ 0.5 originating in the abrupt band structure switch from gapless Dirac-like state to semiconducting state.

Polar nano-region structure in the oxynitride perovskite LaTiO2N

We investigated the multiscale characters of the crystal structure of the oxynitride perovskite LaTiO2N. While X-ray diffraction results identified the average structure as being centrosymmetric, we detected a signature of unknown structural deformation. By viewing the local structure, we unveiled the formation of a polar structure at the nanoscale.

Structural and Thermal Properties in Formamidinium and Cs-Mixed Lead Halides

Formamidinium [FA, HC(NH₂)₂⁺] lead iodide and its cation mixture have attracted interest as potentials in applications for efficient solar cells superior to well-known methylammonium lead iodide. We investigated the crystal structure and thermodynamic properties of high-quality single crystals of FA_1-xCs_xPbI₃ for x = 0 and 0.1 through X-ray diffraction and heat capacity measurements. Both α-FA_0.9Cs_0.1PbI₃ as well as α-FAPbI₃ crystallize in a cubic Pm3̅m structure with orientationally disordered FA molecules confined in the nondistorted Pb-I framework. In FAPbI₃, we observed a second-order transition at 280 K and two first-order transitions at 141.2 and 130.2 K in between β- and γ-phases instead of the previously known single β-γ transition. After doping with 10% Cs, the multiple first-order transitions disappeared, leading to phase transitions emerging at 300 and 149 K with second-order character. We moreover observed low-energy localized modes for both compounds, which is presumably tied to anomalous thermal motion, rattling, of the FA molecule.

Orbital Transitions and Frustrated Magnetism in the Kagome-Type Copper Mineral Volborthite

Volborthite Cu₃V₂O₇(OH)₂·2H₂O is a copper mineral that materializes a two-dimensional quantum magnet comprising a kagome net of spin-¹/₂ Cu²⁺ ions. We prepared single crystals of volborthite using hydrothermal conditions and investigated their crystal structures and magnetic properties. Unusual orbital "switching" and "flipping" transitions were observed: in the former type of transition (switching), the Cu 3d orbital occupied by an unpaired electron changes between the d(3z²-r²) and d(x²-y²) types, and in the latter type of transition (flipping), the d(x²-y²)-type orbitals change their directions. Their origin is ascribed to variations in the orientation of water molecules in the gap between the kagome layers and the accompanying changes of hydrogen bonding. These orbital transitions dramatically modify the magnetic interactions between Cu²⁺ spins, from the anisotropic kagome type to the formation of spin trimers over the kagome net. The effective spin ¹/₂ generated on the trimers exhibits a frustrated magnetism, resulting in a rich phase diagram in the magnetic fields. Volborthite is a unique compound showing an exceptional interplay between the orbital and spin degrees of freedom.

Robust ferromagnetism carried by antiferromagnetic domain walls

Ferroic materials, such as ferromagnetic or ferroelectric materials, have been utilized as recording media for memory devices. A recent trend for downsizing, however, requires an alternative, because ferroic orders tend to become unstable for miniaturization. The domain wall nanoelectronics is a new developing direction for next-generation devices, in which atomic domain walls, rather than conventional, large domains themselves, are the active elements. Here we show that atomically thin magnetic domain walls generated in the antiferromagnetic insulator Cd₂Os₂O₇ carry unusual ferromagnetic moments perpendicular to the wall as well as electron conductivity: the ferromagnetic moments are easily polarized even by a tiny field of 1 mT at high temperature, while, once cooled down, they are surprisingly robust even in an inverse magnetic field of 7 T. Thus, the magnetic domain walls could serve as a new-type of microscopic, switchable and electrically readable magnetic medium which is potentially important for future applications in the domain wall nanoelectronics.

The magnetoresistance effect in a conducting molecular crystal consisting of dicyano(phthalocyaninato)manganese(iii)

A conducting molecular crystal TPP[Mn^III(Pc)(CN)₂]₂ (Mn^III: d⁴, S = 1, TPP = tetraphenylphosphonium and Pc = phthalocyanine) was fabricated. In its crystal structure, the [Mn^III(Pc)(CN)₂] units formed a one-dimensional regular chain along the c-axis with an overlap integral value of 8.6 × 10^-3. TPP[Mn^III(Pc)(CN)₂]₂ showed a semiconducting behaviour that also has been observed for isostructural TPP[Co^III(Pc)(CN)₂]₂ (Co^III: d⁶, S = 0) and TPP[Fe^III(Pc)(CN)₂]₂ (Fe^III: d⁵, S = 1/2) whose ground states are charge-ordered states. In spite of the local magnetic moment of the Mn^III ion (S = 1) at the centre of the Pc ligand, TPP[Mn^III(Pc)(CN)₂]₂ exhibited an almost isotropic and small negative magnetoresistance (MR) effect (the MR ratio was -8.7% under 8 T at 10.7 K), contrarily to the anisotropic giant negative MR effect of TPP[Fe^III(Pc)(CN)₂]₂. The isotropy was found to be due to a (d_xz)¹(d_yz)¹ electronic configuration, and the smaller MR effect was explained by a weaker antiferromagnetic interaction between Mn^III ions than that between Fe^III ions, as suggested by a Weiss temperature Θ of -3.1 K (|J_dd|/k_B = 1.2 K).

Discovery of ferromagnetic-half-metal-to-insulator transition in K2Cr8O16

The hollandite chromium oxide K2Cr8O16 has been synthesized in both powder and single-crystal form under high pressure. Combining electrical resistivity, magnetic susceptibility, and x-ray diffraction, we found that K2Cr8O16 is a ferromagnetic metal (or half-metal) with T(C)=180 K and shows a transition to an insulator at 95 K without any apparent structural change but retaining ferromagnetism. K2Cr8O16 is quite unique in three aspects: It has a rare mixed valence of Cr3+ and Cr4+; it has a metal (or half-metal)-to-insulator transition in a ferromagnetic state; and the resulting low-temperature phase is a rare case of a ferromagnetic insulator. This discovery could open a new frontier on the relation of magnetism and conducting properties in strongly correlated electron systems.

Checkerboard-Type Charge-Ordered State of a Pressure-Induced Superconductor, β-(meso-DMBEDT-TTF)2PF6

To investigate the insulating state of the pressure-induced superconductor, beta-(meso-DMBEDT-TTF)2PF6, we have carried out X-ray analysis at 11.5 K. In an asymmetric unit, there exist two crystallographically independent donor molecules, caused by charge separation. In the column structure, the arrangement of the charge-rich (r) and -poor (p) donor molecules is as rrpprrpp, which affords "checkerboard"-type charge ordering.