Ritsuko Eguchi

Layered inorganic materials can exhibit symmetry-breaking transitions in which electronic instabilities and local structural modes are strongly intertwined. Here we compare the isovalent chalcogenides Bi2Rh3Se2 and Bi2Rh3S2 by synchrotron single-crystal X-ray diffraction and density-functional-theory calculations, and identify a static Bi2Rh4 breathing distortion as the common local motif underlying their distinct low-temperature phases. In Bi2Rh3Se2, the transition near 240 K is characterized by the loss of C-centering without a change in the conventional unit-cell volume. The resulting primitive lattice contains inequivalent Rh4 quadrilaterals formed by alternating expansion and contraction, establishing a C-centering-breaking breathing order. Although previous spectroscopic studies have reported CDW-like signatures in Bi2Rh3Se2, the calculated Fermi surfaces of the high-temperature phase do not reveal an obvious nesting vector expected for a simple nesting-driven Peierls-type CDW scenario. Instead, the Rh 4d-derived electronic states at the Fermi level are naturally sensitive to the observed Rh4 breathing distortion, indicating that the transition is best described as a breathing-order transition accompanied by electronic reconstruction. In Bi2Rh3S2, a comparable Bi2Rh4 breathing distortion appears, but its ordering pattern is switched: when described in the high-temperature-cell setting, the Bi2Rh4 units form a large−small−small sequence along the aHT-direction, accompanied by an antiphase relation between neighboring chains, producing a commensurate 3× superstructure consistent with the first-order character of the transition. Calculations based on the experimentally refined structures further reveal a reduction of the density of states near EF and an anisotropic redistribution of occupied low-energy states in the breathing-distorted phases, supporting the coupling between the Bi2Rh4 motif and electronic reconstruction. These results show that isovalent chalcogen substitution selects the in-plane ordering pattern of an intrinsic Bi2Rh4 breathing distortion, thereby switching the collective symmetry-breaking pathway and associated electronic response.

This study reports the first application of X-ray fluorescence holography (XFH) under high-pressure conditions. We integrated XFH with a diamond anvil cell to investigate the local structure around Sr atoms in single-crystal SrTiO ₃ under high pressure. By utilizing nano-polycrystalline diamond anvils and a yttrium filter, we effectively eliminated significant background noise from both the anvils and the gasket. This optimized experimental configuration enabled the measurement of Sr K α holograms of the SrTiO ₃ sample at pressures up to 13.3 GPa. The variation of lattice constants with pressure was calculated by the shifts of Kossel lines, and real-space images of the atomic structures were reconstructed from the Sr K α holograms at different pressures. This work successfully demonstrates the feasibility of employing XFH under high-pressure conditions as a novel method for visualizing pressure-induced changes in the three-dimensional local structure around the specified element.

Determination of the atomic locations of Bi₂Rh₃Se₂ at 300 and 200 K using X-ray fluorescence holography.

The structural and superconducting properties of the Bi-based superconductor Bi₂Rh₃(Se_1-xS_x)₂ were investigated over a wide pressure range. Bi₂Rh₃Se₂ is a superconductor with a superconducting transition temperature, T_c, of less than ∼1.0 K, whereas Bi₂Rh₃S₂ is a nonsuperconductor. The former shows a clear charge density wave (CDW) phase transition at 240 K and the latter shows a first-order structural phase transition at 165 K, providing the supercell structure. Both compounds have the same crystal structure at ambient temperature and pressure (monoclinic, space group of C2/m (No. 12)). This implies the possible formation of solid solution-like phases Bi₂Rh₃(Se_1-xS_x)₂. In this study, we prepared Bi₂Rh₃(Se_1-xS_x)₂ (x = 0-1.0) and investigated its superconducting properties and phase transitions at different x values. Because the superconducting properties and temperature causing the phase transition (T_pt) are quite different between Bi₂Rh₃Se₂ and Bi₂Rh₃S₂, their variation against x is quite intriguing from the viewpoint of the pursuit of physical properties via the replacement of Se with S. In this study, the features of the superconductivity and crystal structure against x were fully explored to depict the phase diagram. Moreover, we investigated the pressure (p) dependence of superconductivity and phase transition (CDW or the first-order structural transition) to clarify the interplay between the two ordered states.

The local structure of the two-dimensional van der Waals material, Fe5-xGeTe2, which exhibits unique structural/magnetic phase transitions, was investigated by Te K-edge extended X-ray absorption fine structure (EXAFS) and Te K alpha X-ray fluorescence holography (XFH) over a wide temperature range. The formation of a trimer of Te atoms at low temperatures has been fully explored using these methods. An increase in the Te-Fe distance at approximately 150 K was suggested by EXAFS and presumably indicates the formation of a Te trimer. Moreover, XFH displayed clear atomic images of Te atoms. Additionally, the distance between the Te atoms shortened, as confirmed from the atomic images reconstructed from XFH, indicating the formation of a trimer of Te atoms, i.e., a charge-ordered (root 3 x root 3)R30 degrees superstructure. Furthermore, Te K alpha XFH provided unambiguous atomic images of Fe atoms occupying the Fe1 site; the images were not clearly observed in the Ge K alpha XFH that was previously reported because of the low occupancy of Fe and Ge atoms. In this study, EXAFS and XFH clearly showed the local structure around the Te atom; in particular, the formation of Te trimers caused by charge-ordered phase transitions was clearly confirmed. The charge-ordered phase transition is fully discussed based on the structural variation at low temperatures, as established from EXAFS and XFH.

The structural and superconducting properties of the Bi-based compound Bi2Pd3Se2 were investigated over a wide pressure range. The prepared Bi2Pd3Se2 sample was a superconductor with a superconducting transition temperature, T-c, of approximately 3.0 K, which differed from a previous report (T-c of less than 1.0 K). At ambient pressure, the powder X-ray diffraction (XRD) pattern of the Bi2Pd3Se2 sample was consistent with that previously reported for Bi2Pd3Se2. The Rietveld method was used to refine the crystal structure, which had a space group of C2/m (No. 12), as reported previously. This compound showed no clear anomaly due to the charge-density-wave (CDW) transition, as seen from the temperature dependence of magnetic susceptibility. However, the temperature dependence of electrical resistivity indicated a clear anomaly, presumably because of the CDW transition in the low-pressure range; the CDW transition temperature was approximately 230 K. The XRD patterns of the Bi2Pd3Se2 sample were measured at 0.160-22.7 GPa, and the patterns were well analyzed by both the Le Bail and Rietveld refinement methods, showing no structural phase transitions in the above pressure range. The pressure dependence of T-c of Bi2Pd3Se2 was recorded based on the temperature dependence of the electrical resistance, which showed an almost constant T-c at 0-13.7 GPa, and the T-c-pressure (p) behavior was fully discussed.