Anomalous Fine Structure Research Articles

Site-selective analyses of battery reactivities in Li1+xNi1-xO2by powder DAFS

Deep understanding of a battery reactivity of transition metals in a positive electrode is essential for further development of lithium ion batteries (LIBs). Li1-xNi1+xO2(LNO, 0 < x < 1) is one of the typical positive-electrodes with a layered rock-salt structure. LNO often contains significant antisite defects of Ni at Li site, i.e., interlayer gallery, in addition to Ni site in the host layer. Though a relationship between such structural disorder and capacity fading of the battery has been reported[1], its degradation mechanism is still unclear presumably because of insufficient insight into electrochemical behavior of Ni at each site in LNO. In the present work, powder diffraction anomalous fine structure (P-DAFS)[2] has been applied to LNO to analyze the valence and local structure of Ni at both different symmetric sites for tackling this degradation issue. We prepared pellets of as-synthesized and charged LNOs for P-DAFS measurements. The composition of the samples is [LiyNi0.11]3a[Ni]3b[O2]6c(y = 0.89, 0.40) in R-3m space group. DAFS measurements were carried out at undulator beamline BL28XU, SPring-8 in Japan. The DAFS spectra were measured with use of 003 and 104 diffractions in ~1 keV energy range around Ni K-edge (8.331 keV). The XAFS-like spectra were extracted from DAFS spectra with a priori structural information from Rietveld analyses. Figure 1 shows (a) the DAFS spectra of the as-synthesized sample, XAFS-like spectra of Ni occupying the Ni site (b) and the Li site (c) in the as-synthesized (solid line) and the charged (broken line) samples. The spectrum at the Ni site shows a shift to the higher energy with charging, indicating the oxidation of Ni accompanied with Li extraction. In contrast, Ni at the Li site shows no significant shift with charging. Thus, Ni at the Li site hardly compensates the charge for delithiation, which indicates that the electrochemically inactive Ni occupying the Li sites would cause irreversibility in this system.

Structure Determination of Ho2PdSi3

Holmium-Palladium-Silicide Ho2PdSi3 is a member of rare earth-transition metal silicides exhibiting a wide range of interesting magnetic and electrical properties like multiple transition temperatues. The crystal structure results from HoSi2 by substitution of Si by Pd which is ordering commensurably with a 2 × 2 × 8 superstructure confirmed by a previous XRD and a Diffraction Anomalous Fine Structure (DAFS) measurement of the super structure reflection 1/2 1/2 3/8. DAFS is a X-ray method combining the advantages of absorption and diffraction and hence offers the possibility of element and site selective studies. Thus, it was feasible to probe the local environment of Ho and Pd separately. In the following, we will present a comparison of several structure proposals of Ho2PdSi3 with experimental data from beamline E2 and BW1 of the former synchrotron DORIS III at DESY/HASYLAB.

Evaluation of structure models of Ho2PdSi3 using DAFS, inter alia at a satellite reflection

The compounds R2PdSi3, with R = rare earth, exhibit a very interesting magnetic behavior with two phase transitions. Substituting one in four Si atoms by Pd in HoSi2 results in a modulation of the aristotype. There are several different variants discussed in literature about the nature of the modulation of this rare-earth compound. Two of the latest models were compared: a 2 × 2 × 1 layer and a 2 × 2 × 8 stack. The chosen method is Diffraction Anomalous Fine Structure (DAFS) and was applied both experimentally and by simulation at different absorption edges and reflections, i. a. a satellite reflection, aiming on finding the correct crystal structure.

Probing a crystal's short‐range structure and local orbitals by Resonant X‐ray Diffraction methods

Diffraction Anomalous Fine Structure (DAFS) combines the long‐range, crystallographic sensitivity of X‐ray diffraction with the short‐range sensitivity of X‐ray Absorption Spectroscopy (XAS). In comparison to other spectroscopic methods, DAFS can additionally distinguish phases of different translational symmetry by choice of momentum transfer, or isolate spectra from chemically identical atoms on various Wyckoff sites of a crystal's structure using crystallographic weights. The Anisotropy of Anomalous Scattering (AAS) extends the concept of isotropically scattering atoms to a more general case, where the atom's scattering characteristics depend on the polarization as well as the wavevector of the incident and scattered X‐rays. These can be written as tensors that reflect the local site symmetries of the resonant atom. Forbidden Reflection Near‐Edge Diffraction (FRED) is an elegant way to measure AAS by using reflections that are extinguished in the special case of isotropically scattering atoms. They can only be observed due to the non‐isotropic contributions at photon energies in the vicinity of an absorption edge where electronic transitions occur. Combining the site selectivity of DAFS with the information accessible through AAS allows probing the short‐range order and local orbitals of selected atoms in a crystal structure of a chosen phase. The present condensed review gives a brief overview on the pioneer work, the theory and sensitivities as well as selected recent applications of these powerful and promising Resonant X‐ray Diffraction (RXD) methods. Additionally, some recent work of the authors is included exemplarily for the model structure rutile TiO2 presenting the progress in measurement and interpretation.

Ultrathin Oriented BiFeO3 Films from Deposition of Atomic Layers with Greatly Improved Leakage and Ferroelectric Properties

Highly (001)-oriented BiFeO3 ultrathin films of total thickness of less than 10 nm were deposited on Si(001) substrates via deposition of atomic layers (ALD) with a LaNiO3 buffer. A radio-frequency (RF)-sputtered sample of the same thickness was prepared for comparison. The ALD combined with interrupted flow and an exchange reaction between Bi and Fe precursors provides a superior method to grow ternary compounds. According to X-ray diffraction, upon deposition at a temperature of less than 550 °C, the only phase in the film was BiFeO3. Anomalous fine structure from synchrotron X-ray diffraction certified the valence bonding through the BiFeO3 (001) diffraction signal. The stoichiometric ratio of BiFeO3 obtained from X-ray photoelectron spectroscopy indicated that ALD has a proportion much improved over the RF preparation, and this is also in agreement with the results for diffraction anomalous fine structure. The use of high-resolution transmission electron and atomic force microscopes showed that the layer structure and morphology from ALD presented a satisfactory coverage, more conformal than that with the RF method. The BiFeO3 thin film deposited with ALD shows excellent leakage, improved at least 1000 times with respect to the RF preparation, making this method suitable for the fabrication of ferroelectric random-access memory devices. From the hysteresis loop, the largest remanent polarization was observed as 2Pr = 2.0 μC cm(-2).

Synthesis and structural characterization of ZnO doped with Co

Co-doped Zn1−xCoxO nanoparticles were prepared by hydrothermal synthesis. The morphology, solid solubility and local atomic structure around the doping Co atoms were investigated. The morphologies of Co-doped ZnO nanoparticles were observed by scanning electron microscopy. The crystal structure of nanoparticles and the possible impurity phase were analyzed by X-ray diffraction patterns. The local atomic arrangements were studied by X-ray absorption fine structure spectroscopy. The substitution of Co for Zn was confirmed by diffraction anomalous fine structure spectra. The results demonstrated that Zn1−xCoxO can form a single phase of Co-doped ZnO with x⩽0.03. But it is a mixture of Co-doped ZnO and Co(OH)2 phases with x⩾0.05. Hydrothermal temperature can decrease the solid solubility of Co in ZnO lattice. Polar molecules can control the morphology of the nanoparticles. The Co-doped ZnO nanoparticles take a rod-like shape and the axial direction of the nanorods is along the c-axis direction of hexagonal wurtzite unit cell. A mixed-valence state of Co2+ and Co3+ is identified for the Co-doped ZnO nanoparticles. Diffraction anomalous fine structure technique could be used to determine the solid solubility of impurity when only one equivalent crystalline site in the host crystal is replaced by the impurity atoms. The Co-doped ZnO particles were identified to have room-temperature ferromagnetic properties.

Strain assisted inter-diffusion in GaN/AlN quantum dots

The structural and optical properties of high temperature-annealed superlattices of GaN quantum dots embedded in AlN barrier have been studied by a combination of X-ray techniques (reciprocal space mapping, multiwavelength anomalous diffraction, and diffraction anomalous fine structure), high resolution transmission electron microscopy, and photoluminescence spectroscopy. Taking advantage of the disentangling of the chemical and structural information provided by the simultaneous use of X-ray absorption and diffraction data obtained in a synchrotron environment, we provide quantitative determination of strain and composition for each different region of the nanostructures. Eventually, it is shown that strain driven dot/barrier intermixing is present, mostly on top of the dots. These observations have been confirmed by high resolution electron microscopy. A blue shift of photoluminescence peak has been furthermore observed and assigned to GaN/AlN intermixing suggesting a new path for engineering the emission wavelength of such heterostructures.

Multiwavelength anomalous diffraction and diffraction anomalous fine structure to study composition and strain of semiconductor nanostructures

The aim of this paper is to illustrate the use of Multi-Wavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy for the study of structural properties of semiconductor nanostructures. We give a brief introduction on the basic principles of these techniques providing a detailed bibliography. Then we focus on the data reduction and analysis and we give specific examples of their application on three different kinds of semiconductor nanostructures: Ge/Si nanoislands, AlN capped GaN/AlN Quantum Dots and AlGaN/AlN Nanowires. We show that the combination of MAD and DAFS is a very powerful tool to solve the structural problem of these materials of high technological impact. In particular, the effects of composition and strain on diffraction are disentangled and composition can be determined in a reliable way, even at the interface between nanostructure and substrate. We show the great possibilities of this method and give the reader the basic tools to undertake its use.

Complementary information on CdSe/ZnSe quantum dot local structure from extended X-ray absorption fine structure and diffraction anomalous fine structure measurements

The extended X-ray absorption fine structure (EXAFS) and diffraction anomalous fine structure (DAFS) have been applied to investigate a local structure for the CdSe/ZnSe quantum dots grown by molecular beam epitaxy (MBE) and migration-enhanced epitaxy (MEE). The aim was to study the intermixing of Cd and Zn atoms, chemical compositions and strain induced by cap-layer. The EXAFS at the Cd K-edge and DAFS at the Se K-edge proved the intermixing of Cd and Zn atoms. The distances Cd–Se (2.61Å) found from EXAFS and DAFS analysis for h1 region is closer to that in bulk CdSe (2.62Å). The DAFS analysis revealed the differences in the local structure in two investigated regions (i.e. different iso-strain volumes) on the quantum dots. It was found that the investigated areas differ in the Cd concentration. To explain the experimental results the theoretical calculation based on a full valence-force field (VFF) model was performed. The theoretical VFF model fully explains the experimental data.

Diffraction Anomalous Fine Structure study and atomistic simulation of Ge/Si nanoislands

We applied Grazing Incidence Diffraction Anomalous Fine Structure to the study of the structure of Ge dome-shaped nanoislands, grown by Molecular Beam Epitaxy on Si (001) substrates at a temperature of 650°C. We determined the vertical composition of the islands showing the presence of a strong Ge/Si intermixing that is nearly constant from bottom to top. In particular, an abrupt change is found at the substrate interface where the composition switches from pure Si to Ge0.6Si0.4. The analysis of the Diffraction Anomalous Fine Structure oscillations of the spectra is crucial to obtain the true composition profile. We performed atomistic simulations to investigate the role of the strained substrate underneath the dome on the diffraction results and to quantify the resolution of our method. Anomalous Diffraction spectra and Diffraction Anomalous Fine Structure oscillations have been simulated for a real size and real shape cluster including faceting, giving a more detailed data interpretation and understanding of the Ge–Si intermixing mechanism.

Anisotropic local structure of decagonal quasicrystals by DAFS

Diffraction anomalous fine structure (DAFS) analyses were performed on Al–Ni–Co (ANC) and Al–Ni–Fe (ANF) decagonal quasicrystals at the K-edges of Ni, Co and Fe. In ANC, an order–disorder phase transition with anisotropic atomic short-range order (SRO) is clearly recognizable at high temperature. During in situ observation, a zigzag correlation along the periodic direction developed during the early stage of ordering. Far above the phase transition temperature, this weak correlation along the periodic direction is independent of other correlations along the quasiperiodic one. In ANF, SRO of Fe developed along the quasiperiodic direction, although the local environment of Ni was almost random.

Theoretical study on mechanisms of anomalous fine structure in the magnesium isoelectronic sequence

Using the multi-configuration Dirac–Fock self-consistent field method and the relativistic configuration interaction method, the calculations are carried out for the fine structure (FS) energy levels of Mg I, Al II, Si III, S V excited states (3s3d 1,3D, 3s4d 1,3D, 3s4s 1,3S, 3s3p 1,3P and 3s4p 1,3P). Breit interactions and quantum electrodynamics corrections are added as corrections. For Mg and Mg-like ions studied here, electron correlations are treated adequately, intravalence electron correlations in particular. The calculation results are found to be in good agreement with the experimental data and compare favourably with other theoretical results. By means of precise computational methods, we elucidate four competitive mechanisms which affect the interesting FS splitting orderings in the magnesium isoelectronic sequence.

Structural properties of Ge/Si(001) nano-islands and AlGaN nanowires by Diffraction Anomalous Fine Structure and Multiwavelength Anomalous Diffraction

In this paper, we show that combining Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy, in grazing incidence geometry, allows to obtain structural properties (strain and composition) of semiconductor nanostructures. We report results obtained on dome-shaped Ge nano-islands grown on Si(001) surfaces and AlGaN nanowires grown on Si(100). It is shown that, in the case of sharp interfaces, MAD alone can not determine the mean Ge content in the region of the substrate-island interface and needs to be combined with Extented-DAFS measurements.

Observation of inner-shell-excited configurations in triply ionized ceriumCe3+

We report wavelength measurements and energy levels for the three-times ionized cerium atom ${\text{Ce}}^{3+}$. The spectrum is that of a simple one-electron atom overlaid with a more complex three-electron spectrum originating from the excitation of an electron in the inner closed shell to the valence shell. The data provide energy levels for the inner-shell-excited configurations $5{p}^{5}4f5d$ and $5{p}^{5}4f6s$ and solve a long-standing puzzle regarding the fine structure of the $5{p}^{6}6d$ configuration, which is known to be anomalously large and inverted. The data show that this anomalous fine structure is the result of interaction between $5{p}^{6}6d$ and the inner-shell-excited configuration $5{p}^{5}4f5d$. The results are supported by Hartree-Fock calculations and least-squares fits of the energy parameters to the observed levels. From our observed $5{p}^{6}6g$ configuration, we determine the ionization energy as $297670\ifmmode\pm\else\textpm\fi{}70\text{ }{\text{cm}}^{\ensuremath{-}1}$ $(36.906\ifmmode\pm\else\textpm\fi{}0.009\text{ }\text{eV})$.

Diffraction anomalous fine structure study of iron/iron oxide nanoparticles

Diffraction anomalous fine structure is a recently developed technique which can provide a measurement of the local structure of a given element in a particular phase or crystallographic site. Most previous investigations have applied the technique to bulk solids, thin films, and nano-dots or wires on crystalline substrates. In this paper, the technique is applied to highly disordered nanometre-sized Fe/Fe oxide core–shell nanocrystalline powders, the diffraction patterns of which exhibit weak and greatly broadened diffraction peaks. Focusing on the oxide shell diffraction peaks, a qualitative analysis of the near-edge spectral region and a quantitative analysis of the extended energy region are provided; in particular, good quality fittings of the extended range spectra are obtained. The local structure is selectively probed around the tetrahedral and octahedral sites of the oxide shell, finding the presence of a high degree of structural disorder. This study demonstrates that diffraction anomalous fine structure can now be fruitfully applied to nanocrystalline powders.

Structural properties of Ge/Si(001) nano-islands by diffraction anomalous fine structure and multiwavelength anomalous diffraction

In the present paper, we aim to show the interest of combining Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy, in grazing incidence, to obtain structural properties (composition, strain and atomic ordering) of semiconductor heterostructures and nanostructures. As an example we report on preliminary results obtained on a series of Ge/Si(001) nano-island samples: pyramides and domes on nominal and prepatterned surfaces. For free standing domes, it is shown that the Ge content strongly depends on the growth condition with a tendency to increase from the bottom to the top of the nano-islands. There is also some indication of atomic ordering in the upper part of the islands. For small, capped pyramids, we show that the Diffraction Anomalous Fine Structure spectroscopy is the unique non destructive method that allows to recover the actual Ge content, the in-plane and out-of-plane strain and to detect atomic ordering.

Anisotropic strain state of the [11¯00] GaN quantum dots and quantum wires

The strain state of the [11¯00] GaN quantum dots and quantum wires has been studied by a combination of multiwavelength anomalous diffraction and diffraction anomalous fine structure under grazing incidence. The three components of the anisotropic deformation have been independently determined. On one hand, a weak residual strain state along the c-axis, similar for quantum dots and quantum wires, is found. On the other hand, along the [112¯0] direction, quantum wires appear to be more relaxed than quantum dots. Based on high resolution transmission electron microscopy, this is assigned to elastic strain relaxation of GaN governed by either misfit dislocation formation or three-dimensional islanding, depending on the anisotropic strain state of the [11¯00] AlN buffer layer.

Element- and site-specific oxidation state and cation distribution in manganese ferrite films by diffraction anomalous fine structure

Epitaxial manganese ferrite thin films were studied by x-ray diffraction anomalous fine structure to obtain element-specific and site-specific information on site occupancy, local structure, and valency. These properties were introduced to molecular field theory to reproduce thermomagnetization curves and determine superexchange energy, Néel temperature, and spin canting angle.

Raman scattering as a tool for the evaluation of strain inGaN∕AlNquantum dots: The effect of capping

The strain state of $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ quantum dots grown on $6H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ has been investigated as a function of AlN capping thickness by three different techniques. On the one hand, resonant Raman scattering allowed the detection of the ${A}_{1}(\mathrm{LO})$ quasiconfined mode. It was found that its frequency increases with AlN deposition, while its linewidth did not evolve significantly. Available experiments of multiwavelength anomalous diffraction and diffraction anomalous fine structure on the same samples provided the determination of the wurtzite lattice parameters $a$ and $c$ of the quantum dots. A very good agreement is found between resonant Raman scattering and x-ray measurements, especially concerning the in-plane strain state. The results demonstrate the adequacy of Raman scattering, in combination with the deformation potential and biaxial approximations, to determine quantitatively values of strain in GaN quantum dot layers.

Crystallography: past and present

In the 19th century, crystallography referred to the study of crystal shapes. Such studies by Hauy and Bravais allowed the establishment of important hypotheses such as (i) “les molecules integrantes qui sont censees etre les plus petits solides que l’on puisse extraire d’un mineral” [1], (ii) the definition of the crystal lattice and (iii) “le cristal est clivable parallelement a deux ou trois formes cristallines” [2]. This morphological crystallography defined a crystal like “a chemically homogeneous solid, wholly or partly bounded by natural planes that intersect at predetermined angles” [3]. It described the main symmetry elements and operations, nomenclatures of different crystal forms and also the theory of twinning. A breakthrough appeared in 1912 with the use of X-rays by M. von Laue and W.H. and W.L. Bragg. This experimental development allowed the determination of the atomic content of each unit cell constituting the crystal and defined a crystal as “any solid in which an atomic pattern is repeated periodically in three dimensions, that is, any solid that “diffracts” an incident X-ray beam” [3]. Mathematical tools like the Patterson methods, the direct methods, were developed. The way for solving crystalline structure was opened first for simple compounds and at that time crystallography was associated mainly with perfect crystals. In the fifties, crystallographers already had most apparatus and fundamental methods at their disposal; however, we had to wait for the development of computers to see the full use of these tools. Furthermore the development of new sources of neutrons, electrons and synchrotron X-rays allowed the studies of complex compounds like large macromolecules in biology. Nowadays, one of the new frontiers for crystallographers is to relate the crystal structure to its physical-chemical-biological properties, this means that an accurate structural determination is needed to focus on a selective part of the structure (chemical order, anisotropy, charge transfer, magnetic order) versus an external parameter like temperature, pressure, magnetic or electric field. Modern crystallography is also extended to the study of very small crystals, powders, ill-ordered or non-crystallized materials. Thus presently, crystallography is concerned with any solid that “scatters” an incident beam. Nevertheless, as quoted by A. Guinier, “the problems facing crystallographers have only changed, ... new ones have appeared which require reflection and imagination, ... and which in turn may still bring much joy to all those who like crystallography” [4]. Such developments open up crystallography to modern materials like artificial ones and nanostructures with low- and/or multi-scaled-periodicities and/or extremely small “crystal size” and to materials of the “real world”, with mixtures of phases and/or amorphous contribution and/or defects, a common characteristic of ancient materials analysed in patrimonial research. In our contribution we will show by selected examples that these improvements were allowed (i) by the use of powerful sources, apparatus and detectors which allow micro-diffraction, in-situ diffraction, spectroscopy, resonant scattering, inelastic scattering, coherent scattering, (ii) by the development of methods like diffraction anomalous fine structure (DAFS), pair distribution function (PDF), simulated annealing, single object reconstruction, (iii) by combination of scattering and spectroscopy and by combination of scattering and microscopy. Such combination of different approaches is very efficient and, as said by H. Curien at the IUCr Bordeaux Congress in 1990, “in crystallography, there is a constant alternation between the crystal space and its associated reciprocal space, ... the alternation between experiment and model building is another feature of crystallography activity ..., the crystallographer relies both on his computer and on his diffractometer ..., all these dualistic complementarities are responsible for the fact that crystallographers tend to waver between the description of atomic structures and the explanation of macroscopic properties of solid matter” [5].