Bayesian active learning with model selection for spectral experiments.

Active learning is a common approach to improve the efficiency of spectral experiments. Model selection from the candidates and parameter estimation are often required in the analysis of spectral experiments. Therefore, we proposed an active learning with model selection method using multiple parametric models as learning models. Important points for model selection and its parameter estimation were actively measured using Bayesian posterior distribution. The present study demonstrated the effectiveness of our proposed method for spectral deconvolution and Hamiltonian selection in X-ray photoelectron spectroscopy.

Ferroic Order for Anisotropic Magnetic Dipole Term in Collinear Antiferromagnets of (t_{2g})

We present the possibility of x-ray magnetic circular dichroism on RuO_{2} with collinear antiferromagnetism (AFM). Given that the crystal symmetry breaks the time reversal symmetry when the antiparallel spin aligns along the [100] direction, the expectation vector of the anisotropic magnetic dipole operator ⟨t⟩ remains uncanceled along the [010] direction. Our Letter reveals that the magnetic dipole (T_{z}) term in the x-ray magnetic circular dichroism is induced by the residual ⟨t⟩. Because the features of the magnetic moment can be detected via absorption measurements even in the AFM, this technique will be useful for determining the magnetic phase, the Van Vleck-type paramagnet or the excitonic AFM in (t_{2g})^{4} system.

Bayesian framework for analyzing adsorption processes observed via time-resolved X-ray diffraction.

Clarifying dynamic processes of materials is an important research topic in materials science. Time-resolved X-ray diffraction is a powerful technique for probing dynamic processes. To understand the dynamics, it is essential to analyze time-series data using appropriate time-evolution models and accurate start times of dynamic processes. However, conventional analyses based on non-linear least-squares fitting have difficulty both evaluating time-evolution models and estimating start times. Here, we establish a Bayesian framework including time-evolution models. We investigate an adsorption process, which is a representative dynamic process, and extract information about the time-evolution model and adsorption start time. The information enables us to estimate adsorption properties such as rate constants more accurately, thus achieving more precise understanding of dynamic adsorption processes. Our framework is highly versatile, can be applied to other dynamic processes such as chemical reactions, and is expected to be utilized in various areas of materials science.

Topological charge of soft X-ray vortex beam determined by inline holography.

A Laguerre-Gaussian (LG) vortex beam having a spiral wavefront can be characterized by its topological charge (TC). The TC gives the number of times that the beam phase passes through the interval [Formula: see text] following a closed loop surrounding the propagation axis. Here, the TC spectra of soft X-ray vortex beams are acquired using the in-line holography technique, where interference between vortex waves produced from a fork grating and divergent waves from a Fresnel zone plate is observed as a holographic image. The analyses revealed the phase distributions and the TC for the LG vortex waves, which reflects topological number of the fork gratings, as well as for the Hermite-Gaussian (HG) mode waves generated from the other gratings. We also conducted a simulation of the present technique for pair annihilation of topological defects in a magnetic texture. These results may pave the way for development of probes capable of characterizing the topological numbers of magnetic defects.

Giant multiple caloric effects in charge transition ferrimagnet.

Caloric effects of solids can provide us with innovative refrigeration systems more efficient and environment-friendly than the widely-used conventional vapor-compression cooling systems. Exploring novel caloric materials is challenging but critically important in developing future technologies. Here we discovered that the quadruple perovskite structure ferrimagnet BiCu3Cr4O12 shows large multiple caloric effects at the first-order charge transition occurring around 190 K. Large latent heat and the corresponding isothermal entropy change, 28.2 J K-1 kg-1, can be utilized by applying both magnetic fields (a magnetocaloric effect) and pressure (a barocaloric effect). Adiabatic temperature changes reach 3.9 K for the 50 kOe magnetic field and 4.8 K for the 4.9 kbar pressure, and thus highly efficient thermal controls are achieved in multiple ways.

Intermetallic Charge Transfer in V-Substituted PbCrO3.

PbCrO3 features an unusual charge distribution Pb0.52+Pb0.54+Cr3+O3 with Pb charge disproportionation at ambient pressure. A charge transfer between Pb and Cr is induced by the application of pressure resulting in Pb2+Cr4+O3 charge distribution and a large volume collapse. Here, structural and charge distribution changes in PbCr1-xVxO3 are investigated. Despite a cubic crystal structure in 0 ≤ x ≤ 0.60, discontinuous reduction in the unit cell volume was observed between x = 0.35 and 0.40. Hard X-ray photoemission spectroscopy confirmed the change in Pb charge state from the coexisting Pb2+ and Pb4+ at x = 0.35 to single Pb2+ at x = 0.40. This indicates that V substitution stabilizes the high pressure cubic Pb2+Cr4+O3-type phase. With further increase in the V substitution, the PbVO3-type polar tetragonal phase appeared at x = 0.80.

Tilting and rotational motions of silver halide crystal with diffracted X-ray blinking.

The dynamic properties of crystalline materials are important for understanding their local environment or individual single-grain motions. A new time-resolved observation method is required for use in many fields of investigation. Here, we developed in situ diffracted X-ray blinking to monitor high-resolution diffraction patterns from single-crystal grains with a 50 ms time resolution. The diffraction spots of single grains of silver halides and silver moved in the θ and χ directions during the photolysis chemical reaction. The movements of the spots represent tilting and rotational motions. The time trajectory of the diffraction intensity reflecting those motions was analysed by using single-pixel autocorrelation function (sp-ACF). Single-pixel ACF analysis revealed significant differences in the distributions of the ACF decay constants between silver halides, suggesting that the motions of single grains are different between them. The rotational diffusion coefficients for silver halides were estimated to be accurate at the level of approximately 0.1 to 0.3 pm2/s. Furthermore, newly formed silver grains on silver halides correlated with their ACF decay constants. Our high-resolution atomic scale measurement-sp-ACF analysis of diffraction patterns of individual grains-is useful for evaluating physical properties that are broadly applicable in physics, chemistry, and materials science.

X-ray microscope for imaging topological charge and orbital angular momentum distribution formed by chirality.

The distribution of topological charges on X-ray vortices was measured by differential Fourier space filtering microscope, differential radial Hilbert transform microscope. It was experimentally verified for the first time using a Spiral Fresnel zone plate objective lens. This X-ray microscope is highly sensitive to X-ray topological defects, such as edges and vortices, at the exit-face wave field of objects. Its efficient use is also discussed.

Enhanced Negative Thermal Expansion Induced by Simultaneous Charge Transfer and Polar-Nonpolar Transitions.

Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar-nonpolar transitions, was observed for the first time in BiNi1-xFexO3 (0.25 ≤ x ≤ 0.5). The low-temperature phase was found to have a polar structure (space group of R3c) with a Bi3+0.5(1+x)Bi5+0.5(1-x)Ni2+1-xFe3+xO3 charge distribution and short-range ordering of Bi3+ and Bi5+. The volume reduction upon heating that was induced by charge transfer between Bi5+ and Ni2+ decreased with increasing x because of the reduction in the amount of Ni2+. Simultaneous polar-nonpolar transition also contributed to NTE, and a composition-independent enhanced volume reduction of ∼2% was observed.

Formation of an Intermediate Valence Icosahedral Quasicrystal in the Au-Sn-Yb System.

We report on the formation of a new icosahedral quasicrystal (iQC) in the Au-Sn-Yb alloy system. This iQC has a primitive icosahedral lattice with a lattice constant aico of 0.5447(7) nm and a composition that was determined to be Au60.0Sn26.7Yb13.3. X-ray absorption spectroscopy measurement of the near Yb L3 edge demonstrates that the Yb valence in the iQC is an intermediate valence between divalent (4f14) and trivalent (4f13) at ambient pressure and was determined to be 2.18+. The results are compared to those for a corresponding 2/1 cubic approximant crystal. The formation of this new iQC is discussed in terms of the atomic size factor (δ) and the valence electron-to-atom ratio (e/a).

Emergence of a new valence-ordered structure and collapse of the magnetic order under high pressure in EuPtP.

The layered hexagonal EuPtP is a rare substance that exhibits two successive valence transitions occurring simultaneously with valence ordering transitions and an antiferromagnetic order. Anticipating that the application of pressure to this sample would induce a new valence-ordered structure and/or a new phenomenon associated with valence fluctuation, we examined the electrical resistivity ρ, the Eu L3-edge x-ray absorption spectroscopy, and the powder x-ray diffraction under high pressure. We found a new valence transition at around P = 2.5 GPa. After the transition, a new valence-ordered structure is realized at the lowest temperature. The valence-ordered structure is inferred to be stacking of [Formula: see text] (2+: Eu2+ layer, 3+: Eu3+ layer) along the c-axis. Upon further increases in pressure, the valence-ordered structure is suppressed and another valance-ordered phase is realized up to P = 6 GPa. The antiferromagnetic order collapses in the pressure range between 6 GPa and 8 GPa.

Systematic charge distribution changes in Bi- and Pb-3d transition metal perovskites.

Charge distribution changes in Bi- and Pb-3d transition metal perovskite type oxides were examined by comprehensive precise structural analysis, spectroscopy, and theoretical investigations. The change in the depth of the d level of the transition metal caused the intermetallic charge transfer. A temperature-induced charge-transfer transition in chemically modified BiNiO3 results in technologically important negative thermal expansion.

A-Site and B-Site Charge Orderings in an s-d Level Controlled Perovskite Oxide PbCoO3.

Perovskite PbCoO3 synthesized at 12 GPa was found to have an unusual charge distribution of Pb2+Pb4+3Co2+2Co3+2O12 with charge orderings in both the A and B sites of perovskite ABO3. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb2+Pb4+3Co2+2Co3+2O12 quadruple perovskite structure. It is shown that the average valence distribution of Pb3.5+Co2.5+O3 between Pb3+Cr3+O3 and Pb4+Ni2+O3 can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.

Pressure-Driven Spin Crossover Involving Polyhedral Transformation in Layered Perovskite Cobalt Oxyfluoride.

We report a novel pressure-driven spin crossover in layered cobalt oxyfluoride Sr2CoO3F with a distorted CoO5 square pyramid loosely bound with a fluoride ion. Upon increasing pressure, the spin state of the Co(III) cation gradually changes from a high spin state (S = 2) to a low spin state (S = 0) accompanied by a anomalously large volume contraction (bulk modulus, 76.8(5) GPa). The spin state change occurs on the CoO5 pyramid in a wide pressure range, but the concomitant gradual shrinkage of the Co-F bond length with pressure gives rise to a polyhedral transformation to the CoO5F octahedron without a structural phase transition, leading to the full conversion to the LS state at 12 GPa. The present results provide new effective strategy to fine-tune electronic properties of mixed anion systems by controlling the covalency in metal-ligand bonds under pressure.

Geometrical Spin Frustration of Unusually High Valence Fe(5+) in the Double Perovskite La2LiFeO6.

A double perovskite-structure oxide La2LiFeO6 with unusually high-valence Fe(5+) was synthesized using a high-pressure technique. The Li(+) and Fe(5+) ions at the B site in the rhombohedral R3̅ perovskite structure are ordered in a rock salt manner, and the resultant tetrahedral network of Fe(5+) gives geometrical spin frustration, which is consistent with a large frustration index f (|θ|/TN) ≈ 10. Mg(2+) substitution for Li(+) produces Fe(4+) from some Fe(5+) and changes the magnetic properties. The Weiss temperature is increased from -119 to 21 K by the substitution of only 1%, significantly decreasing the frustration index. The geometrical frustration of the Fe(5+) spin sublattice cannot be tolerant for even a very small amount of Fe(4+) disturbance.

Ferromagnetism Induced by Substitution of the Iron(IV) Ion by an Unusual High-Valence Nickel(IV) Ion in Antiferromagnetic SrFeO3.

Novel cubic perovskites SrFe(1-x)Ni(x)O3 (0≤x≤0.5) with unusual high-valence iron(IV) and nickel(IV) ions were obtained by high-pressure and high-temperature synthesis. Substantial magnetic moments of Ni(IV), which is intrinsically nonmagnetic with a nominal d(6) electron configuration, were induced by the large magnetic moments of Fe(IV) through orbital hybridization with oxygen. As a result, ferromagnetism with the transition temperature (T(c)) above room temperature could be induced.

Melting of Pb Charge Glass and Simultaneous Pb-Cr Charge Transfer in PbCrO3 as the Origin of Volume Collapse.

A metal to insulator transition in integer or half integer charge systems can be regarded as crystallization of charges. The insulating state tends to have a glassy nature when randomness or geometrical frustration exists. We report that the charge glass state is realized in a perovskite compound PbCrO3, which has been known for almost 50 years, without any obvious inhomogeneity or triangular arrangement in the charge system. PbCrO3 has a valence state of Pb(2+)(0.5)Pb(4+)(0.5)Cr(3+)O3 with Pb(2+)-Pb(4+) correlation length of three lattice-spacings at ambient condition. A pressure induced melting of charge glass and simultaneous Pb-Cr charge transfer causes an insulator to metal transition and ∼10% volume collapse.

Rattling in the Quadruple Perovskite CuCu3 V4 O12.

Of particular interest is a peculiar motion of guest atoms or ions confined to nanospace in cage compounds, called rattling. While rattling provides unexplored physical properties through the guest-host interactions, it has only been observed in a very limited class of materials. Herein, we introduce an A-site-ordered quadruple perovskite, CuCu3 V4 O12 , as a new family of cage compounds. This novel AA'3 B4 O12 -type perovskite has been obtained by a high-pressure synthesis technique and structurally characterized to have cubic Im$\bar 3$ symmetry with an ionic model of Cu(2+) Cu(2+) 3 V(4+) 4 O12 . The thermal displacement parameter of the A-site Cu(2+) ion is as large as Uiso ≈0.045 Å(2) at 300 K, indicating its large-amplitude thermal oscillations in the oversized icosahedral cages. Remarkably, the presence of localized phonon modes associated with rattling of the A-site Cu(2+) ion manifests itself in the low-temperature specific heat data. This work sheds new light on the structure-property relations in perovskites.

Mapping platinum species in polymer electrolyte fuel cells by spatially resolved XAFS techniques.

There is limited information on the mechanism for platinum oxidation and dissolution in Pt/C cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) under the operating conditions though these issues should be uncovered for the development of next-generation PEFCs. Pt species in Pt/C cathode catalyst layers are mapped by a XAFS (X-ray absorption fine structure) method and by a quick-XAFS(QXAFS) method. Information on the site-preferential oxidation and leaching of Pt cathode nanoparticles around the cathode boundary and the micro-crack in degraded PEFCs is provided, which is relevant to the origin and mechanism of PEFC degradation.

Multiple magnetic interactions in A-site-ordered perovskite-structure oxides.

Multiple magnetic interactions in A-site-ordered perovskite-structure oxides AA'3B2B'2O12 with A'-site Cu and B-site Fe ions are highlighted here. Several new compounds with this structure type were obtained by high-pressure synthesis and have been given unusual magnetic properties due to multiple interactions of Cu and Fe ions (A'-A', A'-B, A'-B', B-B, B-B', and B'-B' interactions). The magnetic interaction is discussed here in light of the results of magnetic structure analysis with neutron powder diffraction data and x-ray magnetic circular dichroism spectra obtained in x-ray absorption experiments. The characteristic structural framework with ordered cation arrangements and the variation in the oxidation state of the ions at the A' and B sites are shown to play roles crucial for the diverse and intriguing physical properties of these new compounds.