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We incorporated atomic force microscopy images of polymer : non-fullerene acceptor organic photovoltaics into machine learning, where fast Fourier transform and grey-level co-occurrence matrix were utilized to predict power conversion efficiencies.

We report the observation of superconductivity with multiple magnetic ordering and Rashba-type spin-orbit coupling in a layered polar semimetal EuAuBi. Magnetic transition is observed at 4 K, followed by a superconducting transition at 2.2 K, which is sensitive to the crystal surface conditions. The upper critical field Hc2 of 9.8 T for the out-of-plane field is three times higher than that for the in-plane field, which can be associated with the two-dimensional structure or the surface state. On the basis of first-principles calculations, it is found that the characteristic Hc2 possibly reflects the anisotropic modification of the Fermi surface by the effective combination of Rashba-type spin splitting and Zeeman spin splitting enhanced by Eu moments., 12 pages, 5 figures

Given the rarity of metallic systems that exhibit ferroelectric-like transitions, it is apparently challenging to find a system that simultaneously possesses superconductivity and ferroelectric-like structural instability. Here, we report the observation of superconductivity at 2.4 K in a layered semimetal SrAuBi characterized by strong spin-orbit coupling (SOC) and ferroelectric-like lattice distortion. Single crystals of SrAuBi have been successfully synthesized and found to show a polar-nonpolar structure transition at 214 K, which can be associated with the buckling of Au-Bi honeycomb lattice. On the basis of the band calculations considering SOC, we found significant Rashba-type spin splitting and symmetry-protected multiple Dirac points near the Fermi level. We believe that this discovery opens up new possibilities of pursuing exotic superconducting states and relevant novel responses inherent to the strong SOC and topological band structure.

Two-dimensional magnetic materials have recently attracted great interest due to their unique functions as the electric field control of a magnetic phase and the anomalous spin Hall effect. For such remarkable functions, a spin-orbit coupling (SOC) serves as an essential ingredient. Here we report a giant positive magnetoresistance in a layered magnetic semiconductor AgCrSe2, which is a manifestation of the subtle combination of the SOC and Zeeman-type spin splitting. When the carrier concentration approaches the critical value of 2.5\times10^18 cm^-3, a sizable positive magnetoresistance of ~400 % emerges upon the application of magnetic fields normal to the conducting layers. Based on the magneto-Seebeck effect and the first-principles calculations, the unconventional magnetoresistance is ascribable to the enhancement of effective carrier mass in the SOC induced J = 3/2 state, which is tuned to the Fermi level through the Zeeman splitting enhanced by the p-d coupling. This study demonstrates a new aspect of the SOC-derived magnetotransport in two-dimensional magnetic semiconductors, paving the way to novel spintronic functions., Comment: 8 papges, 5 figures

We systematically synthesized perovskite-type oxides Sr1-xCaxCoO3 containing unusually high valence Co4+ ions by a high pressure technique, and investigated the effect of systematic lattice change on the magnetic and electronic properties. As the Ca content x exceeds about 0.6, the structure changes from cubic to orthorhombic, which is supported by the first-principles calculations of enthalpy. Upon the orthorhombic distortion, the ground state remains to be apparently ferromagnetic with a slight drop of the Curie temperature. Importantly, the compounds with x larger than 0.8 show antiferromagnetic behavior with positive Weiss temperatures and nonlinear magnetization curves at lowest temperature, implying that the ground state is noncollinear antiferromagnetic or helimagnetic. Considering the incoherent metallic behavior and the suppression of the electronic specific heat at high x region, the possible emergence of a helimagnetic state in Sr1-xCaxCoO3 is discussed in terms of the band-width narrowing and the double-exchange mechanism with the negative charge transfer energy as well as the spin frustration owing to the next-nearest neighbor interaction., Comment: 13 pages, 4 figures

Single crystal and polycrystalline samples of MoP$_4$ with a black-phosphorus-derived structure have been successfully synthesized by a high-pressure technique. The polycrystalline samples show a large positive magnetoresistance and a small negative Seebeck coefficient at low temperatures, reflecting a semi-metallic nature with high-mobility electrons. Consistent with the transport properties, the band structure calculation reveals a semi-metallic state with the presence of two types of Dirac nodes slightly below the Fermi level. The Dirac node along the $\Gamma$-X direction normal to the phosphorus layers is gapped out in the presence of spin-orbit coupling (SOC), whereas the band crossing at the Z-point is immune to SOC because of the non-symmorphic symmetry. This work demonstrates a great potential of phosphorus-based layered Zintl compounds for topological semimetal candidates allowing chemical band engineering., Comment: 14 pages, 4 figures, 2 tables

A new perovskite-type cuprate PrCuO3 has been synthesized by high-pressure oxygen annealing. Synchrotron X-ray powder diffraction and absorption spectroscopy revealed that PrCuO3 crystallizes in the GdFeO3-type structure with cooperative Jahn–Teller distortion, forming one-dimensional chains of corner-shared CuO4 plaquettes with nearly divalent Cu ions.

The control of acoustic phonons, which are the carriers of sound and heat, has become the focus of increasing attention because of a demand for manipulating the sonic and thermal properties of nanometric devices. In particular, the photoacoustic effect using ultrafast optical pulses has a promising potential for the optical manipulation of phonons in the picosecond time regime. So far, its mechanism has been mostly based on the commonplace thermoelastic expansion in isotropic media, which has limited applicability. In this study, we investigate a conceptually new mechanism of the photoacoustic effect involving a structural instability that utilizes a transition-metal dichalcogenide VTe

Topologically nontrivial materials host protected edge states associated with the bulk band inversion through the bulk-edge correspondence. Manipulating such edge states is highly desired for developing new functions and devices practically using their dissipation-less nature and spin-momentum locking. Here we introduce a transition-metal dichalcogenide VTe$_2$, that hosts a charge density wave (CDW) coupled with the band inversion involving V3$d$ and Te5$p$ orbitals. Spin- and angle-resolved photoemission spectroscopy with first-principles calculations reveal the huge anisotropic modification of the bulk electronic structure by the CDW formation, accompanying the selective disappearance of Dirac-type spin-polarized topological surface states that exist in the normal state. Thorough three dimensional investigation of bulk states indicates that the corresponding band inversion at the Brillouin zone boundary dissolves upon CDW formation, by transforming into anomalous flat bands. Our finding provides a new insight to the topological manipulation of matters by utilizing CDWs' flexible characters to external stimuli., Comment: 46 pages, 6 main figures, 12 supplementary figures

The competing magnetic ground states of the itinerant magnet EuCuSb, which has a hexagonal layered structure, were studied via magnetization, resistivity, and neutron diffraction measurements on single-crystal samples. EuCuSb has a three-dimensional semimetallic band structure as confirmed by band calculation and angle-resolved photoelectron spectroscopy, consistent with the nearly isotropic metallic conductivity in the paramagnetic state. However, below the antiferromagnetic transition temperature of TN1 (8.5 K), the resistivity, especially along the hexagonal axis, increases significantly. This implies the emergence of anisotropic magnetic ordering coupled to the conducting electrons. Neutron diffraction measurements show that the Eu spins, which order ferromagnetically within each layer, are collinearly modulated (up-up-down-down) along the hexagonal axis below TN1, followed by the partial emergence of helical spin modulation below TN2 (6 K). Based on the observation of anomalous magnetoresistance with hysteretic behavior, we discuss the competing nature of the ground state inherent in a frustrated Heisenberg-like spin system with a centrosymmetric structure., Comment: 12 pages, 4 figures, Physical review B in press

We found that a high mobility semimetal 1T'-MoTe2 shows a significant pressure-dependent change in the cryogenic thermopower in the vicinity of the critical pressure, where the polar structural transition disappears. With the application of a high pressure of 0.75 GPa, while the resistivity becomes as low as 10 {\mu}{\Omega}cm, thermopower reached the maximum value of 60 {\mu}VK-1 at 25 K, leading to a giant thermoelectric power factor of 300 {\mu}WK-2cm-1. Based on semiquantitative analyses, the origin of this behavior is discussed in terms of inelastic electron-phonon scattering enhanced by the softening of zone center phonon modes associated with the polar structural instability., Comment: 13 pages, 4 figures Physical review B (accepted)

A new perovskite-type cuprate PrCuO

The effects of Rh and Ru doping for SrFeO3, a helimagnetic metal with a cubic perovskite structure, are studied by magnetic and resistivity measurements. Although SrRhO3 is a paramagnetic metal and SrRuO3 is a ferromagnetic one, the Rh doping induces a nearly ferromagnetic metallic state, whereas the Ru doping induces a spin-glass insulating state. Mossbauer measurements evidence a marked difference between SrFe0.8Rh0.2O3 and SrFe0.8Ru0.2O3 in the formal valences of Fe, which are estimated to be 4+ and 3.75+, respectively. The contrasting magnetic behaviors of Rh- and Ru-doped SrFeO3 are discussed in terms of the subtle balance between the double-exchange ferromagnetism and the superexchange antiferromagnetism.

Helimagnetic materials are identified as promising for novel spintronic applications. Since helical spin order is manifested as a compromise of competing magnetic exchange interactions, its emergence is limited by unique constraints imposed by the crystalline lattice and the interaction geometries as exemplified by the multiferroic perovskite manganites with large orthorhombic distortion. Here we show that a simple cubic perovskite ${\mathrm{SrCoO}}_{3}$ with room-temperature ferromagnetism has the potential to host helimagnetic order upon isotropic lattice expansion. Increasing the Ba content $x$ in ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CoO}}_{3}$ continuously expands the cubic lattice, eventually suppressing the ferromagnetic order near $x=0.4$ where helimagnetic correlations are observed as incommensurate diffuse magnetic scattering by neutron-diffraction measurements. The emergence of helimagnetism is semiquantitatively reproduced by first-principles calculations, leading to the conjecture that a simple cubic lattice with strong $d\text{\ensuremath{-}}p$ hybridization can exhibit a variety of novel magnetic phases originating from competing exchange interactions.

We report spin-split Landau levels of quasi-two-dimensional Dirac fermions in a layered antiferromagnet EuMnBi$_2$, as revealed by interlayer resistivity measurements in a tilted magnetic field up to $\sim$35 T. The amplitude of Shubnikov-de Haas (SdH) oscillation in interlayer resistivity is strongly modulated by changing the tilt angle of the field, i.e., the Zeeman-to-cyclotron energy ratio. The effective $g$ factor estimated from the tilt angle, where the SdH oscillation exhibits a phase inversion, differs by approximately 50% between two antiferromagnetic phases. This observation signifies a marked impact of the magnetic order of Eu sublattice on the Dirac-like band structure. The origin may be sought in strong exchange coupling with the local Eu moments, as verified by the first-principles calculation., Comment: 13 pages, 4 figures, submitted

We investigated the effect of pressure on the magnetic and thermoelectric properties of Sr$_{3.1}$Y$_{0.9}$Co$_{4}$O$_{10+\delta }$. The magnetization is reduced with the application of pressure, reflecting the spin-state modification of the Co$^{3+}$ ions into the nonmagnetic low-spin state. Accordingly, with increasing pressure, the Seebeck coefficient is enhanced, especially at low temperatures, at which the effect of pressure on the spin state becomes significant. These results indicate that the spin-orbital entropy is a key valuable for the thermoelectric properties of the strongly correlated cobalt oxides., Comment: 5 pages, 5 figures

The relation between the polar structural instability and superconductivity in a Weyl semimetal candidate MoTe2 has been clarified by finely controlled physical and chemical pressure. The physical pressure as well as the chemical pressure, i.e., the Se substitution for Te, enhances the superconducting transition temperature Tc at around the critical pressure where the polar structure transition disappears. From the heat capacity and thermopower measurements, we ascribe the significant enhancement of Tc at the critical pressure to a subtle modification of the phonon dispersion or the semimetallic band structure upon the polar-to-nonpolar transition. On the other hand, the physical pressure, which strongly reduces the interlayer distance, is more effective on the suppression of the polar structural transition and the enhancement of Tc as compared with the chemical pressure, which emphasizes the importance of the interlayer coupling on the structural and superconducting instability in MoTe2., 5 pages, 4 figures

For the innovation of spintronic technologies, Dirac materials, in which the low-energy excitation is described as relativistic Dirac fermions, are one of the most promising systems, because of the fascinating magnetotransport associated with the extremely high mobility. To incorporate Dirac fermions into spintronic applications, their quantum transport phenomena are desired to be manipulated to a large extent by magnetic order in a solid. We here report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi$_2$, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions. In addition to the high mobility more than 10,000 cm$^2$/Vs, Landau level splittings presumably due to the lifting of spin and valley degeneracy are noticeable even in a bulk magnet. These results will pave a route to the engineering of magnetically functionalized Dirac materials., Comment: 26 pages, 4 figures, 1 table

Chirality--that is, left- or right-handedness--is an important concept in a broad range of scientific areas. In condensed matter, chirality is found not only in molecular or crystal forms, but also in magnetic structures. A magnetic skyrmion is a topologically stable spin vortex structure, as observed in chiral-lattice helimagnets, and is one example of such a structure. The spin swirling direction (skyrmion helicity) should be closely related to the underlying lattice chirality via the relativistic spin-orbit coupling. Here, we report on the correlation between skyrmion helicity and crystal chirality in alloys of helimagnets Mn(1-x)Fe(x)Ge with varying compositions by Lorentz transmission electron microscopy and convergent-beam electron diffraction over a broad range of compositions (x = 0.3-1.0). The skyrmion lattice constant shows non-monotonous variation with composition x, with a divergent behaviour around x = 0.8, where the correlation between magnetic helicity and crystal chirality changes sign. This originates from continuous variation of the spin-orbit coupling strength and its sign reversal in the metallic alloys as a function of x. Controllable spin-orbit coupling may offer a promising way to tune skyrmion size and helicity.

The electron mobility is one of the key parameters that characterize the charge-carrier transport properties of materials, as exemplified by the quantum Hall effect as well as high-efficiency thermoelectric and solar energy conversions. For thermoelectric applications, introduction of chemical disorder is an important strategy for reducing the phonon-mediated thermal conduction, but is usually accompanied by mobility degradation. Here, we show a multilayered semimetal β-CuAgSe overcoming such a trade-off between disorder and mobility. The polycrystalline ingot shows a giant positive magnetoresistance and Shubnikov de Haas oscillations, indicative of a high-mobility small electron pocket derived from the Ag s-electron band. Ni doping, which introduces chemical and lattice disorder, further enhances the electron mobility up to 90,000 cm(2) V(-1) s(-1) at 10 K, leading not only to a larger magnetoresistance but also a better thermoelectric figure of merit. This Ag-based layered semimetal with a glassy lattice is a new type of promising thermoelectric material suitable for chemical engineering.

Ferroelectrics with spontaneous electric polarization play an essential role in today's device engineering, such as capacitors and memories. Their physical properties are further enriched by suppressing the long-range polar order, as is exemplified by quantum paraelectrics with giant piezoelectric and dielectric responses at low temperatures. Likewise in metals, a polar lattice distortion has been theoretically predicted to give rise to various unusual physical properties. So far, however, a "ferroelectric"-like transition in metals has seldom been controlled and hence its possible impacts on transport phenomena remain unexplored. Here we report the discovery of anomalous enhancement of thermopower near the critical region between the polar and nonpolar metallic phases in 1T'-Mo$_{1-x}$Nb$_{x}$Te$_2$ with a chemically tunable polar transition. It is unveiled from the first-principles calculations and magnetotransport measurements that charge transport with strongly energy-dependent scattering rate critically evolves towards the boundary to the nonpolar phase, resulting in large cryogenic thermopower. Such a significant influence of the structural instability on transport phenomena might arise from the fluctuating or heterogeneous polar metallic states, which would pave a novel route to improving thermoelectric efficiency., Comment: 26 pages, 4 figures

We investigate the surface electronic structures of polar 1T'-MoTe2, the Weyl semimetal candidate realized through the nonpolar-polar structural phase transition, by utilizing the laser angle-resolved photoemission spectroscopy (ARPES) combined with first-principles calculations. Two kinds of domains with different surface band dispersions are observed from a single-crystalline sample. The spin-resolved measurements further reveal that the spin polarizations of the surface and the bulk-derived states show the different domain-dependences, indicating the opposite bulk polarity. For both domains, some segment-like band features resembling the Fermi arcs are clearly observed. The patterns of the arcs present the marked contrast between the two domains, respectively agreeing well with the slab calculation of (0 0 1) and (0 0 -1) surfaces. The present result strongly suggests that the Fermi arc connects the identical pair of Weyl nodes on one side of the polar crystal surface, whereas it connects between the different pairs of Weyl nodes on the other side., 13 pages, 4 figures

Multiferroics, in which dielectric and magnetic orders coexist and couple with each other, attract renewed interest for their cross-correlated phenomena, offering a fundamental platform for novel functionalities. Elementary excitations in such systems are strongly affected by the lattice-spin interaction, as exemplified by the electromagnons and the magneto-thermal transport. Here we report an unprecedented coupling between magnetism and phonons in multiferroics, namely, the giant thermal Hall effect. The thermal transport of insulating polar magnets (Zn

Phonon transport is an essential property of thermoelectric materials. Although the phonon carries heat, which reduces the thermoelectric efficiency, it contributes positively to the Seebeck coefficient S through the phonon-drag effect, as typified by the high-purity semiconductors, which show fairly large S at cryogenic temperatures. Although such a large S is attractive in terms of Peltier cooling, a clear guiding principle for designing thermoelectric materials enriched by the phonon-drag effect remains to be established. Here we demonstrate that a correlated semiconductor, FeSb2, is a promising thermoelectric material featuring quasi-ballistic phonons dragging d electrons with large effective mass. By changing the sample size within the sub-millimetre order for high-purity single crystals, we succeed in substantially increasing S to as much as −27 mV K−1 at low temperatures. Our results exemplify a strategy for exploring phonon-drag-based thermoelectric materials, the performance of which can be maximized by combining heavy electrons with ballistic phonons., Thermoelectric performances depend on phonon and electron transport. Here, Takahashi et al. show that the large Seebeck coefficient observed in high-purity single-crystal FeSb2 is due to the phonon-drag effect and to the high effective mass of d electrons interacting with quasi-ballistic phonons.

Unusual enhancement of cryogenic thermopower manifests itself around the critical point of polar order in a metal., Ferroelectrics with spontaneous electric polarization play an essential role in today’s device engineering, such as capacitors and memories. Their physical properties are further enriched by suppressing the long-range polar order, as exemplified by quantum paraelectrics with giant piezoelectric and dielectric responses at low temperatures. Likewise in metals, a polar lattice distortion has been theoretically predicted to give rise to various unusual physical properties. However, to date, a “ferroelectric”-like transition in metals has seldom been controlled, and hence, its possible impacts on transport phenomena remain unexplored. We report the discovery of anomalous enhancement of thermopower near the critical region between the polar and nonpolar metallic phases in 1T′-Mo1−xNbxTe2 with a chemically tunable polar transition. It is unveiled from the first-principles calculations and magnetotransport measurements that charge transport with a strongly energy-dependent scattering rate critically evolves toward the boundary to the nonpolar phase, resulting in large cryogenic thermopower. Such a significant influence of the structural instability on transport phenomena might arise from the fluctuating or heterogeneous polar metallic states, which would pave a novel route to improving thermoelectric efficiency.

There is still no general consensus on how one can describe the out-of-equilibrium phenomena in matter induced by an ultrashort light pulse. We investigate the pulse-induced dynamics in a layered Dirac semimetal ${\mathrm{SrMnBi}}_{2}$ by pump-and-probe photoemission spectroscopy. At $\ensuremath{\lesssim}1$ ps, the electronic recovery slowed upon increasing the pump power. Such a bottleneck-type slowing is expected in a two-temperature model (TTM) scheme, although opposite trends have been observed to date in graphite and in cuprates. Subsequently, an unconventional power-law cooling took place at $\ensuremath{\sim}100$ ps, indicating that spatial heat diffusion is still ill defined at $\ensuremath{\sim}100\phantom{\rule{0.28em}{0ex}}\mathrm{ps}$. We identify that the successive dynamics before the emergence of heat diffusion is a canonical realization of a TTM scheme. Criteria for the applicability of the scheme is also provided.

We have studied magnetic excitations in a field-induced noncollinear commensurate ferrimagnetic phase of Ba2Mg2Fe12O22 by means of polarized inelastic neutron scattering (PINS) and terahertz (THz) time-domain optical spectroscopy under magnetic field. A previous THz spectroscopy study reported that the field-induced phase exhibits electric-dipole-active excitations with energies of around 5 meV [Kida et al., Phys. Rev. B 83, 064422 (2011)]. In the present PINS measurements, we observed inelastic scattering signals around 5 meV at the zone center in the spin-flip channel. This directly shows that the electric-dipole-active excitations are indeed of magnetic origin, that is, electromagnons. In addition, the present THz spectroscopy confirms that the excitations have oscillating electric polarization parallel to the c axis. In terms of the spin-current model (Katsura-Nagaosa-Balatsky model), the noncollinear magnetic order in the field-induced phase can induce static electric polarization perpendicular to the c axis, but not dynamic electric polarization along the c axis. Furthermore, we suggest that the electromagnon excitations can be explained by applying the magnetostriction model to the out-of-phase oscillations of the magnetic moments, which is deduced from the present experimental results.

Perovskite-type manganites exhibit various interesting phenomena arising from complex interplay among spin, charge, orbital, and lattice degrees of freedom. One such example is the keen competition between phases with different spin/charge/orbital orders. Keen competition between antiferromagnetic metal and orbital-ordered insulator is found in the slightly electron-doped regime near Mn4+ state which is stabilized by the high oxygen-pressure condition. Another one is the emergence of ferroelectricity either induced by the magnetic ordering or independently of the magnetic ordering. As the respective examples, perovskite-type YMnO3 and Sr1−xBaxMnO3 are discussed. In the YMnO3, the ferroelectric lattice distortion associated with the E-type spin order is observed for the first time. Displacement-type ferroelectricity with off-center magnetic ions is discovered for Sr0.5Ba0.5MnO3, which shows both large polarization value and strong coupling between ferroelectricity and magnetism.

We investigate photo-induced lattice strains of a monoclinic VTe2 thin flake using ultrafast electron diffraction. After photoexcitation by a 190 fs pulse, we observe diffraction intensity oscillations with periods of 35 and 75 ps, which indicate coherent acoustic phonons of two distinct branches. The oscillations of and diffraction intensities have opposite signs, indicating the change of the diffraction angle due to the shear strain. By numerically simulating these diffraction intensities as a function of the monoclinic angle, we evaluate the amplitude of the photo-induced shear strain.

A-site ordered perovskites CaCu3Co4O12 and YCu3Co4O12, and their solid solutions Ca1−xYxCu3Co4O12 (x = 0.25, 0.50, and 0.75), were synthesized under high pressure (9 GPa) and high temperature (1273 K). They all have a CaCu3Ti4O12-type structure (cubic, space group: Im3) and the lattice constant a slightly decreased from 7.1226(5) A of CaCu3Co4O12 to 7.1195(3) A of YCu3Co4O12. Rietveld refinement based on synchrotron powder X-ray diffraction suggests that the valence state of CaCu3Co4O12 is close to CaCu3+3Co3.25+4O12, which differs from those of other CaCu2+3B4+4O12 perovskites. Substitution of Ca with Y at the A-site changes the metallic CaCu3Co4O12 to an insulating YCu3Co4O12, and a metal−insulator transition occurs at x = 0.50−0.75. The electrical resistivity, thermoelectricity, and specific heat results reveal that electrons are doped into the Co 3d band and that the valence state changes from CaCu3+3Co3.25+4O12 to YCu3+3Co3+4O12.

The mutual control of the electric and magnetic properties of a solid is currently of great interest because of the possible application for novel electronic devices. We report on the low-magnetic-field (for example, B values of ±30 milliteslas) control of the polarization ( P ) vector in a hexaferrite, Ba 2 Mg 2 Fe 12 O 22 , which shows the helimagnetic spin structure with the propagation vector k 0 parallel to [001]. The B -induced transverse conical spin structure carries the P vector directing perpendicular to both B and k 0 , in accord with the recently proposed spin-current model. Then, the oscillating or multidirectionally rotating B produces the cyclic displacement current via the flexible handling of the magnetic cone axis.

A new magnetoplumbite-type hexagonal cobalt oxide, SrCo12O19, has been synthesized at a high pressure of 3 GPa and its structural and magnetic properties have been studied using single crystalline samples. Bond-valence calculations for five crystallographically independent Co sites show a rather wide charge distribution from +2.15 to +3.50. Magnetic measurements have revealed anisotropic magnetic behavior below a magnetic ordering temperature of 80 K. Comparative discussions on the structure–property relationship for SrCo12O19 and related compounds suggest the presence of localized spins with a uniaxial anisotropy at the trigonal bipyramidal site.

The valence state change of BiNiO3 perovskite under pressure has been investigated by a powder neutron diffraction study and electronic-state calculations. At ambient pressure, BiNiO3 has the unusual charge distribution Bi(3+)(0.5)Bi(5+)(0.5)Ni(2+)O3 with ordering of Bi(3+) and Bi(5+)charges on the A sites of a highly distorted perovskite structure. High-pressure neutron diffraction measurements and bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high-pressure phase is metallic Bi(3+)Ni(3+)O3. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable.