Here we investigate the electronic structure of the tetragonal tungsten bronze Ba3-xYbxTa5O15 by making use of hard x-ray photoemission spectroscopy. The core level spectroscopy shows that the substitution with Yb ions in the series first occurs on the compact S1 site. For x <= 1, Yb is found to be dominantly Yb2+ with a small mixing of Yb3+, while for x > 1, a significant increase of Yb3+ is found, suggesting not only that site S2 favours Yb3+, but also that their presence affects also the valency of the ions in site S1. The valence band spectra show a relatively deep Yb2+ doublet, but at the same time indications of a Ta 5d-Yb 4f interaction are found, suggesting the presence of Yb 4f carriers at the Fermi level through this hybridization. Our results thus point towards an exotic form of d-f electronic interplay that together with the structural degrees of freedom can result in the unusual trends observed in the physical properties of Ba3-xYbxTa5O15.

BaV10-xTixO15 with x = 0.15 exhibits coexisting orbital-ordered (OO) and orbital-disordered (OD) phases as well as slow dynamics during the phase transition from the OD phase to the OO phase. Owing to these slow dynamics and the fact that these two phases exhibit antiferromagnetic ordering with different transition temperatures, two-phase coexisting states can be considered magnetic nanoparticles in a crystal, whose size can be controlled relatively easily. Measuring the magnetic susceptibility of these states reveals that the Neel temperature of the OO phase decreases as the size of the OO phase decreases, similarly to conventional magnetic nanoparticles.

We have investigated the electronic structure of the rocksalt TiO x and VO x systems using polarization-dependent hard X-ray photoelectron spectroscopy. The ability to disentangle the various subshell contributions allowed us to unveil the presence of cation 4s molecular orbitals centered around anion vacancies. We observed a close relationship between the presence of these molecular orbitals and the conductive properties of the oxide material, exemplifying the importance of the 4s degrees of freedom for low-valent transition metal compounds. The experimentally vanishing intensity at the Fermi level classifies TiO x and VO x as bad metals and strongly suggests that stoichiometric and defect-free TiO and VO are Mott-Hubbard insulators.

A series of mixed compounds, Co1-xMnxV2O4, was investigated by strain and magnetization measurements. It was found that a small distortion of the crystal observed in CoV2O4 (x = 0) below the ferrimagnetic ordering temperature T-N persists for 0 < x < 1, whereas a structural transition likely accompanied by V orbital ordering occurs as a first-order phase transition at T(s )lower than T-N for 0.8 <= x < 1 and T(N )and T(s )merge at x=1. These characteristics can be reproduced by a Landau model considering the magnetization M, the orbital Q, and aquadratic coupling between M and Q.

We have investigated the electronic structure of the rocksalt TiOx and VOx systems using polarization-dependent hard X-ray photoelectron spectroscopy. The ability to disentangle the various subshell contributions allowed us to unveil the presence of cation 4s molecular orbitals centered around anion vacancies. We observed a close relationship between the presence of these molecular orbitals and the conductive properties of the oxide material, exemplifying the importance of the 4s degrees of freedom for low-valent transition metal compounds. The experimentally vanishing intensity at the Fermi level classifies TiOx and VOx as bad metals and strongly suggests that stoichiometric and defect-free TiO and VO are Mott-Hubbard insulators.

It is found that a small distortion caused by magnetostriction and a structural phase transition caused by Jahn-Teller distortion are continuously connected in Co_{1-x}Fe_{x}V_{2}O_{4} under variation of x. Spin-orbit coupling in Jahn-Teller-active Fe^{2+} ions gives rise to this novel correlation between spins, orbitals, and the lattice distortion in this series of compounds.

The A-site cation-ordered Ruddlesden-Popper compound Gd3Ba2Fe4O12.5 was synthesized using the topotactic chemical reaction of Gd3Ba2Fe4O12. The crystal structure was determined through single-crystal X-ray diffraction and scanning transmission electron microscopy. Gd3Ba2Fe4O12.5 crystallizes in a tetragonal unit cell with P42/mnm symmetry, where a = 5.57240(10) Å and c = 35.3289(8) Å, differing from the P42/ncm space group of Gd3Ba2Fe4O12. The topotactically introduced oxide ions randomly occupy half of the atomic sites on the central layer (Gd layer) of the perovskite block, causing the iron ions on both sides of the layer to form six-coordinate octahedra and five-coordinate pyramids. Consequently, these polyhedra are randomly distributed within the perovskite blocks. Mössbauer spectroscopy revealed the presence of three independent trivalent Fe sites, corresponding to two FeO6 octahedra and one FeO5 pyramid. The introduction of oxide ions into Gd3Ba2Fe4O12 to form Gd3Ba2Fe4O12.5 altered the polyhedral rotation pattern from the same direction to the opposite direction relative to the ⟨110⟩ direction at the center of each perovskite block, although both compounds equally exhibited the a-b0c0/b0a-c0 manner. The occupation of O ions in the Gd layers of Gd3Ba2Fe4O12 induces a change in the crystal symmetry, which is associated with the octahedral rotation pattern and coordination environment.

57 Fe Mössbauer spectroscopy of the A-site ordered spinel LiFeCr 4 O 8 has been conducted in order to investigate the local structure and magnetic structure. The paramagnetic spectrum is composed of a singlet, reflecting the cubic local symmetry. Below T N ≈ 94 K, the spectrum becomes a very broad hyperfine sextet, which is interpreted as a distribution of the hyperfine field. The arrangement of the half-filled A-site Fe ions and the breathing pyrochlore Cr ions may produce a wide distribution in the magnetic interaction. Below T MS ≈ 23 K, the spectral shape abruptly changes to a well-resolved sextet, which can be fitted by one component sextet with a very small quadrupole shift. We found that the high-temperature magnetic phase (ferrimagnetic) and the low-temperature magnetic phase (conical) coexist even at 40 K. The spin-gap transition around T SG ≈ 60 K is not observed. The reported decrease in the magnetization below around 50 K may be a result of increasing magnetic anisotropy due to the appearance of the conical magnetic phase.

The electronic structure of spinel-type AlV2O4 is investigated using bulk -sensitive hard x-ray photoemission spectroscopy (HAXPES). The main and shoulder peaks of the V 2p3/2 core-level HAXPES spectra indicate charge disproportionation between V2+ and V3+, consistent with the average V valence of +2.5. The estimated V2+/V3+ ratio is compatible with the formation of V trimers and tetramers in AlV2O4, providing a hint to develop a model for charge and orbital orderings in the V pyrochlore lattice. We propose that V2+ tetramers and V3+ trimers on the pyrochlore lattice are naturally explained by an orbitally induced Peierls mechanism.

We investigated the electronic structure of BaEu2Nb5O15 and Eu3Nb5O15 by means of hard x-ray photoelectron spectroscopy. The Eu 3d core level spectra are primarily dominated by Eu2+ signals. The well -screened feature in the Nb 3d core level spectra of BaEu2Nb5O15 is suppressed in Eu3Nb5O15, consistent with the insulating behavior of Eu3Nb5O15 due to the atomic disorder and/or the rattling effect of the small Eu ions in the spacious A2 site. In the valence band spectra, the Nb 4d states at the Fermi level are close to the Eu 4 f states located around 2 eV below it. This suggests that the degree of localization of the Nb 4d electrons can be enhanced through the Eu 4 f-Nb 4d hybridization with the atomically and magnetically disordered Eu ions at the A2 site.

We have grown single crystals of mixed -valence pseudobrookite titanate MgTi 2 O 5 -Ti 3 O 5 systems (Mg 1 - x Ti 2 + x O 5 ) and have systematically investigated their physical properties as a novel canonical mixedvalence system which shows a successive change from the nonmagnetic band insulating phase (MgTi 2 O 5 ) to the charge -ordered spin -singlet dimer phase (Ti 3 O 5 ). In comparison with Al 1 - x Ti 2 + x O 5 [R. Takahama et al. , Phys. Rev. Mater. 4 , 074401 (2020)], we have clarified that, in the lower x region of Mg 1 - x Ti 2 + x O 5 , the physical properties are greatly affected by the change of electron density with a variation of x and we have found that the doped electrons enhance the optical conductivity along a Ti -Ti ladder direction. However, we have also clarified that, in the higher x region, almost all Ti 3 + ions produced by an increase in x contribute not to an increase in itinerant electrons but to a development of Ti 3 + -Ti 3 + dimer correlation. These observations indicate a crossover from the conductance along the ladder direction to a nontrivial conductance under the background of the dimer correlation.

The photoinduced dynamics of Ti2O3 with a corundum structure have been investigated by pump-probe optical spectroscopy. It is found that a peak in the optical spectrum corresponding to excitations within Ti-Ti dimers is suppressed when a laser pulse is applied at room temperature, indicating a photoinduced phase transition from the low-temperature insulating state to the high-temperature conducting state in Ti2O3. In addition, the oscillation of reflectivity with time is observed, which can be explained by a strain wave generated by a laser pulse applied to the sample surface. The dependence of the oscillation on the direction of the surface indicates that the change in the c lattice constant parallel to the Ti-Ti dimers plays an important role in the phase transition.

Barium tantalate with a tetragonal tungsten bronze structure, ${\mathrm{Ba}}_{3}{\mathrm{Ta}}_{5}{\mathrm{O}}_{15}$, and compounds where Ba is partly substituted by various rare earths ($R$), ${\mathrm{Ba}}_{3\ensuremath{-}x}{R}_{x}{\mathrm{Ta}}_{5}{\mathrm{O}}_{15}$, were synthesized as single crystals, and their transport, magnetic, and magnetotransport properties were investigated. It was found that the compounds with Eu substitution show a relatively large negative magnetoresistance arising from the interaction between the conduction electrons in the Ta $5d$ orbital and the localized spins in the Eu $4f$ orbital.

Single crystals of Ba3-xYbxTa5O15 with a tetragonal tungsten bronze structure were grown using a floating-zone technique, and their transport and magnetic properties, optical spectra, and specific heat were measured. It was found that this series of compounds exhibits a valence change of Yb ions from 2+ to 3+ with increasing x. This can be explained by assuming that the Yb2+ level exists immediately below the Fermi level in Ta 5d states, which decreases in energy with increasing x. It was also found that a positive magnetoresistance up to 3 T and a negative magnetoresistance above 3 T appear at low temperatures for x >= 1.0, indicating a peculiar state of Yb in this series of compounds.

(Received 5 March 2024; accepted 23 April 2024; published 7 May 2024) Barium tantalate with a tetragonal tungsten bronze structure, Ba 3 Ta 5 O 15 , and compounds where Ba is partly substituted by various rare earths ( R ), Ba 3 - x R x Ta 5 O 15 , were synthesized as single crystals, and their transport, magnetic, and magnetotransport properties were investigated. It was found that the compounds with Eu substitution show a relatively large negative magnetoresistance arising from the interaction between the conduction electrons in the Ta 5 d orbital and the localized spins in the Eu 4 f orbital.

We investigated the electronic structure of ${\mathrm{BaEu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ and ${\mathrm{Eu}}_{3}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ by means of hard x-ray photoelectron spectroscopy. The Eu $3d$ core level spectra are primarily dominated by ${\mathrm{Eu}}^{2+}$ signals. The well-screened feature in the Nb $3d$ core level spectra of ${\mathrm{BaEu}}_{2}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ is suppressed in ${\mathrm{Eu}}_{3}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$, consistent with the insulating behavior of ${\mathrm{Eu}}_{3}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ due to the atomic disorder and/or the rattling effect of the small Eu ions in the spacious $A2$ site. In the valence band spectra, the Nb $4d$ states at the Fermi level are close to the Eu $4f$ states located around 2 eV below it. This suggests that the degree of localization of the Nb $4d$ electrons can be enhanced through the Eu $4f$--Nb $4d$ hybridization with the atomically and magnetically disordered Eu ions at the $A2$ site.

Slow dynamics have been experimentally studied in various spin systems, but most of the studies so far have been conducted on the systems with spin frustration. In the present study, the dynamics of magnetization in a series of La5Mo4-xMnxO16 have been investigated, in which the spins on quasi-square lattices have extremely large two-dimensional anisotropy in the magnetic interactions. It was found that slow dynamics are dominated by a nucleation-growth process with interfacial energy caused by in-plane magnetic interactions. A detailed analysis suggests the formation of domain walls at the interface.

To study the photoinduced processes in strongly correlated electron systems occurring coherently on various time scales, we conducted the pump-probe optical spectroscopy of hexagonal YMnO3 with ranges of delay time between 0.1 and 5000 ps and the energy of light between 1 and 3 eV. We found that the photoinduced changes in reflectivity were dominated by the intra-atomic excitation of a d electron in Mn, by the relaxation of energy from electronic excitations to phonon excitations, by the propagation of a strain wave, and by thermal conduction inside a sample, which occur successively with time. We also found that the sound velocity and thermal conductivity of the sample including their anisotropy can be obtained by this technique.