Kazunari Yamaura

A ferroelectric-like structural transition in a metal

Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric (R3c) to non-centrosymmetric (R3c) transition in metallic LiOsO3 that is structurally equivalent to the ferroelectric transition of LiNbO3 (ref. 3). The transition involves a continuous shift in the mean position of Li(+) ions on cooling below 140 K. Its discovery realizes the scenario described in ref. 2, and establishes a new class of materials whose properties may differ from those of normal metals.

High-temperature ferrimagnetism driven by lattice distortion in double perovskite Ca2FeOsO6.

5d and 3d hybrid solid-state oxide Ca2FeOsO6 crystallizes into an ordered double-perovskite structure with a space group of P2₁/n with high-pressures and temperatures. Ca2FeOsO6 presents a long-range ferrimagnetic transition at a temperature of ~320 K (T(c)) and is not a band insulator, but is electrically insulating like the recently discovered Sr2CrOsO6 (T(c) ~725 K). The electronic stat of Ca2FeOsO6 is adjacent to a half-metallic state as well as that of Sr2CrOsO6. In addition, the high-T(c) ferrimagnetism was driven by lattice distortion, which was observed for the first time among double-perovskite oxides and represents complex interplays between spins and orbitals. Unlike conventional ferrite and garnet, the interplays likely play a pivotal role of the ferrimagnetism. A new class of 5d-3d hybrid ferrimagnetic insulators with high-T(c) is established to develop practically and scientifically useful spintronic materials.

High-pressure high-temperature transitions in MgCr2O4 and crystal structures of new Mg2Cr2O5 and post-spinel MgCr2O4 phases with implications for ultrahigh-pressure chromitites in ophiolites

Abstract We determined phase relations in MgCr2O4 at 12-28 GPa and 1000-1600 °C using a multi-anvil apparatus. At 12-15 GPa, spinel-type MgCr2O4 (magnesiochromite) first decomposes into a mixture of new Mg2Cr2O5 phase + corundum-type Cr2O3 at 1100-1600 °C, but it dissociates first into MgO periclase + corundum-type Cr2O3 at l000 °C. At about 17-19 GPa, the mixture of Mg2Cr2O5 phase + corundum-type Cr2O3 transforms to a single MgCr2O4 phase. Structure refinements using synchrotron X-ray powder diffraction data indicated that the high-pressure MgCr2O4 phase has a CaTi2O4-type structure (Cmcm), and that the basic structure of the Mg2Cr2O5 phase is the same as that of recently found modified ludwigite-type Mg2Al2O5 and Fe2Cr2O5 (Pbam). The phase relations in this study may suggest that natural chromitites in the Luobusa ophiolite regarded as the deep-mantle origin were derived from the mantle shallower than the depths corresponding to pressure of 12-15 GPa because of absence of the assemblage of (Mg,Fe)2Cr2O5 + Cr2O3 in the chromitites.

High-pressure synthesis of 5d cubic perovskite BaOsO3 at 17 GPa: ferromagnetic evolution over 3d to 5d series.

In continuation of the series of perovskite oxides that includes 3d(4) cubic BaFeO3 and 4d(4) cubic BaRuO3, 5d(4) cubic BaOsO3 was synthesized by a solid-state reaction at a pressure of 17 GPa, and its crystal structure was investigated by synchrotron powder X-ray diffraction measurements. In addition, its magnetic susceptibility, electrical resistivity, and specific heat were measured over temperatures ranging from 2 to 400 K. The results establish a series of d(4) cubic perovskite oxides, which can help in the mapping of the itinerant ferromagnetism that is free from any complication from local lattice distortions for transitions from the 3d orbital to the 5d orbital. Such a perovskite series has never been synthesized at any d configuration to date. Although cubic BaOsO3 did not exhibit long-range ferromagnetic order unlike cubic BaFeO3 and BaRuO3, enhanced feature of paramagnetism was detected with weak temperature dependence. Orthorhombic CaOsO3 and SrOsO3 show similar magnetic behaviors. CaOsO3 is not as conducting as SrOsO3 and BaOsO3, presumably due to impact of tilting of octahedra on the width of the t2g band. These results elucidate the evolution of the magnetism of perovskite oxides not only in the 5d system but also in group 8 of the periodic table.

Superconductivity in Bismuth Oxysulfide Bi4O4S3

Bismuth oxysulfide Bi4O4S3, which has recently been claimed to be an exotic superconductor (Tc = 4.5 K), was investigated by magnetic susceptibility and electrical resistivity measurements as well as by electron probe microanalysis. Single-phase Bi4O4S3 was successfully prepared by a high-pressure method, and its lattice parameters and normal-state resistivity, as well as the density of states at the Fermi level, were found to be comparable to those determined earlier. However, the observed superconductivity was most likely impurity-driven, strictly contradictory to the observations in ongoing experiments. The present results indicate that the superconductivity of Bi4O4S3 does not truly reflect the bulk nature of the BiS2 layered phase, regardless of the manner in which Bi4O4S3 is synthesized. We discuss possible superconducting impurities.

Extended Ni(III) oxyhalide perovskite derivatives: Sr2NiO3X (X = F, Cl).

Extended layered oxyhalide compounds, Sr2NiO3X (X = F, Cl), with the square pyramidal coordination around the trivalent nickel ions in the low spin state (S = 1/2), are successfully synthesized by a high-pressure and high-temperature reaction. Both these compounds crystallize in the n = 1 Ruddlesden-Popper type structure, but the difference of halogen anions incorporated dictate the anion-site ordering patterns and the magnetic ground states. Sr2NiO3F adopts the tetragonal cell in the space group I4/mmm (a = b = 3.79125(2) Å and c = 13.13754(9) Å), with O/F anions being disordered at the apical sites, while the crystal structure of Sr2NiO3Cl is described in the tetragonal space group P4/nmm (a = b = 3.85566(1) Å and c = 14.43240(6) Å) with O/Cl anions being fully ordered at the apical sites. Additionally, Sr2NiO3Cl undergoes a long-range antiferromagnetic order below TN = 33 K, while the fluorine counterpart does not exhibit a long-range ordering but spin glass transition at T(SG) = 11 K. In light of the positive Weiss temperatures for both X = F and Cl, the unpaired electron likely occupies a d(xy) orbital. Namely, the superexchange interaction mediated by d(xy)-Opπ-d(xy) in the NiO2 basal plane is antiferromagnetic, while the direct exchange interaction between d(xy)-d(xy) along the diagonal directions is ferromagnetic. The origin of spin glass behavior observed in X = F is probably due to randomness of the direct d(xy)-d(xy) bonds caused by off-centering nickel ions and O/F site disordering.

Direct observation of the depairing current density in single-crystalline Ba0.5K0.5Fe2As2 microbridge with nanoscale thickness

We investigated the critical current density (Jc) of Ba0.5K0.5Fe2As2 single-crystalline microbridges with thicknesses ranging from 276 to 18u2009nm. The Jc of the microbridge with thickness down to 91u2009nm is 10.8 MA/cm2 at 35u2009K, and reaches 944.4 MA/cm2 by extrapolating Jc(T) to Tu2009=u20090u2009K using a two-gap s-wave Ginzburg-Landau model, well in accordance with the depairing current limit. The temperature, magnetic field, and angular-dependence of Jc(T,H,θ) indicated weaker field dependence and weakly anisotropic factor of 1.15 (1u2009T) and 1.26 (5u2009T), which also yielded the validity of the anisotropic Ginzburg-Landau scaling.

Unusual magnetic hysteresis and the weakened transition behavior induced by Sn substitution in Mn3SbN

Substitution of Sb with Sn was achieved in ferrimagnetic antiperovskite Mn3SbN. The experimental results indicate that with an increase in Sn concentration, the magnetization continuously decreases and the crystal structure of Mn3Sb1-xSnxN changes from tetragonal to cubic phase at around x of 0.8. In the doping series, step-like anomaly in the isothermal magnetization was found and this behavior was highlighted at xu2009=u20090.4. The anomaly could be attributed to the magnetic frustration, resulting from competition between the multiple spin configurations in the antiperovskite lattice. Meantime, Hc of 18 kOe was observed at xu2009=u20090.3, which is probably the highest among those of manganese antiperovskite materials reported so far. With increasing Sn content, the abrupt change of resistivity and the sharp peak of heat capacity in Mn3SbN were gradually weakened. The crystal structure refinements indicate the weakened change at the magnetic transition is close related to the change of c/a ratio variation from tetrago...

Synthesis, crystal structure, and electronic properties of high-pressure PdF2-type oxides MO2 (M = Ru, Rh, Os, Ir, Pt).

The polycrystalline MO2s (HP-PdF2-type MO2, M = Rh, Os, Pt) with high-pressure PdF2 compounds were successfully synthesized under high-pressure conditions for the first time, to the best of our knowledge. The crystal structures and electromagnetic properties were studied. Previously unreported electronic properties of the polycrystalline HP-PdF2-type RuO2 and IrO2 were also studied. The refined structures clearly indicated that all compounds crystallized into the HP-PdF2-type structure, M(4+)O(2-)2, rather than the pyrite-type structure, M(n+)(O2)(n-) (n < 4). The MO2 compounds (M = Ru, Rh, Os, Ir) exhibited metallic conduction, while PtO2 was highly insulating, probably because of the fully occupied t2g band. Neither superconductivity nor a magnetic transition was detected down to a temperature of 2 K, unlike the case of 3d transition metal chalcogenide pyrites.

Size dependence of structural, magnetic, and electrical properties in corundum-type Ti2O3 nanoparticles showing insulator–metal transition

Abstract Corundum-type Ti2O3 has been investigated over the last half century because it shows unusual insulator–metal (I-M) transition over a broad temperature range (420–550 K). In this work, we successfully synthesized Ti2O3 nanoparticles (20, 70, 300 nm in size) by the low-temperature reduction between precursors of rutile-type TiO2 and the reductant CaH2, in a non-topotactic manner. The reaction time required for obtaining the reduced phase increases with increasing the particle size. Synchrotron X-ray powder diffraction and electron microscopy studies reveal that the symmetry of all the present samples remains the same as that of bulk samples. However, the particle-size reduction results in three important features compared with bulk samples as follows, (i) color shift from dark brown to bluish black, (ii) anisotropic volume contraction involving the shrinkage of Ti–Ti bonds in the ab plane and along the c axis, (iii) reduction of the I-M transition temperature from 420 K to 350 K. These suggest that the a1g band broadening caused by the surface strain effects, which favors narrowing of the band gap, may play a critical role in the suppression of IM transition.