Superlattice Reflections Research Articles

Depth Profile of Induced Magnetic Polarization in Au Layers of Fe/Au(001) Metallic Superlattice by Resonant X-ray Magnetic Scattering at High Angle Region

Spin polarization of 5d conduction electrons induced in the Au layers of antiferromagnetically coupled Fe/Au metallic superlattice was investigated by resonant X-ray magnetic diffraction at the Au L3 absorption edge. Superlattice reflections around the Au 002 Bragg peak were measured in the magnetic saturation state with circularly polarized synchrotron radiation X-rays of 11921 eV. The magnetic diffraction profile was extracted with the combination of helicity switching and reversing the direction of magnetic field applied along the line of intersection between the film surface and the scattering plane. The depth distribution of resonant magnetic scattering length in the Au layers was estimated from the magnetic diffraction profile. Both resonant magnetic diffraction from the Au layer and non-resonant magnetic diffraction from the Fe layer were considered in the analysis. We demonstrated that the depth distribution of the Au magnetic scattering length was expressed with sum of the interface-concentrated ...

A neutron diffraction study of lattice distortion, mismatch and misorientation in a single-crystal superalloy after different heat treatments

Two-dimensional neutron diffraction measurements of superlattice and fundamental lattice reflections from a single-crystal superalloy confirm the existence of angular distortion and shear stress in the γ′ phase, and reveal its correlation with the morphology and deformation of the γ′ phase during heat treatment. The tetragonal distortions with c/a>1 are developed and retained during subsequent heat treatments, whereas the angular distortions are enhanced for the γ′ phase during the aging treatments. The evolution of the lattice mismatch and anisotropy during the subsequent heat treatments are consistent with the transformation of certain macroscopic anisotropic compositional and stress distributions in the as-cast sample to the microscopic level at the γ/γ′-interface upon reprecipitation and the growth of the γ′ from the supersaturated γ-matrix. The high negative γ/γ′ lattice mismatch of the alloy is consistent with the high levels of refractory elements, in particular Cr, in the alloy. Comparison between superlattice and fundamental lattice reflections is revealing. Firstly, the existence of lattice misorientation at the γ/γ′-interface and the discrete misorientations from the splitting of coherently aligned aggregates of γ′-precipitates on the smoothly distributed γ -matrix is shown. Secondly, the measured lattice misorientation correlates with the spacing of dislocations at the γ/γ′-interface and evidenced by the transmission electron microscopy observation. Furthermore, it indicates the anisotropic distribution of dendrites along crystal growth axis and the presence of small misoriented higher-order dendrite arms in variously heat-treated samples.

Oxygen-Vacancy-Induced Orbital Reconstruction of Ti Ions at the Interface ofLaAlO3/SrTiO3Heterostructures: A Resonant Soft-X-Ray Scattering Study

Resonant soft-x-ray scattering measurements have been performed to investigate interface electronic structures of (LaAlO(3)/SrTiO(3)) superlattices. Resonant scattering intensities at superlattice reflections show clear evidence of degeneracy lifting in t(2g) states of interface Ti ions. Polarization dependence of intensities indicates the energy of d(xy) states is lower by ~1 eV than two other t(2g) states. The energy splitting is insensitive to epitaxial strain. The orbital reconstruction is induced by oxygen vacancies and confined to the interface within two unit cells, indicating charge compensation at the polar interfaces.

Optical and x-ray time resolved study of the structural transition in mixed valence manganites

Time resolved optical reflectivity and x-ray diffraction techniques are employed to study the laser-induced structural response in two charge and orbitally ordered manganites. Optical data indicate a non-thermal nature of the laser-triggered phase transition via the disappearance of an optical phonon related to the charge and orbitally ordered phase. The x-ray diffraction measurements on superlattice reflections confirm the non-thermal time scale of the initial step of this phase transition but also show that the complete change of structural symmetry is not instantaneous.

Anisotropic Form Factors of Neutron Scattering by Magnetic Octupole in CeB6

The form factor of elastic neutron scattering is calculated for the quadrupole order phase of CeB6 in magnetic fields. It is shown that the dependence of the form factor on the direction of neutron momentum transfer is very small for the Bragg reflections, whereas the scattering due to field-induced octupoles gives rise to a significant anisotropy of the form factor for the super-lattice reflections. These results are discussed in quantitative comparison with recent exprimental results. The similarity and dissimilarity with the results for phase IV in Ce1−xLaxB6 is also discussed.

New structural features of the high-pressure synthetic sheet-disilicate Phase-X, K(2-x)Mg2Si2O7Hx

The structure of the synthetic high-pressure sheet-disilicate Phase-X (PhX), a possible host of H 2 O and K in the mantle, has been determined for a crystal synthesized at 16 GPa/1300 °C/23 h. The composition of the sample is close to K 1.5 Mg 2 Si 2 O 7 H 0.5 , which is 50% PhX/50% Anhydrous-PhX and has 25% of interlayer K sites vacant. The structures of four crystals were determined by singlecrystal X-ray diffraction and had very similar diffraction characteristics and structural results; the structure of one of the larger crystals is reported here. Reflection intensity statistics strongly indicate that PhX is centrosymmetric, space group P6 3 /mcm, in contrast to other studies that have reported non-centrosymmetric space group P63cm. While it was possible to obtain good agreement indices for refinements in P63cm, there were strong correlations between atoms that are equivalent in P6 3 /mcm, suggesting that the correct structure is centrosymmetric. Full anisotropic refinement in space group P6 3 /mcm gave R 1 = 0.036, wR 2 = 0.079, GoF = 1.467. As with all previous studies of PhX, the H atom was not located. Difference-Fourier maps of the residual electron density indicated that the K atom is displaced from the 4c site lying on the sixfold axis on to three split 12j sites 0.2 Å away, each having ¼ occupancy, giving a total of 3 K atoms per unit cell and corresponding to 1.5 K apfu, in good agreement with the content derived from electron microprobe analysis. Diffraction patterns of all four crystals examined, reconstructed from the full-intensity data collection, consistently show the presence of a large hexagonal superstructure with dimensions 8a sub × 8a sub × c sub , having Z = 128, compared with Z = 2 for the two-layer subcell. Complex arrays of superlattice reflections occur in layers with l = 2n, but are absent from l = 2n + 1 layers. Unpolarized infrared spectra of single crystals of PhX were obtained that are similar to those reported previously in the literature. Spectra in the OH-stretching region consist of a major absorption band at 3595 cm -1 and three much weaker bands at 3690, 3560, and 3405 cm -1 . Bond-valence analysis of PhX indicates that O1 is very over-bonded, whereas O2 is slightly under-bonded and a possible site for protonation. We present geometrical and crystal-chemical arguments that exclude O1 as a candidate for protonation, whereas a much better case can be made for O2. In PhX structures, H must be located at a partially occupied site with a multiplicity 4 ≤ m ≤ 24 in P6 3 /mcm or 4 ≤ m ≤ 12 in P6 3 cm. Such low occupancies for H sites are the likely reason for their invisibility to diffraction. We outline a model for the incorporation of H into PhX of composition K 1.5 Mg 2 Si 2 O 7 H 0.5 that suggests a mechanism for ordering based upon avoidance of H and K, coupled with K-site vacancies. Such behavior may also be the origin of the superstructure. The P6 3 /mcm structure and the presence of an underlying superstructure may well be characteristic of ordered intermediate compositions at or near PhX 50 /Anhydrous-PhX 50 . Identification of a new space group and recognition of a previously unobserved superstructure point to new possibilities for PhX and its derivatives that may bear significantly upon their stability at mantle conditions.

Complex structural phase transitions in slightly Ca modified Na0.5Bi0.5TiO3

A temperature dependent neutron powder diffraction study, in conjunction with dielectric and ferroelectric characterization, of slightly Ca modified Na0.5Bi0.5TiO3 (NBT) revealed an instability with regard to a non-polar orthorhombic (Pbnm) distortion above room temperature. This intermediate orthorhombic phase has earlier been reported for unmodified NBT by electron diffraction studies, but has never been captured by global (x-ray/neutron) diffraction techniques. Calcium substitution seems to amplify the magnitude of this intermediate orthorhombic distortion thereby making the corresponding superlattice reflections become visible in the neutron diffraction pattern. The study revealed the following sequence of very complex structural evolution with temperature: .

Distinct Charge Orders in the Planes and Chains of Ortho-III-OrderedYBa2Cu3O6+δSuperconductors Identified by Resonant Elastic X-ray Scattering

Recently, charge density wave (CDW) order in the CuO(2) planes of underdoped YBa(2)Cu(3)O(6+δ) was detected using resonant soft x-ray scattering. An important question remains: is the chain layer responsible for this charge ordering? Here, we explore the energy and polarization dependence of the resonant scattering intensity in a detwinned sample of YBa(2)Cu(3)O(6.75) with ortho-III oxygen ordering in the chain layer. We show that the ortho-III CDW order in the chains is distinct from the CDW order in the planes. The ortho-III structure gives rise to a commensurate superlattice reflection at Q=[0.33 0 L] whose energy and polarization dependence agrees with expectations for oxygen ordering and a spatial modulation of the Cu valence in the chains. Incommensurate peaks at [0.30 0 L] and [0 0.30 L] from the CDW order in the planes are shown to be distinct in Q as well as their temperature, energy, and polarization dependence, and are thus unrelated to the structure of the chain layer. Moreover, the energy dependence of the CDW order in the planes is shown to result from a spatial modulation of energies of the Cu 2p to 3d(x(2)-y(2)) transition, similar to stripe-ordered 214 cuprates.

Phase transformations in Ni + Al and Ni + Al + Cr mixtures during mechanochemical synthesis and subsequent heating

Mechanochemical processing of elemental mixtures with the compositions Ni75Al25, Ni70Al25Cr5, and Ni75Al20Cr5 (5 at % Cr in the mixtures instead of the equivalent amount of Ni or Al) leads to the formation of nanocrystalline nickel-based solid solutions (crystallite size in the range ≃ 7–12 nm). Comparison of experimentally determined lattice parameters of the solid solutions with Vegard’s law values and with the lattice parameters evaluated using the Bozzolo-Ferrante rule, which takes into account the bulk moduli of constituent elements, suggests that the atoms in the solid solutions are bonded more strongly. Heating the synthesized ternary solid solutions in a calorimeter to 1000°C leads to the formation of an ordered γ′-phase (L12). Analysis of the relative intensity ratio of superlattice and fundamental reflections indicates that the Cr atoms always reside in the Al sublattice, independent of the composition of the starting mixture. When 5 at % Cr is incorporated instead of Ni, the chromium atoms force out aluminum from the Al sublattice, and the Ni deficiency in the Ni sublattice is compensated by the Al atoms. The ordered phases remain nanocrystalline (crystallite size in the range ≃ 40–70 nm).

Direct observation of anti-phase boundaries in heteroepitaxy of GaSb thin films grown on Si(001) by transmission electron microscopy

Unambiguous identification of anti-phase boundaries (APBs) in heteroepitaxial films of GaSb grown on Si has been so far elusive. In this work, we present conventional transmission electron microscopy (TEM) diffraction contrast imaging using superlattice reflections, in conjunction with convergent beam electron diffraction analysis, to determine a change in polarity across APBs in order to confirm the presence of anti-phase disorder. In-depth analysis of anti-phase disorder is further supported with atomic resolution high-angle annular dark-field scanning transmission electron microscopy. The nature of APBs in GaSb is further elucidated by a comparison to previous results for GaAs epilayers grown on Si.

Effects of order–disorder reactions on rapidly quenched Fe–6.5%Si alloy

Abstract Deposits of the Fe–6.5wt.%Si alloy rapidly quenched by spray forming were investigated. The order phase can be either B2 or DO3 depending on annealing treatment conditions. The observation of pairs dislocations indicates the presence of super dislocations and B2 antiphase boundaries (APBs) which affects significantly the soft magnetic properties. The dislocations bound the APBS which yield δ fringes when image 200 superlattice reflections. Samples treated at 700 °C for 1 h were oil quenched, this has induced a decreasing of power loss and the TEM micrographs have showed developed 1/4〈1 1 1〉 anti phase domain structure in the B2 phase. The magnetic properties were: power loss of 1.59 W/kg and coercive force of 76 A/m, at B = 1 T, f = 60 Hz. The samples annealed at 1250 °C for 1 h presented the same interaction between the APBs and magnetic properties presenting better power losses. The magnetic properties were: power loss of 1.30 W/kg and coercive force of 40 A/m, at B = 1 T, f = 60 Hz. This suggests a strong interaction between magnetic properties and anti phase domain structure in the B2 ordered phase. Optical microscopy observations corroborate the magnetic measurement conclusions.

Incommensurate Orbital Modulation behind Ferroelectricity inCuFeO2

CuFeO(2) is one of the multiferroic materials and is the first case that the electric polarization is not explained by the magnetostriction model or the spin-current model. We have studied this material using soft x-ray resonant diffraction and found that superlattice reflection 0 1-2q 0 appears in the ferroelectric and incommensurate magnetic ordered phase at the Fe L(2,3) absorption edges and moreover that the rotation of the x-ray polarization such as from σ to π or from π to σ is allowed at this reflection. These findings definitely provide direct evidence that the 3d t(2g↓) orbital state of Fe ions has a long-range order in the ferroelectric state. The spin-orbit interaction in Fe ions plays a crucial role to the ferroelectricity in CuFeO(2), coupling two nontrivial spin and orbital orders, both of which break the crystal symmetry.

Residual disorder and diffusion in thin Heusler alloy films

Co2FeSi/GaAs(110) and Co2FeSi/GaAs(111)B hybrid structures were grown by molecular-beam epitaxy and characterized by transmission electron microscopy (TEM) and X-ray diffraction. The films contained inhomogeneous distributions of ordered L2_1 and B2 phases. The average stoichiometry was controlled by lattice parameter measurements, however diffusion processes lead to inhomogeneities of the atomic concentrations and the degradation of the interface, influencing long-range order. An average long-range order of 30-60% was measured by grazing-incidence X-ray diffraction, i.e. the as-grown Co2FeSi films were highly but not fully ordered. Lateral inhomogeneities of the spatial distribution of long-range order in Co2FeSi were found using dark-field TEM images taken with superlattice reflections.

Evolution of three-dimensional correlations during the photoinduced melting of antiferromagnetic order in La0.5Sr1.5MnO4

Using time-resolved resonant soft x-ray diffraction, we measure the evolution of the full three-dimensional scattering volume of the antiferromagnetic superlattice reflection in the single-layer manganite La${}_{0.5}$Sr${}_{1.5}$MnO${}_{4}$on femtosecond time scales following photoexcitation. We find that the in-plane correlations are unchanged as a metastable state is entered, however there are subtle changes in the $c$-axis correlations. We observe a transient shift of the scattering ellipsoid along ($00L$) at very short times, and at longer time scales the short-range $c$-axis correlations are more robust than they are in equilibrium. Such results are not obtainable with any other techniques and hint at previously unresolved processes in the dynamics of photomelting in strongly correlated systems.

Negative linear compressibility and complex high-Pstructure in Zn[Au(CN)2]2

The structure of the framework material zinc dicyanoaurate is built up of tetrahedrally-coordinated Zn cations linked by linear [NC-Au-CN]- complex anions, which form a six-fold interpenetrated -quartz-type network. This material exhibits strong negative linear thermal expansion along the c direction due to framework flexibility [1]. In situ, single-crystal and powder x-ray diffraction under high pressure in diamond anvil cells indicate that the c lattice parameter increases by 8% from ambient pressure up to 1.8 GPa, which is the highest negative linear compressibility (NLC) known for any material. Structural data obtained from single crystal x-ray diffraction indicate that the NLC mechanism is linked to the closing of N-Zn-N angles and the bending of Au-C-N-Zn linkages. A large number of superlattice reflections are observed above 2 GPa, indicating that the unit cell is doubled in all three directions. This phase transition is accompanied by a strong decrease in NLC. Systematic absences indicate that the space group changes from P6222 to P6422 or vice versa depending on the absolute configuration of the initial crystal corresponding to an instability at one of the L points in the Brillouin zone. The resulting unit cell has the following lattice parameters at 3.6 GPa: a=14.485(2)A, c=45.796(6)A with 528 atoms per unit cell. The ZnAu2 sublattice of the new, high-pressure form was solved by direct methods and followed by the use of difference Fourier maps to locate the C and N atoms. A significant change occurs in the Au sublattice with the Au-Au interatomic vector becoming canted with respect to c. This provides an additional compression mechanism, which reduces the need for the structure to expand along c. These results provide a detailed structural mechanism for the exceptional and unusual mechanical properties of this material. [1] Goodwin, A. L., Kennedy, B. J.. & Kepert, C. J. (2009). J. Am. Chem. Soc. 131, 6334-6335. Keywords: structural phase transitions; high-pressure structure determination; mechanical properties of crystals

The thermodynamics of the I $$ \overline{1} $$ –P $$ \overline{1} $$ phase transition in Ca-rich plagioclase from an assessment of the spontaneous strain

In-situ powder diffraction measurements between 90 and 935 K on four anorthite-rich plagioclase samples (An100, An96Ab4, An89Ab11 and An78Ab22) were used to determine the detailed evolution of these samples through the \(I \overline{1} \)–\(P \overline{1} \) phase transition. The c-type reflections indicative of \(P \overline{1} \) symmetry were detected only in An100, An96Ab4, whereas deviations in the evolution of the unit-cell parameters with temperature were observed in all samples, most prominently in the β unit-cell angle. The c-type reflections disappear at ~510 and ~425 K in An100 and An96Ab4 respectively, and their intensity decreases according to a tricritical trend \( I^{2} \propto \left( {T - T_{\text{c}} } \right) \). The cell parameter changes were used to determine the spontaneous strains arising from the transition which were modelled with Landau theory, allowing for low-temperature quantum saturation, in order to determine the thermodynamic behaviour. In An100 tricritical behaviour was observed [Tc = 512.7(4) K; θs = 394(4)] in good agreement with previous studies, and the c-type superlattice reflections indicative of \(P \overline{1} \) symmetry persist up to the Tc determined from the spontaneous strain, and then disappear. The evolution of the spontaneous strain in An96Ab4 is tricritical at low temperatures [Tc = 459(1) K, θs = 396(5)] up to the temperature of disappearance of c-type reflections, but becomes second order beyond ~440 K. In An89Ab11 the strain displays second-order behaviour throughout [Tc = 500(1) and θs = 212(5)], and the c-type reflections are not detected in the powder diffraction patterns at any temperature. The apparent discrepancy between the absence of c-type reflections in temperature ranges where the cell parameters display significant spontaneous strain is resolved through consideration of the sizes of the anti-phase domains within the crystals. It is deduced that the tricritical phase transition occurs in well-ordered crystals with large domains in which the behavior of individual domains is dominant (i.e. in pure anorthite) or where the \(P \overline{1} \) distortions within the domains are large enough to dominate the structural coherency strains between the domains. When both the magnitude of the \(P \overline{1} \) pattern of displacements of the tetrahedral framework become smaller and the influence of the structural coherency between anti-phase domains becomes significant, the thermodynamic behavior becomes 2nd-order in character, the c-type reflections disappear, and the orientation of the spontaneous strain changes.

Surfactant-free microwave-assisted hydrothermal synthesis of BaMoO4 hierarchical self-assemblies and enhanced photoluminescence properties

BaMoO4 with 3D hierarchical multilayer disk-like and nest-like architectures self-assembled from 2D nanosheets was successfully synthesized via a microwave-assisted hydrothermal route without any surfactant. The as-prepared products were characterized by X-ray powder diffractometer (XRD), scanning electron microscope (SEM), field emission transmission electron microscope (FE-TEM), and photoluminescence (PL) spectrometer. The results show that the reaction parameters, including pH value, reactant concentration, and molar ratio of [Ba2+]/[MoO42-], played important roles on the morphologies of the final products. And the formation mechanism of 3D hierarchical architectures is a stepwise oriented aggregation-based self-assembly process. The superstructure characteristic of 3D nest-like BaMoO4 architecture was observed in HRTEM image and the corresponding fast Fourier transform (FFT) for the first time, and the superlattice reflections with non-integer indices occurred around the subcell reflections at ±1/6(2a*+2c*). Room temperature photoluminescence spectra of 3D BaMoO4 architectures reveal a strong and broad blue emission, and the 3D nest-like architectures own the enhanced intensity than multilayer disks.

Distinct oxygen hole doping in different layers ofSr2CuO4−δ/La2CuO4superlattices

X-ray absorption in $\rm Sr_{2}CuO_{4-\delta}-La_{2}CuO_4$ (SCO-LCO) superlattices shows a variable occupation with doping of a hole state different from holes doped for $x \lesssim x_{optimal}$ in bulk $\rm La_{2-x}Sr_{x}CuO_4$ and suggests that this hole state is on apical oxygen atoms and polarized in the $a-b$ plane. Considering the surface reflectivity gives a good qualitative description of the line shapes of resonant soft X-ray scattering. The interference between superlattice and surface reflections was used to distinguish between scatterers in the SCO and the LCO layers, with the two hole states maximized in different layers of the superlattice.

Current status of a time-of-flight single crystal neutron diffractometer SENJU at J-PARC

SENJU is a state-of-the-art single crystal time-of-flight Laue diffractometer in Materials and Life Science Experimental Facility at Japan Proton Accelerator Research Complex (J-PARC). The diffractometer is designed for precise crystal and magnetic structure analyses under multiple extreme conditions, such as low temperature, high-pressure, high-magnetic field. Measurements using small sample less than 1.0 mm3 will be also realized, which allows us to study wide variety of materials. SENJU is using a poisoned decoupled moderator to obtain peak profiles of Bragg reflection, and intensity distributions of superlattice reflections and diffuse scatterings with good accuracy. At the beginning, the diffractometer will have 31 two-dimensional scintillator detectors to cover wide area of reciprocal lattice space by a single measurement. The instrument is currently under construction and is scheduled to start on-beam commissioning in February 2012.

Hydrogenation and structural properties of Gd2Ni7 with superlattice structure

The crystal structure and hydrogenation properties of Ce2Ni7-type Gd2Ni7 were investigated by X-ray diffraction (XRD) and the hydrogen pressure–composition (P–C) isotherm. Ce2Ni7-type Gd2Ni7 was obtained by annealing at 1523 K for 12 h and quenching in ice water. Two superlattice reflections (002 and 004) of the Ce2Ni7-type were clearly observed at 2θ = 7.3° and 14.6° in the XRD profile. The refined lattice parameters were a = 0.49662(9) nm and c = 2.4255(3) nm, respectively. Two plateaus were clearly observed during the absorption–desorption process in the P–C isotherm. The first and second plateaus were at 0.015 and 0.13 MPa, respectively, in the first desorption. The maximum hydrogen capacity reached was 1.13 H/M. The enthalpy and entropy were calculated as −20 kJ/mol H2 and −80 J/mol H2 K, respectively, from the van’t Hoff plot. After the P–C isotherm, the GdNi5 cell expanded by 2.15%, but the Gd2Ni4 cell shrank by 2.83%.