Superstructure Reflections Research Articles

Morphology and spatial distribution of ordered domains in GaInP/GaAs(001) according to transmission electron microscopy

The structure of epitaxial films of the GaInP solid solution, in which ordering occurs, was studied using transmission electron microscopy. The films were grown by metalorganic vapor phase epitaxy on GaAs (001) substrates near the half-composition point. During the study, dark-field images obtained using superstructure reflections for cross-sectional and plan-view specimens of films were analyzed. The morphology and relative spatial arrangement of ordered domains have been determined. The phenomenon of spontaneous self-organization of regions with CuPt–B+ and CuPt–B– ordering near the surface was discovered, while in the bulk of the film the domains are uniformly located and mutually overlap each other. The effect of spatial separation of domains is associated with the lattice relaxation, leading to a change in the surface topology.

Superstructure reflections in tilted perovskites.

The superstructure spots that appear in diffraction patterns of tilted perovskites are well documented and easily calculated using crystallographic software. Here, by considering a distortion mode as a perturbation of the prototype perovskite structure, it is shown how the structure-factor equation yields Boolean conditions for the presence of superstructure reflections. This approach may have some advantages for the analysis of electron diffraction patterns of perovskites.

Revealing the phase transition scenario in antiferroelectric thin films by x-ray diffuse scattering

There is no consensus among researchers regarding how phase transitions occur in antiferroelectric (AFE) epitaxial heterostructures, in particular, in heterostructures based on model AFE lead zirconate. The questions about the number of phase transitions in such films and by what mechanism they occur remain controversial. This paper presents a look at the phase transition scenario in two types of epitaxial heterostructures: PbZrO3/Ba[La–Sn]O3/MgO (001) thin films with thicknesses from 25 to 1000 nm and PbZrO3/SrRuO3/SrTiO3 (001) thin film 100 nm thick using the diffuse x-ray scattering in the grazing incidence setup. We register the characteristic butterfly-shaped diffuse scattering (DS) intensity distribution in the HK pseudocubic planes, which corresponds to the anisotropic ferroelectric soft mode. No incommensurate soft mode was observed in the cuts of reciprocal space parallel to the film surface by diffuse scattering. We reproduce the shape of DS distribution at different temperatures by the model based on the dielectric stiffness and the electric polarization correlation tensor in the cubic approximation. Such modeling allows not only to characterize the DS parameters from the challengingly low signal-to-background data set, but also to extract experimentally the sensitivity of the materials with respect to inhomogeneous polarization. While the observed temperature evolution of DS is consistent with the dielectric measurements, the correlation between the DS and the phase transition sequence observed by superstructure reflections is yet to be understood better.

Superstructure reflections in 40% Sn(II)-substituted BaZr0.5Ti0.5O3 perovskite modeled with a Bayesian method for crystallographic refinement

Superstructure reflections in 40% Sn(II)-substituted BaZr0.5Ti0.5O3 perovskite modeled with a Bayesian method for crystallographic refinement

Structure and phase changes of alumina produced by flame hydrolysis.

Fumed alumina from the combustion of AlCl3 produced nano particles with specific areas from 30 to 220m2g-1 (BET) which were characterized by powder X-ray diffraction, 27Al solid-state NMR and transmission electron microscopy. During the short-lived synthesis, highly disordered γ-alumina progressively transforms into a mixture of δ and θ-alumina. For the γ-alumina particles, only for particles with the highest specific area a significant amount of five-coordinated Al can be found which is only partially located in the particle surface. Water can be bound reversibly and increases the coordination number of aluminium atoms in the particle surface. For the well-crystallized mixture of δ and θ-alumina, high resolution and high S/N powder XRD pattern features a large number of superstructure reflections along with the commonly observed diffuse reflections. 27Al MQMAS NMR provides a total of 8 crystallographic sites with an unusually high resolution in the tetrahedral region, with 4 distinct AlO4 sites pertaining to the δ phase alone. The results suggest that the δ-alumina phase produced in this process can be described as an ordered structure.

As predicted and more: modulated channel occupation in YZn5+x.

Like many complex intermetallic phases, the crystal structures of REZn5+x compounds (RE = lanthanide or Group 3 element) based on the EuMg5 type have gradually unfolded. The original reports described a complex hexagonal structure with an unusual combination of tetrahedrally close-packed regions and open spaces, as well as observations of superstructure reflections. More recently, we reinvestigated the structure of YZn5, reclassifying it as the EuMg5+x-type compound YZn5+x (x ≃ 0.2), in which disordered channels run along c through the spaces formerly considered open. In addition, DFT-chemical pressure (DFT-CP) analysis of ordered models of YZn5+x highlighted paths for communication between neighboring channels setting the stage for superstructure formation. Herein, the experimental elucidation of this effect is presented with the synthesis and structure determination of a modulated form of YZn5+x. By slow-cooling samples of YZn5+x from the annealing temperature, crystals were obtained that exhibit satellite reflections with the modulation wavevector q = {1\over 3}a* + {1\over 3}b* + 0.3041c*. Structure solution and refinement using a (3+1)D model in superspace group P31c({1\over 3}\,\!{1\over 3}σ3)00s reveals incommensurate order in the structure's channels. Here, two Zn sites associated with the channels are present, each with discontinuous atomic domains that are slanted in the x3x4 plane. Their slanting corresponds to adjustments along the c axis for the presence or absence of close neighbors along that axis, while the occupation patterns of neighboring channels are shifted by {1\over 3} of the modulation period. These features follow earlier predictions from CP analysis, highlighting how this approach can be used predictively in search of new phenomena.

Highly mismatched antiferroelectric films: Transition order and mechanical state

Epitaxial ${\mathrm{PbZrO}}_{3}/{\mathrm{SrRuO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures are among the most widely studied thin-film antiferroelectrics. This paper explores their temperature-induced phase transitions and the characteristics of the domain structure by means of high-resolution synchrotron x-ray diffraction. The antiferroelectric transition order appears to change from the first to the second; peculiar in-plane $M$-point superstructures develop at high temperatures, manifesting a new phase; the $R$- and $\mathrm{\ensuremath{\Sigma}}$-point superstructure reflections demonstrate much reduced splitting as compared to the demands of the mechanical compatibility. We discuss the energetics of the reduced around-the-normal antiferroelectric domain tilts, its relation to the observed changes in phase transitions, and point to the possible contribution to the observed effect from an unusual diffraction mechanism related to the partial coherence between domains in a random nanodomain structure.

The Crystal Structure of Mg–Al–CO3 Layered Double Hydroxide

The crystal structure of quintinite, Mg4Al2(OH)12(CO3)·3H2O, from the Jacupiranga alkaline complex (Cajati, São Paulo, Brazil), was refined for two samples (91002 and C7029) using single-crystal X-ray diffraction data. The mineral crystallizes in the P-3c1 space group, a = 5.246/5.298, c = 15.110/15.199 Å for samples 91002/C7029. The crystal structure consists of octahedral sheets with Mg and Al ordering according to a 3 × 3 superstructure. The Mg and Al atoms are coordinated by six hydroxylated oxygen atoms; the average <Mg–O> and <Al–O> bond distances are in the ranges 2.022–2.053 Å and 1.974–1.978 Å, respectively. The interlayer structures are identical (in contradiction to the previous assumptions), and consist of disordered (CO3)2− groups and (H2O)0 molecules. The samples from Jacupiranga can be identified as quintinite-2T, which is the second finding of this polytype after the Kovdor alkaline complex (Kola peninsula, Russia). The powder X-ray diffraction pattern of quintinite-2T contains weak superstructure reflection at 4.57 Å (010), indicative of Mg and Al ordering. An important crystal-chemical criterion of quintinite is the interlayer distance (d00n-value) of ~7.56 Å, which is steady among natural specimens from various findings worldwide.

Transport and thermoelectric properties of melt spinning synthesized M2FeNiSb2 (M = Ti, Hf) double half Heusler alloys

In this study, we report on structural and thermoelectric properties of novel Heusler compounds Hf2FeNiSb2 and Ti2FeNiSb2. Polycrystalline samples were successfully synthesized by arc melting and melt spinning techniques and consolidated by spark plasma sintering method. The studied compounds were found to crystallize in a face centered cubic crystal structure, as can be judged by superstructural reflections, clearly seen on the X-ray diffraction patterns. Morphology and grain sizes were examined by SEM analysis, showing homogeneity and high density of grains in the synthesized samples. Electrical conductivity and Seebeck coefficient were investigated in a range of temperatures between 300 and 900 K. Thermal conductivity was measured in the same temperature range and found to be significantly lower than that previously reported for related half-Heusler alloys such as TiCoSb. The thermoelectric figure of merit of the concerned Hf2FeNiSb2 and Ti2FeNiSb2 samples was found at 0.082 and 0.027, respectively, at 874 K.

Synthesis and characterization of AsO[(W,Mo)O3]13, a new (6)-intergrowth tungsten bronze (ITB)

Crystals of a new arsenic tungsten-molybdenum oxide, AsO[(W,Mo)O3]13, have been grown through vapour phase transport at high temperature (1123 ​K). The crystal structure was investigated by combining electron and single-crystal X-ray diffraction techniques, electron microprobe chemical analysis and X-ray absorption spectroscopy. Single-crystal XRD yielded the orthorhombic unit-cell parameters a ​= ​25.0895, b ​= ​7.3061, c ​= ​3.9089 ​Å. Although electron diffraction indicated a doubled translation along [001], superstructure reflections were undetectable with X-ray diffraction so that only the average structure (sub-cell) was determined. The most reliable structural model was obtained in the P222 space group (R1 ​= ​6.96%). It exhibits an octahedral framework which forms six-octahedra thick perovskite slabs (PTB) alternated to single hexagonal tungsten bronze (HTB) modules, indicating that the compound corresponds to a (6)-ITB phase. As3+ was found to be disordered on two off-centred, symmetry-related sites into the hexagonal channels with the typical pyramidal AsO3 geometry. This is in agreement with the results of the extended X-ray absorption fine structure that indicate an As–O bond distance of 1.77 ​Å. The ordered distribution of As within the tunnels, likely related to the doubling of periodicity along [001], could not be determined.

High-Voltage Layered NaNi0.5Co0.1Ti0.3Sb0.1O2 Cathode for Sodium-Ion Batteries

Developing a high-voltage cathode material for sodium-ion batteries is a great challenge. Herein, we report an O3-type layered NaNi0.5Co0.1Ti0.3Sb0.1O2 cathode composition, which can operate at a high voltage (4.35 V). Inclusion of different valence state transition metals, like Ni, Co, and Ti, and a metalloid Sb in the sodium metal oxide composition leads to the formation of a cationic disordered structure, which enhances redox, electronic conductivity, cycling stability, and working voltage. The structural characterizations reveal that the composition is free from superstructure reflections, and electrochemical studies show that the composition suppresses multiphase transitions. As a result, NaNi0.5Co0.1Ti0.3Sb0.1O2, exhibits initial charge and discharge capacities as 129 and 112 mAh/g, respectively, at the current rate of 9.1 mA/g for 2.0–4.35 V. Thus, the high voltage, improved capacity, and retention characteristics of NaNi0.5Co0.1Ti0.3Sb0.1O2 would open up the way for bulk production of Na-ion batteries.

Soft-Phonon and Charge-Density-Wave Formation in Nematic BaNi2As2

We use diffuse and inelastic x-ray scattering to study the formation of an incommensurate charge-density-wave (I-CDW) in BaNi_{2}As_{2}, a candidate system for charge-driven electronic nematicity. Intense diffuse scattering is observed around the modulation vector of the I-CDW, Q_{I-CDW}. It is already visible at room temperature and collapses into superstructure reflections in the long-range ordered state where a small orthorhombic distortion occurs. A clear dip in the dispersion of a low-energy transverse optical phonon mode is observed around Q_{I-CDW}. The phonon continuously softens upon cooling, ultimately driving the transition to the I-CDW state. The transverse character of the soft-phonon branch elucidates the complex pattern of the I-CDW satellites observed in the current and earlier studies and settles the debated unidirectional nature of the I-CDW. The phonon instability and its reciprocal space position are well captured by our abinitio calculations. These, however, indicate that neither Fermi surface nesting, nor enhanced momentum-dependent electron-phonon coupling can account for the I-CDW formation, demonstrating its unconventional nature.

Local Stability to Periodicity in the EuMg5+x Type: Chemical Pressure, Disordered Channels, and Predicted Superstructure in YZn5.225

AbstractA central theme in the structural chemistry of intermetallic phases is that complex structures can be derived from variations on simpler ones. This is vividly demonstrated by the variety of structure types that can be connected to chemical pressure (CP)‐driven transformations of the simple CaCu5 type. In this Article, we investigate an intriguing addition to this family: the EuMg5‐type intermetallics, as exemplified by YZn5. As expected from the large negative CPs around the cations in CaCu5‐type structures, YZn5 exhibits tightened coordination environments around the cations. However, it also contains an unusually inhomogeneous atomic packing, particularly in channels running between the Y atoms along c. Our structural reinvestigation of YZn5 reveals a disordered occupation pattern of Zn atoms within these channels, consistent with the EuMg5+x type, a disordered variant of the EuMg5 type. DFT‐CP analysis indicates that the transition from the CaCu5 type to the YZn5+x structure indeed creates more compact Y environments, but strong tensions remain within the Zn sublattice. These include CP features on the channel walls that provide a mechanism for the communication of structural information between the channels and favorable cooperation in their occupation patterns. Based on these results, a structural model is proposed that explains an earlier observation of superstructure reflections in the diffraction patterns of ErZn5 corresponding to a √3×√3×3 supercell.

Crystal structure and magnetic properties of double perovskite Sr2FeReO6 ceramics synthesized by a combined molten-salt synthesis and solid-state reaction method

Sr2FeReO6 (SFRO) double perovskite ceramics were synthesized by a combined molten-salt synthesis and solid-state reaction (SSR) method under N2 (sample A) and Ar (sample B), and also by direct SSR method under Ar (sample C). All the SFRO ceramics crystallized in a tetragonal structure with I4/m space group. Superstructural reflections such as (011) peak appeared in A-B samples but absent in sample C. Their average crystallite sizes were ∼ 50 nm, and the compositions of the three samples were determined as Sr2Fe1+xRe1−xO6 (x = 0.197, 0.142, and 0.087). XPS spectra revealed that Re existed in dual valence states in sample B, but the dual oxide states of Re and Fe in sample C. Oxygen existed as lattice oxygen in B–C samples. Ferromagnetic behavior was observed in the A-C samples at 2 K and 300 K, and their saturation magnetizations at 2 K were 1.259, 1.477, and 0.842 μB/f.u., respectively.

Hidden Charge Order in an Iron Oxide Square-Lattice Compound.

Since the discovery of charge disproportionation in the FeO_{2} square-lattice compound Sr_{3}Fe_{2}O_{7} by Mössbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained "hidden" to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge order in the FeO_{2} planes that vanishes at a sharp second-order phase transition upon heating above 332K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty of detecting them by conventional probes. We discuss the implications of these findings for research on "hidden order" in other materials.

Crystal structures of K-cymrite and kokchetavite from single-crystal X-ray diffraction

Abstract We determined for the first time the crystal structures of high-pressure K-cymrite (KAlSi3O8⋅H2O) and its dehydrated form kokchetavite (KAlSi3O8) using single-crystal X-ray diffraction. The K-cymrite structure has been successfully refined in the hexagonal space group P6/mmm [a = 5.3361(3) Å, c = 7.7081(7) Å, V = 190.08(3) Å3, R1 = 0.036 for 127 unique observed reflections], which is in agreement with previous models from powder X-ray diffraction. In contrast, kokchetavite shows superstructural reflections, suggesting a different unit cell and a space group of P6/mcc [a = 10.5757(3) Å, c = 15.6404(6) Å, V = 1514.94(10) Å3, R1 = 0.068 for 1455 unique observed reflections]. Upon dehydration, single-crystal grains of K-cymrite transform into single-crystal grains of kokchetavite. The latter questions a previous interpretation of kokchetavite crystals in mineral inclusions as a product of direct crystallization from fluid/melt. The Raman spectrum of K-cymrite shows a strong polarization dependence, which is important in identification of the mineral inclusions.

Space weathering of iron sulfides in the lunar surface environment

Alteration of iron sulfides on the lunar surface by space weathering is poorly understood. We examined space weathering features of iron sulfides in lunar mature soil grains using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM observations reveal that iron sulfides have vesicular textures and iron whiskers on their surfaces. Iron sulfides observed using TEM are troilite and NC-pyrrhotite. The space-weathered rim on the iron sulfides is characterized by crystallographic misorientations and the disappearance of superstructure reflections of troilite in electron diffraction patterns. These crystallographic modifications are probably produced by solar wind irradiation. The rim contains opened vesicles that are aligned along the c-plane of the sulfides, as well as numerous tiny vesicles. The Fe/S ratio at the surface of the rim is higher than in non-altered regions, indicating selective sulfur loss from the surface. Iron whiskers protrude from the space weathered rim and consist of polycrystalline metallic iron. The sulfide rims and the iron whiskers are both coated with vapor-deposited materials rich in O and Si. The combined processes driven by the solar wind irradiation, heating during impact events, solar UV radiation, and the thermal cycling may cause vesicular textures, selective sulfur escape from the iron sulfides, and the formation of the iron whiskers. The rim textures support the notion that the enrichment of heavy sulfur isotopes in mature lunar soils is caused by space weathering of iron sulfides. The space weathered rims on lunar iron sulfides are similar to those observed in regolith samples from asteroid Itokawa. Therefore, alterations of sulfide surface might be common among airless bodies in the solar system.

Antimony substitution leading to structural transformation (Bixbyite→ Fluorite) and altering the optical band gap in Y2O3

The transformation of ordered bixbyite structured Y2O3 to cation disordered fluorite structure is demonstrated by antimony substitution in the present work. The samples are prepared by the wet chemical method and characterized by employing various physicochemical techniques. The superstructure reflections of bixbyite were observed for samples prepared with 10 mol% of antimony, beyond which the structure transformed into a disordered fluorite structure. The diffracted peaks became broad in the powder X-ray diffraction pattern with increasing antimony concentration. The Williamson-Hall analysis indicated compressive strain in antimony substituted samples. Microscopy and Raman spectral measurements confirmed this structural transformation. The antimony existed in the III oxidation state, as revealed by XPS measurements. The optical bandgap of Y2O3 changed from 4.81 to 3.62, and 3.20 eV for 20 and 50 mol% antimony substituted samples. In the photoluminescence emission spectrum, the transitions of the excited Jahn-Teller state of Sb3+ are observed in the visible region. 50 mol% antimony substituted yttria sample catalyzed the complete degradation of aqueous rhodamine-6G (Rh-6G) in 105 min, assisted by the UV–visible radiation. The reaction followed pseudo-first-order kinetics, and the catalyst showed nearly the same efficiency up to five times of its reuse. The recovered catalyst did not undergo any change in its structure.

Synthesis and characterization of a hexagonal polymorph of GdAlO3

The structure and basic properties of a hexagonal polymorph of GdAlO3 aluminate (GAH) were studied using a set of experimental methods (XRD, FTIR, Raman spectroscopy, diffuse reflectance spectroscopy, luminescence spectroscopy, magnetic susceptibility, and TEM). The GAH crystallized at 900 °C from an amorphous precursor and decomposed into perovskite GdAlO3 (GAP) at 1000 °C. XRD pattern of polycrystalline GAH could be indexed in P63/mmc space group with unit cell parameters a = 0.3734 nm, c = 1.0535 nm, following literature data. The electron diffraction revealed, however, the presence of well-defined superstructure reflections due to defects ordering. Contrary to a recent report suggesting the presence of CO3 groups in the structure of similar hexagonal YAlO3, we did not observe such groups in the FTIR spectra of GAH. On the other hand, the stoichiometry of GAH differs from ideal GdAlO3, with Al/Gd atomic ratio close to 1.2, similar to reported for hexagonal YAlO3. GAH exhibits paramagnetic behavior over 1.8–300 K temperature range, with the effective Gd magnetic moment μeff = 7.90 μB, close to the theoretical value for free Gd3+ ion. No paramagnetic – antiferromagnetic phase transition at ~4 K, characteristic for perovskite-type GdAlO3, was observed. The reason is probably a less "dense" structure of GAH, with longer Gd-Gd and Gd-O-Gd interatomic distances. Emission spectra of Dy doped (~1 at%) GAH and GAP revealed dramatic differences in Dy3+ emission bands, despite similar mean crystallite size of the host material. An almost structureless 4F9/2→6H13/2 and 4F9/2→6H15/2 emission bands, observed for GAH, indicate, following TEM data, that Dy ions occupy numerous nonequivalent sites due to the presence of a large concentration of crystal defects.

Modulated and Incommensurate Superstructures of Atomic–Vacancy Ordering in Refractory Transition Metal Carbides

The formation of partially disordered modifications in refractory nonstoichiometric carbides prone to M6X5-type ordering is considered. In M6X5 superstructures the ray $${\mathbf{k}}_{9}^{{(3)}}$$ of the Lifshitz star {k9} with a current parameter μ9 = 1/2 in the disorder–order phase transition channel is replaced by the rays $${\mathbf{k}}_{5}^{{(6)}}$$ and $${\mathbf{k}}_{5}^{{(5)}}$$ of the non-Lifshitz star {k5} with a variable current parameter 0 < μ5 < 1/2. Depending on the specific value of the index μ5, this replacement leads to various modulated structures that differ by the concentration of vacancies in the defective ( $$1\bar {1}1$$ ) planes of the carbon fcc sublattice and by the modulation period. In the diffraction spectra the positions of the superstructure reflections due to the star {k5} at μ5 ≈ 0.473 correspond to the incommensurate ordered phase experimentally detected in a nonstoichiometric tantalum carbide. The incommensurate phase is close in short-range order in the first coordination sphere to the initial M6X5 superstructures with a reduced long-range order parameter η ≈ 0.6.