Superstructure Reflections Research Articles

Topotactic synthesis, crystal structure and oxygen non-stoichiometry of ordered NdBaMnFeO6-δ

The NdBaMnFeO6-δ layered perovskite was synthesized by a topotactic reaction in the N2/H2 atmosphere for the first time while controlling the sample mass during the experiment. The X-ray powder diffraction (XRPD) patterns of the samples with different oxygen content refined by the Rietveld method revealed the YBaCuFeO5-type tetragonal structure of NdBaMnFeO6-δ (sp. gr. P4/mmm). The results obtained by selected area electron diffraction (SAED) and by high-resolution transmission electron microscopy (HRTEM) clearly evidenced the doubling of the c parameter without any additional superstructure reflections. The NdBaMnFeO6-δ phase exhibited narrower variation in oxygen content compared with that for NdBaMn2O6-δ and NdBaFe2O6-δ. This was explained by a higher distortion of square-pyramidal/octahedral surrounding of 3d-metals in NdBaMnFeO6-δ, which significantly increased with oxygen content, reducing the upper limit of oxygen content down to ≈5.45.

Three-dimensional atomic scale electron density reconstruction of octahedral tilt epitaxy in functional perovskites

Octahedral tilts are the most ubiquitous distortions in perovskite-related structures that can dramatically influence ferroelectric, magnetic, and electronic properties; yet the paradigm of tilt epitaxy in thin films is barely explored. Non-destructively characterizing such epitaxy in three-dimensions for low symmetry complex tilt systems composed of light anions is a formidable challenge. Here we demonstrate that the interfacial tilt epitaxy can transform ultrathin calcium titanate, a non-polar earth-abundant mineral, into high-temperature polar oxides that last above 900 K. The comprehensive picture of octahedral tilts and polar distortions is revealed by reconstructing the three-dimensional electron density maps across film-substrate interfaces with atomic resolution using coherent Bragg rod analysis. The results are complemented with aberration-corrected transmission electron microscopy, film superstructure reflections, and are in excellent agreement with density functional theory. The study could serve as a broader template for non-destructive, three-dimensional atomic resolution probing of complex low symmetry functional interfaces.

Metastable Bi2Zr2O7 with Pyrochlore-like Structure: Stabilization, Oxygen Ion Conductivity, and Catalytic Properties.

Equimolar concentrations of Zr4+ and Bi3+ were chelated with ethylenediaminetetraacetic acid ligand with the purpose of using it as a precursor to generate pyrochlore-like Bi2Zr2O7. When the X-ray amorphous precursor was calcined at 750 °C for 3 h in air, pyrochlore-like product with superstructure reflections was identified by powder X-ray diffraction (PXRD) along with one minor reflection due to β-Bi2O3. This phase was found to be metastable from additional experiments conducted by varying calcination conditions. Structural refinement of PXRD pattern by Le Bail method in Fd3̅ m space group yielded cubic lattice constant of 10.8421(27) Å. Flower-petal-like morphology of the sample was evident in its field-emission scanning electron microscopy image and energy-dispersive X-ray analysis performed at various locations of the specimen confirmed nearly equal concentration of zirconium and bismuth. Six bands at 260, 320, 448, 531, 597, and 828 cm-1 were observed for this sample in its Raman spectrum and supported our claim of pyrochlore-like structure. Indexation of bright spots present in selected area electron diffraction pattern and observed distances of lattice fringes in high-resolution transmission electron microscopy image were in conformity with the results from PXRD measurements. Absorbance maxima at 312, 372, and 423 nm with a broad tailing stretching up to visible region was noticed in the UV-visible spectrum of this sample. Direct band gap of 2 eV was estimated for this sample from Tauc plot. The oxygen ion conductivity of the sample in the temperature range of 333-773 K was examined, and the highest conductivity at 773 K was 3.071 × 10-6 S/cm. From activation energy estimation and dielectric loss analysis, thermally activated process related to the mobility of oxygen ion vacancy was found responsible for the observed ionic conductivity. A similar conclusion was reached after careful analysis of dielectric spectroscopy data of this sample. High surface area (125.04 m2/g) and mesoporosity (pore diameter of 3.81 nm) were possessed by this sample, which paved way for studying its catalytic role in the reduction of nitroaromatics and carcinogenic Cr6+. Cyclability experiments showed the retainment of catalytic activity up to five cycles by the sample without undergoing any structural change.

Magnetic Properties of a Layered Cobaltite Sr1–xYxCoO3–δ (x = 0.1)

The structure, the magnetic and magnetotransport properties of perovskite Sr0.9Y0.1CoO2.63 have been studied. The sample is shown to have a two-phase structure. The main phase has a tetragonally distorted unit cell and is described by space group I4/mmm. The broadening of the reflections with indices corresponding to doubling unit cell parameter c indicates the absence of the rigorous translation symmetry along axis c. The existence of the broadened superstructure reflection observed in the diffraction pattern at small angles at temperature lower than 400 K is explained by the existence of the monoclinic phase whose content is significantly lower than that of the tetragonal phase, but is dominant in the Sr0.8Y0.2CoO3–δ composition. The spontaneous magnetization appears as the monoclinic phase forms. The magnetic structure is mainly G-type antiferromagnetic with magnetic moments 1.5μB in the layers of CoO6 octahedra and 2μB in the anion-deficit CoO4 + γ layers. The conduction of the Sr0.9Y0.1CoO2.63 composition has a semiconducting character. The magnetoresistance is 57% in a field of 14 T at a temperature of 5 K and strongly decreases with the temperature increase.

Intermediate phase with orthorhombic symmetry displacement patterns in epitaxial PbZrO3 thin films at high temperatures

Antiferroelectric PbZrO3 epitaxial thin film with thickness of 50 nm grown on the SrTiO3 substrate with SrRuO3 buffer layer was studied by Grazing incidence X-ray diffraction in a wide range of temperatures. Apart from the superstructure reflections characteristic for the antiferroelectric phase, we identify the reflections with pseudocubic coordinates of the form (H + 0.5, K + 0.5, L). This indicates the presence of structures having orthorhombic symmetry as an effect of anti-phase Pb atoms displacements, which are apparently similar to the ones previously proposed for the rhombohedral high-temperature phase in PbZrxTi1-xO3. For these structures only the domain orientations with displacements parallel to the surface are observed in contrast to the antiferroelectric domains where the parallel to surface Pb displacements are absent. The antiferroelectric phase coexists with the one characterized by (H + 0.5, K + 0.5, L) reflections in a broad temperature range with the maximal amount of the latter being present at T = 150 °C.

The hydrocarbon-bearing clathrasil chibaite and its host-guest structure at low temperature.

The natural sII-type clathrasil chibaite [chemical formula SiO2·(M12,M16), where Mx denotes a guest mol-ecule] was investigated using single-crystal X-ray diffraction and Raman spectroscopy in the temperature range from 273 to 83 K. The O atoms of the structure at room temperature, which globally conforms to space group [V = 7348.9 (17) Å3, a = 19.4420 (15) Å], have anomalous anisotropic displacement parameters indicating a static or dynamic disorder. With decreasing temperature, the crystal structure shows a continuous symmetry-lowering transformation accompanied by twinning. The intensities of weak superstructure reflections increase as temperature decreases. A monoclinic twinned superstructure was derived at 100 K [A2/n, V = 7251.0 (17) Å3, a' = 23.7054 (2), b' = 13.6861 (11), c' = 23.7051 (2) Å, β' = 109.47°]. The transformation matrix from the cubic to the monoclinic system is ai ' = (½ 1 ½ / ½ 0 -½ / ½ -1 ½). The A2/n host framework has Si-O bond lengths and Si-O-Si angles that are much closer to known values for stable silicate-framework structures compared with the averaged model. As suggested from band splitting observed in the Raman spectra, the [512]-type cages (one crystallographically unique in , four different in A2/n) entrap the hydro-carbon species (CH4, C2H6, C3H8, i-C4H10). The [51264]-type cage was found to be unique in both structure types. It contains the larger hydro-carbon mol-ecules C2H6, C3H8 and i-C4H10.

Investigation of local structural phase transitions in 95Na0.5Bi0.5TiO3-5BaTiO3 piezoceramics by means of in-situ transmission electron microscopy

The temperature-dependence of superstructure reflections, diffuse scattering intensity, and ferroelectric domain morphology for 95Na0.5Bi0.5TiO3-5BaTiO3 ceramics was investigated by in-situ transmission electron microscopy. A subtle local structural phase transition was observed around 110 °C. This local phase transition is characterized by a transformation in the dominant octahedral-tilt system from antiphase to in-phase tilting and was correlated with the depolarization mechanism common in this type of materials. Simulations of the octahedral-tilt disorder were developed at different temperatures which enabled us to provide a comprehensive structural model that explains the temperature-dependence of superstructure reflections and electron diffuse scattering intensity. This model revealed that plate-like in-phase nanodomains tilted about three orthogonal axes coexist with an antiphase tilted matrix, even at room temperature. With increasing temperature, the concentration of in-phase nanodomains is increased till a critical volume fraction is reached. We propose that these plate-like in-phase nanodomains are an inherent characteristic of NBT-based compounds and that the local phase transition observed around 110 °C leads to a disruption in the long-range ferroelectric order and is responsible for the depolarization mechanism.

Commensurate versus incommensurate charge ordering near the superconducting dome in Ir1−xPtxTe2 revealed by resonant x-ray scattering

The electronic-structural modulations of Ir$_{1-x}$Pt$_x$Te$_2$ (0 $\leqq x\leqq$ 0.12) have been examined by resonant elastic x-ray scattering (REXS) and resonant inelastic x-ray scattering (RIXS) techniques at both the Ir and Te edges. Charge-density-wave-like superstructure with wave vectors of $\mathbf{Q}$=(1/5 0 $-$1/5), (1/8 0 $-$1/8), and (1/6 0 $-$1/6) are observed on the same sample of IrTe$_2$ at the lowest temperature, the patterns of which are controlled by the cooling speeds. In contrast, superstructures around $\mathbf{Q}$=(1/5 0 $-$1/5) are observed for doped samples (0.02 $\leqq x\leqq$ 0.05). The superstructure reflections persist to higher Pt substitution than previously assumed, demonstrating that a charge density wave (CDW) can coexists with superconductivity. The analysis of the energy-dependent REXS and RIXS lineshape reveals the importance of the Te 5$p$ state rather than the Ir 5$d$ states in the formation of the spatial modulation of these systems. The phase diagram re-examined in this work suggests that the CDW incommensurability may correlate the emergence of superconducting states as-like Cu$_x$TiSe$_2$ and Li$_x$TaS$_2$.

Single-crystal study of the charge density wave metal LuNiC2

We report on single crystal growth, single crystal x-ray diffraction, physical properties and density functional theory (DFT) electronic structure as well as Fermi surface calculations for two ternary carbides, LuCoC2 and LuNiC2. Electrical resistivity measurements reveal for LuNiC2 a charge density wave (CDW) transition at T_{CDW}~ 450 K and, for T > T_{CDW}, a significant anisotropy of the electrical resistivity, which is lowest along the orthorhombic a-axis. The analysis of x-ray superstructure reflections suggest a commensurate CDW state with a Peierls-type distortion of the Ni atom periodicity along the orthorhombic a-axis. DFT calculations based on the CDW modulated monoclinic structure model of LuNiC2 as compared to results of the orthorhombic parent-type reveal the formation of a partial CDW gap at the Fermi level which reduces the electronic density of states from N(E_{F})= 1.03 states/eV f.u. without CDW to N(E_{F})= 0.46 states/eV f.u. in the CDW state. The corresponding bare DFT Sommerfeld value of the latter, gamma_{DFT}^{CDW}= 0.90 mJ/molK^2, reaches reasonable agreement with the experimental value gamma= 0.83(5) mJ/mol\,K^2 of LuNiC2. LuCoC2 displays a simple metallic behavior with neither CDW ordering nor superconductivity above 0.4 K. Its experimental Sommerfeld coefficient, gamma= 5.9 (1) mJ/molK^2, is in realistic correspondence with the calculated, bare Sommerfeld coefficient, gamma_{DFT}= 3.82 mJ/molK^2, of orthorhombic LuCoC2.

Ferromagnetism and Charge Order from a Frozen Electron Configuration in Strained Epitaxial LaCoO_{3}.

We report ordering of the cobalt electron configuration in ferromagnetic strained epitaxial LaCoO_{3}. Specifically, the presence of charge order is demonstrated from distinct features of the resonant cobalt contribution to superstructure reflections. Density functional theory calculations show that the observed order is consistent with the spin-state periodicity predicted to give rise to ferromagnetism in LaCoO_{3}. Through the modification of symmetry by strain, concurrent frozen charge and spin-state order are stabilized, giving rise to long-range magnetic order.

Exploring YSZ/ZrO2 - PuO2 systems: Candidates for inert matrix fuel

The investigations in phase behavior of plutonia-based system are generally carried out by employing surrogates for the ease of handling. However, the curiosity to understand the phase relations by employing PuO2 directly instead of surrogates led to the present work on phase relations in yttria stabilized zirconia (YSZ)–PuO2 and ZrO2–PuO2 systems. These systems have relevance to inert matrix fuel concept. Phase relations have been discussed in both the systems under different conditions. The nominal compositions in YSZ-PuO2 system under vacuum exhibit fluorite structure for low PuO2 content (up to 20 mol %) while the pyrochlore structure was observed for the samples with 30–50 mol% PuO2 content. Annealing under oxidizing conditions did not alter the F-type phases but decreased the ordering in pyrochlore phases. The samples with composition Zr0.8Pu0.2O2 and Zr0.5Pu0.5O2 in ZrO2–PuO2 systems under reduced condition form F-type phase and pyrochlore, respectively. The F-type phase, upon oxidation, transformed to tetragonal phase while the pyrochlore phase retains its identity with concomitant decrease in intensity of superstructure reflections indicating the gradual pyrochlore to fluorite phase transition. The sintering studies on YSZ-PuO2 samples show that no appreciable shrinkage were observed on the samples beyond 1773 K. Linear thermal expansion on the representative nominal compositions was measured in a vertical dilatometer over the temperature range from RT to 1573 K in argon atmosphere. Though the coefficients of linear thermal expansion of 0.6 YSZ-0.4 PuO2 (40 mol% PuO2 containing YSZ) is less compared to YSZ (x = 0.0) at lower temperature, the trend reverses beyond 1473 K.

The influence of potassium content on octahedral-tilt disorder in Na0.5Bi0.5TiO3-solid solutions near morphotropic phase boundary

A semi-quantitative comparison of 3D electron diffraction data revealed structural differences at the nanoscale for three (95-x)Na0.5Bi0.5TiO3-xK0.5Bi0.5TiO3-5BaTiO3 solid solutions close to the morphotropic phase boundary, with x=0, 5, 10. Using a novel rotation electron diffraction technique, diffuse scattering intensity was recorded in 3D as continuous rods along hk0.5 reciprocal-planes. By analyzing the different superstructure reflections and the intensity/morphology of diffuse scattering, a structural model of the local octahedral-tilt disorder for the different ceramics was developed. A good agreement was obtained between the experimental and simulated data with a model comprising of antiphase (a−a−a−) and plate-like in-phase (a0a0c+) nanodomains.

Formation of a Stepped Si(100) Surface and Its Effect on the Growth of Ge Islands

The transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 A/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 A/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I2 × 1/I1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.

Effect of a Stepped Si(100) Surface on the Nucleation Process of Ge Islands

Nucleation of Ge islands on a stepped Si(100) surface is studied. It is shown by diffraction of fast electrons that at a temperature of 600°C, constant flux of Si, and deposition rate of 0.652 A/s, a series of the 1×2 superstructure reflections completely disappears, if the Si (100) substrate deviated by an angle of 0.35° to the (111) face is preliminarily heated to 1000°C. The disappearance of the 1×2 superstructure reflexes is due to the transition from the surface with monoatomic steps to that with diatomic ones. Investigations of the Ge islands’ growth were carried out on the Si(100) surface preliminarily annealed at temperatures of 800 and 1000°C. It is shown that the islands tend to nucleate at the step edges.

Distribution of Vacancies in a Hybrid M(5–11/18)X(5–11/18) Superstructure of a High-Temperature Ordered β-TiO Phase

A new structural model of the high-temperature ordered β-TiO phase of titanium monoxide with vacancies in metallic and nonmetallic sublattices has been proposed. The model suggests that the vacancies are simultaneously distributed over sites of two partially disordered superstructures: monoclinic M5X5(mon) (space group C2/m (A2/m)) and cubic M5X5(cub) (space group Pm\(\overline 3 \)m). The hybrid superstructure belongs to space group P1m1 of the monoclinic system. The proposed notation M(5–11/18)X(5–11/18) of the hybrid superstructure takes into account an expansion of the vacancy sublattice due to the superposition of initial \({M_5}{X_{{5^ - }}}\) type superstructures. It is shown that the arrangements of the superstructure reflections in the diffraction spectra of the M(5–11/18)X(5–11/18) and M5X5(cub) superstructures are identical.

Ammonothermal Synthesis and Crystal Structure of the Nitridoalumogermanate Ca1–xLixAl1–xGe1+xN3 (x ≈ 0.2)

The new nitridoalumogermanate Ca1–xLixAl1–xGe1+xN3 (x ≈ 0.2) was synthesized from supercritical ammonia at 925 K and 185 MPa starting from the intermetallic precursor Ca3Al2Ge2 and Li as mineralizer. The crystal structure was refined by the Rietveld method; Ca1–xLixAl1–xGe1+xN3 with x ≈ 0.2 [a = 9.9822(5), b = 5.7763(2), c = 5.1484(1) Å, Z = 4, Rwp = 0.0492] crystallizes in orthorhombic space group Cmc21 (no. 36). Needle‐shaped crystals with up to 15 µm in length were detected by scanning electron microscopy. Possible cation ordering was analyzed by transmission electron microscopy. No superstructure reflections were observed in electron diffraction patterns and the structure model was confirmed by Z‐contrast imaging. The composition was verified by energy‐dispersive X‐ray spectroscopy on single crystals and inductively coupled plasma optical emission spectrometry. Optical properties were studied by diffuse reflectance spectroscopy, showing a broad absorption band between 250 and 300 nm. The optical band gap was estimated to be 4.3 eV at room temperature using the Tauc method.

Another step toward the solution of the real structure of zinkenite

Abstract The crystal structure of Cu-bearing zinkenite from Saint-Pons (Alpes Maritimes department, France), having idealized chemical composition Cu0.7Pb9.7Sb21.3S42, has been studied. It has a pronounced hexagonal sub-cell, with unit-cell parameters a=22.1219(11), c=4.3207(3) Å, V=1831.2(2) Å3, space group P63, Z=1. The sub-cell crystal structure was refined to R 1=0.072 on the basis of 3905 reflections with F o>4σ(F o) and 133 refined parameters. It can be described as formed by one kind of rod, with walls of columns of (Pb/Sb)-centered polyhedra flanking both the rods and the sites located along the 63 screw axis. Minor Cu is hosted in the tetrahedral voids between the rods and the walls of polyhedra. Alternatively, the crystal structure of zinkenite can be described as formed by trigonal rods, delimited by lone electron-pair micelles, and tunnels hosting (Pb/Sb) atoms. The occurrence of weak superstructure reflections points to a triclinic unit cell with parameters a=38.271(2), b=22.1219(13), c=8.6475(5) Å, α=89.931(3), β=90.030(3), γ=89.957(3)°, V=7323.6(7) Å3, space group P1, Z=4. The twin laws making the twin lattice hexagonal have been taken into account and the crystal structure has been solved and refined. Notwithstanding the very low R 1 value (R 1=0.038 on the basis of 22563 reflections with F o>4σ(F o) and 1194 refined parameters), several shortcomings, mainly due to the low diffraction quality of the available crystals, allow only the description of the main structural features of the superstructure of zinkenite, indicating the correctness of the triclinic model hypothesized by previous authors.

Распределение вакансий в гибридной сверхструктуре M-=SUB=-(5-11/18)-=/SUB=-X-=SUB=-(5-11/18)-=/SUB=- высокотемпературной упорядоченной фазы beta-TiO

AbstractA new structural model of the high-temperature ordered β-TiO phase of titanium monoxide with vacancies in metallic and nonmetallic sublattices has been proposed. The model suggests that the vacancies are simultaneously distributed over sites of two partially disordered superstructures: monoclinic M_5X_5(mon) (space group C 2/ m ( A 2/ m )) and cubic M_5X_5(cub) (space group Pm $$\overline 3 $$ 3 ¯ m ). The hybrid superstructure belongs to space group P 1 m 1 of the monoclinic system. The proposed notation M_(5–11/18)X_(5–11/18) of the hybrid superstructure takes into account an expansion of the vacancy sublattice due to the superposition of initial $${M_5}{X_{{5^ - }}}$$ M 5 X 5 − type superstructures. It is shown that the arrangements of the superstructure reflections in the diffraction spectra of the M_(5–11/18)X_(5–11/18) and M_5X_5(cub) superstructures are identical.

Формирование ступенчатой поверхности Si(100) и ее влияние на рост островков Ge

AbstractThe transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 Å/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 Å/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I _2 × 1/ I _1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.

Магнитные свойства слоистого кобальтита Sr-=SUB=-1-x-=/SUB=-Y-=SUB=-x-=/SUB=-CoO-=SUB=-3-delta-=/SUB=- (x = 0.1)

AbstractThe structure, the magnetic and magnetotransport properties of perovskite Sr_0.9Y_0.1CoO_2.63 have been studied. The sample is shown to have a two-phase structure. The main phase has a tetragonally distorted unit cell and is described by space group I 4/ mmm . The broadening of the reflections with indices corresponding to doubling unit cell parameter c indicates the absence of the rigorous translation symmetry along axis c . The existence of the broadened superstructure reflection observed in the diffraction pattern at small angles at temperature lower than 400 K is explained by the existence of the monoclinic phase whose content is significantly lower than that of the tetragonal phase, but is dominant in the Sr_0.8Y_0.2CoO_3–δ composition. The spontaneous magnetization appears as the monoclinic phase forms. The magnetic structure is mainly G-type antiferromagnetic with magnetic moments 1.5μ_B in the layers of CoO_6 octahedra and 2μ_B in the anion-deficit CoO_4 + γ layers. The conduction of the Sr_0.9Y_0.1CoO_2.63 composition has a semiconducting character. The magnetoresistance is 57% in a field of 14 T at a temperature of 5 K and strongly decreases with the temperature increase.