X-ray Fluorescence Holography Research Articles

Superstructure of Fe5-xGeTe2 Determined by Te K-Edge Extended X-ray Absorption Fine Structure and Te Kα X-ray Fluorescence Holography.

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 Kα 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 superstructure. Furthermore, Te Kα XFH provided unambiguous atomic images of Fe atoms occupying the Fe1 site; the images were not clearly observed in the Ge Kα 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.

Observation of the Superstructure in Fe5-xGeTe2 by X-ray Fluorescence Holography.

Fe5-xGeTe2 is a two-dimensional van der Waals material that exhibits ferromagnetic order with a high Curie temperature (TC) of around room temperature. In addition to TC, two magnetic transitions occur with decreasing temperature, and a charge-ordered state is observed at low temperatures. We employed Ge Kα X-ray fluorescence holography (XFH) for Fe5-xGeTe2 to directly investigate the local structure in the charge-ordered state, i.e., the superstructure. The Ge Kα XFH results revealed local atomic structures around the Ge atom, thus clarifying the simultaneous locations and arrangements of the Te, Fe, and Ge atoms. The atomic positions relative to the Ge atom are useful for understanding the coexistence of the ideal 1 × 1 structure and superstructure found in the charge-ordered state.

Local atomic structure of V-doped BiFeO3 thin films measured by X-ray fluorescence holography

Bismuth ferrite (BiFeO3: BFO) is a multiferroic material that exhibits ferroelectricity, antiferromagnetism, and ferroelasticity simultaneously at RT. BFO holds great promise as a ferroelectric semiconductor because of its ability to alter conductivity by reversing its spontaneous polarization. Moreover, BFO thin films doped with transition metals such as Mn or V can modulate their conductivity. Nevertheless, the mechanism of this conductivity change remains unclear because the effects of dopants on the local atomic structure of BFO are not fully understood. In this study, we investigated the local atomic structure around the Fe site in a V-doped BFO thin film by X-ray fluorescence holography. Reconstructed atomic structures from the Fe Kα hologram patterns revealed that the atomic structure stability of the V-doped BFO thin film differs from that of previously reported Mn-doped BFO thin films. The results provide important insights into the mechanism of controlling the conductivity of BFO thin films by dopants.

Local Structures of Fe<sub>0.08</sub>Co<sub>0.92</sub> Studied by X-ray Fluorescence Holography

Local Structures of Fe<sub>0.08</sub>Co<sub>0.92</sub> Studied by X-ray Fluorescence Holography

Direct Observation of Group-V Dopant Substitutional Defects in CdTe Single Crystals.

Point defect chemistry strongly affects the fundamental properties of materials and has a decisive impact on device performance. The Group-V dopant is prominent acceptor species with high hole concentration in CdTe; however, its local atomic structure is still not clear owing to difficulties in definitive measurements and discrepancies between experimental observations and theoretical models. Herein, we report on direct observation of the local structure for the As dopant in CdTe single crystals by the X-ray fluorescence holography (XFH) technique, which is a powerful tool to visualize three-dimensional atomic configurations around a specific element. The XFH result shows the As substituting on both Cd (AsCd) and Te (AsTe) sites. Although AsTe has been well known as a shallow acceptor, AsCd has not attracted much attention and been discussed so far. Our results provide new insights into point defects by expanding the experimental XFH study in combination with theoretical first-principles studies in II-VI semiconductors.

Local Structural Analysis around Solute-Element in Annealed Mg<sub>99.2</sub>Zn<sub>0.2</sub>Y<sub>0.6</sub> Alloy Using X-ray Fluorescence Holography

X-ray fluorescence holography was applied to dilute Mg99.2Zn0.2Y0.6 alloy annealed at 520 °C for 5 h to obtain atomic images around Zn atom. In spite of the extremely low concentration of the solute-element, clear hologram patterns were obtained. The reconstructed atomic images revealed that the Zn atom in this annealed Mg99.2Zn0.2Y0.6 alloy mainly occupy the hcp Mg site. This finding is consistent with the transmission electron microscope image, where fcc-type stacking faults are hardly observed. These results are in contrast to the previous report that Zn and Y form short-range-ordered solute clusters at the fcc-type stacking fault in Mg-Zn-Y alloy. The effect of heat treatment on the atomic arrangement of solute-elements in this dilute alloy is discussed in relation to the previously reported mechanical properties.

Multiple-site Ag doping in Bi[formula omitted]Se[formula omitted]: Compositional crossover from substitution to intercalation as revealed by photoelectron diffraction and X-ray fluorescence holography

The local structure around Ag atoms in Ag-doped Bi2Se3 (AgxBi2−ySe3) was investigated by photoelectron diffraction and X-ray fluorescence holography to understand the manner of Ag atom doping. At a low Ag concentration (x=0.05), photoelectron diffraction indicated that Ag atoms occupied Bi substitution sites. However, a mere accumulation of holes via Ag substitution for Bi fails to explain the observed variation previously reported in the transport properties of Ag-doped Bi2Se3 with different x values. In particular, simple Ag substitution for Bi fails to explain the pinning of the Fermi level at the bottom of conduction band, as suggested by the observed transport properties. In the case of a high Ag concentration (x=0.2), photoelectron diffraction suggested that the Ag atoms occupied not only the substitutional Bi site but also multiple interstitial sites, namely, the octahedral site in the van der Waals interlayer and the interstitial site in the Se layer. Ag 3d photoelectron spectra revealed that the Ag atoms had the same oxidation state, +1, regardless of the type of occupied site. Furthermore, X-ray fluorescence holography was employed for a model-free local structural analysis that refined the exact locations of Ag atoms in the Bi2Se3 crystal lattice. The behavioral crossover documented herein from hole doping at the substitutional site to electron doping at multiple sites reasonably explains the dependence of electronic structures and transport properties of Ag-doped Bi2Se3 on dopant concentration.

Development of serial X-ray fluorescence holography for radiation-sensitive protein crystals.

X-ray fluorescence holography (XFH) is a powerful atomic resolution technique capable of directly imaging the local atomic structure around atoms of a target element within a material. Although it is theoretically possible to use XFH to study the local structures of metal clusters in large protein crystals, the experiment has proven difficult to perform, especially on radiation-sensitive proteins. Here, the development of serial X-ray fluorescence holography to allow the direct recording of hologram patterns before the onset of radiation damage is reported. By combining a 2D hybrid detector and the serial data collection used in serial protein crystallography, the X-ray fluorescence hologram can be directly recorded in a fraction of the measurement time needed for conventional XFH measurements. This approach was demonstrated by obtaining the Mn Kα hologram pattern from the protein crystal Photosystem II without any X-ray-induced reduction of the Mn clusters. Furthermore, a method to interpret the fluorescence patterns as real-space projections of the atoms surrounding the Mn emitters has been developed, where the surrounding atoms produce large dark dips along the emitter-scatterer bond directions. This new technique paves the way for future experiments on protein crystals that aim to clarify the local atomic structures of their functional metal clusters, and for other related XFH experiments such as valence-selective XFH or time-resolved XFH.

Basics and Front of X-ray Fluorescence Holography

Basics and Front of X-ray Fluorescence Holography

Atomic Displacement in Pb-Free Piezo-Electric Material Visualized by X-ray Fluorescence Holography

Atomic Displacement in Pb-Free Piezo-Electric Material Visualized by X-ray Fluorescence Holography

Local structure analysis around Y in Mg99.7Y0.3 single crystal using x-ray fluorescence holography

Mg-Y alloys exhibit a good room-temperature ductility even at fairly large grain sizes and a moderately strong basal texture. Since a clarification of this structural origin is contributed for developing high-performance, affordable, lightweighted structural metals, it is desired to reveal the structural origin. The objective of this study is to reveal the structural origin of an enhanced ductility in a Mg99.7Y0.3 single crystal using an x-ray fluorescence holography (XFH) analysis. It was found that a part of Y atoms was located at the ideal positions in pure Mg but others formed Y dimers in the hexagonal close-packed structure of Mg99.7Y0.3. It was considered that the dimer formation plays an important role in activating non-limited the plastic slip in the crystallographic z-direction of Mg-Y alloys although the crystal structure is still hexagonal by close-packed.

Elucidation of the Local Structure of the Topological Insulator β-PdBi2 by X-ray Fluorescence Holography

The local structures around the Bi and Pd atoms in β-PdBi2 were determined from Bi Lα and Pd Kα X-ray fluorescence holography (XFH) images measured with synchrotron radiation at room temperature, demonstrating clear atomically resolved images along the c-axis (z-axis). More precisely, the location of atoms in the atomic layer at z = 0 Å was unambiguously determined from each hologram; that is, the location of Bi atoms at z = 0 Å was determined from Bi Lα XFH, whereas the location of Pd atoms at z = 0 Å was unambiguously determined from Pd Kα XFH. Moreover, the Bi atoms in the atomic plane at z = 3.6 Å were clearly reconstructed from Bi Lα XFH with Barton’s algorithm. However, clear atomic images of the planes at z = 1.8 Å and z = 2.9 Å were not obtained from the Bi Lα holograms, nor were they observed at z = 1.8 Å in the case of the Pd Kα holograms; some weak atomic images of Bi at z = 4.7 Å were observed from Pd Kα holograms. Thus, two different patterns can be distinguished for the atomic images in the plane at z ≠ 0 Å: a clear atomic image and an ambiguous atomic image, owing to the order or disorder between the atomic layers. The small fluctuation or the small disorder between the atomic layers formed through the attractive interaction of Bi––Pd2+–Bi– provides clear atomic images, whereas the atomic layers formed via weak van der Waals interactions lead to ambiguous atomic images. Thus, the atomic images of β-PdBi2 obtained by XFH clearly reproduce not only the crystal structure determined by single-crystal X-ray diffraction but also provide information on thermal fluctuation and static disorder between the atomic layers.

X-ray fluorescence holography of biological metal sites: Application to myoglobin

X-ray fluorescence holography (XFH) is a relatively new technique capable of providing unique three-dimensional structural information around specific atoms that act as a light source in crystalline samples. So far, XFH has typically been applied to inorganic materials such as dopants in metals and semiconductors. Here, we investigate the possibility of using XFH to visualize the metal active site in sperm whale myoglobin (Mb), a monomeric oxygen storage heme protein. We demonstrate that the atomic images reconstructed from the hologram data of crystals of carbonmonoxy myoglobin (MbCO) are moderately consistent with the crystal structure, which is also determined in this study by X-ray crystallography in the near-atomic resolution, as well as simulation results. These results open up a new avenue for the application of XFH to local atomic and electronic structure imaging of metal-sites in biomolecules.

X-ray Fluorescence Holography as a Probe of Hyper-Ordered Structures

X-ray Fluorescence Holography as a Probe of Hyper-Ordered Structures

Atomic positions and displacements in piezoelectric materials Ca3TaGa3Si2O14 and Ca3TaGa1.5Al1.5Si2O14 investigated by Ta-Lα X-ray fluorescence holography

The atomic positions and displacements of atoms around the Ta atom in piezoelectric materials Ca3TaGa3Si2O7 (CTGS) and Ca3TaGa1.5Al1.5Si2O7 (CTGAS) were investigated at 100 K by Ta-Lα X-ray fluorescence holography (XFH). The experimental atomic images were compared with the simulated ones using the crystal structures of CTGS and CTGAS, which were determined by single crystal X-ray diffractometry (SC-XRD). The atomic positions agreed between XFH and SC-XRD experiments. With the help of XFH simulation, the displacements of Ta, Ca, Si, and Ga atoms relative to the Ta atom were qualitatively analyzed using experimental atomic image intensities. The relative displacement of the Ca atom was increased by Al substitution, while those of the Ta and Ga atoms were decreased. The first principles calculation based on density-functional theory (DFT) was performed to understand bonding character between constituents. The evaluation of the crystal orbital Hamilton population (COHP) clarified that the Ca-O bond has strong ionic character different with the other bonds, suggesting that the positional shift of the Ca atom is responsible for the piezoelectricity in CTGS. The effect of Al substitution on piezoelectricity was also considered based on the change in the Ca-O bond.

Anomalous atomic fluctuations in the local structure around Mn of (Zn,Sn,Mn)As2 thin films

Local atomic structures around Mn in $(\mathrm{Zn},\mathrm{Sn},\mathrm{Mn}){\mathrm{As}}_{2}$ thin films grown on InP (001) substrates were studied using x-ray fluorescence holography. The reconstructed three-dimensional atomic images showed that the addition of Mn resulted in further distortions of the As sublattice from the pure sphalerite ${\mathrm{ZnSnAs}}_{2}$. The first-principles calculations showed that the further distortion could be attributed to the shortening of the Mn-As bond length induced by the p-d exchange mechanism, which mediates the ferromagnetism in this material. In addition, we found that the positional fluctuation of the cations was maximized at the second nearest neighbor cation shell from detailed analysis of the atomic images. This anomaly in the fluctuation could be explained by the calculated forces acting on the cations.

Epitaxial growth of a homogeneous anatase TiO2 thin film on LaAlO3 (0 0 1) using a solvothermal method with anticorrosive ligands

TiO2 is one of the most extensively investigated oxides owing to its numerous potential applications. However, most methods reported for the growth of epitaxial thin films of anatase TiO2 are costly and laborious. In this study, a solvothermal method being simple and inexpensive was used for epitaxial growth of anatase TiO2 thin films on a LaAlO3 substrate. Solvothermal methods usually use highly acidic conditions to stabilize Ti ions in water. The acids used tend to corrode the substrate. Hence, acetylacetone was used as a stabilizer to grow epitaxial thin films. The fabricated epitaxial thin film had the orientation of (001) [100] TiO2 || (001) [100] LaAlO3 with 2.5 μm thickness. The fabricated thin film can be used as applications such as photocatalysts, electrodes, and optical devices. Substitution of Ti atom with dopant atom was clarified using X-ray fluorescence holography.

Parameter estimation for X-ray scattering analysis with Hamiltonian Markov Chain Monte Carlo.

Bayesian-inference-based approaches, in particular the random-walk Markov Chain Monte Carlo (MCMC) method, have received much attention recently for X-ray scattering analysis. Hamiltonian MCMC, a state-of-the-art development in the field of MCMC, has become popular in recent years. It utilizes Hamiltonian dynamics for indirect but much more efficient drawings of the model parameters. We described the principle of the Hamiltonian MCMC for inversion problems in X-ray scattering analysis by estimating high-dimensional models for several motivating scenarios in small-angle X-ray scattering, reflectivity, and X-ray fluorescence holography. Hamiltonian MCMC with appropriate preconditioning can deliver superior performance over the random-walk MCMC, and thus can be used as an efficient tool for the statistical analysis of the parameter distributions, as well as model predictions and confidence analysis.

Local Structure Analysis on Yttria-Stabilized Zirconia by X-ray Fluorescence Holography

Local Structure Analysis on Yttria-Stabilized Zirconia by X-ray Fluorescence Holography

Significant displacement of calcium and barium ions in ferroelectric (Ba0.9Ca0.1)TiO3 revealed by x-ray fluorescence holography

The x-ray fluorescence holography technique is applied to perovskite (Ba0.9Ca0.1)TiO3 ferroelectrics in order to investigate the local structures around the Ca and Ba ions at the A-site. While the A-site atomic images around the Ba ion can be well understood based on the BaTiO3 structure, those around the Ca ion are located at the closer positions to the emitter. Furthermore, the shape of the atomic image around the Ca ion is elongated in the radial direction. These features clearly show the local lattice contraction around the Ca ion and a significant displacement of the Ca ions from the ideal position. On the basis of the detailed analysis of the position and the shape of the atomic image around the Ca ion, it is found that the Ca ions are displaced by ∼0.36 Å in the ⟨111⟩ direction, and the surrounding Ba ions are displaced by ∼0.22 Å in the ⟨100⟩ direction. The present results indicate that the doped Ca ions activate the A-site polarization of this compound, which can lead to an enhancement of the ferroelectric and piezoelectric performances.