Commensurate Superstructure Research Articles

Ordered structure and thermal expansion in tungsten bronze Pb₂K(0.5)Li(0.5)Nb₅O₁₅.

The crystal structure and thermal expansion behaviors of a new tetragonal tungsten bronze (TTB) ferroelectric, Pb2K(0.5)Li(0.5)Nb5O15, were systematically investigated by selected-area electron diffraction (SAED), neutron powder diffraction, synchrotron X-ray diffraction (XRD), and high-temperature XRD. SAED and Rietveld refinement reveal that Pb2K(0.5)Li(0.5)Nb5O15 displays a commensurate superstructure of simple orthorhombic TTB structure at room temperature. The structure can be described with space group Bb2₁m. The transition to a paraelectric phase (P4/mbm) occurs at 500 °C. Compared with Pb2KNb5O15 (PKN), the substitution of 0.5K(+) with small 0.5Li(+) into PKN causes the tilting of NbO6 octahedra away from the c axis with Δθ ≈ 10° and raises the Curie temperature by 40 °C, and the negative thermal expansion coefficient along the polar b axis increases more than 50% in the temperature range 25-500 °C. We present that, by introduction of Li(+), the enhanced spontaneous polarization is responsible for the enhanced negative thermal expansion along the b axis, which may be caused by more Pb(2+) in the pentagonal caves.

Structural modulations in the rare-earth metal digermanides REAl1-xGe2 (RE = Gd-Tm, Lu, Y; 0.8 < x < 0.9). Correlations between long- and short-range vacancy ordering.

Rare-earth metal aluminum germanides with the general formula REAl(1-x)Ge(2) (RE = Gd, Tb, Dy, Ho, Er, Tm, Lu, and Y) have been synthesized by direct fusion of the corresponding elements. The structures have been studied by single-crystal X-ray diffraction and selected-area electron diffraction (SAED). The average structure represents a randomly "stuffed" variant of the orthorhombic ZrSi(2) structure type, also known as the CeNi(1-x)Si(2) type (Pearson symbol oC16; space group Cmcm). The SAED patterns for selected members of the family suggest the coexistence of commensurate and incommensurate structural modulations. The most prominent model for long-range vacancy ordering is the Tb(4)FeGe(8) type (Pearson symbol mP26; space group P21/n), which is the commensurate 4-fold superstructure of CeNi(1-x)Si(2) (x = (3)/4). Short-range correlations cause additional deviations in the 4-fold superlattice. These results shed more light on the structural complexity as a function of the aluminum vacancies and size of the rare-earth metal. Magnetic susceptibility measurements are presented and discussed. The measured ordering temperatures and calculated ones based on empirical rules and Ruderman-Kittel-Kasuya-Yosida interactions are shown to be in close agreement.

Core–shell cermet condensates by pulsed-laser ablation on Zn in TEOS

Spherical cermet condensates with metallic Zn core and specific shell types, both having significant internal compressive stress, were fabricated by pulsed-laser ablation on Zn target in TEOS for X-ray/electron diffraction and spectroscopic characterizations. The nanosized Zn cores have 1-D commensurate (0001) superstructure and wurtzite-type ZnO shell following almost parallel epitaxy relationship, i.e., basal planes exactly in parallel but others slightly off, across a semicoherent interface. The submicron-sized Zn condensates were free of superstructure and encapsulated by a Si–H-signified turbostratic graphite shell. The defective cermet condensates thus fabricated showed UV–Vis photoemission and absorption with a minimum band gap of 1.95 eV for potential optoelectronic and catalytic applications.

Spin Exchange and Magnetic Dipole–Dipole Interactions Leading to the Magnetic Superstructures of MAs2O6 (M = Mn, Co, Ni)

The three isostructural magnetic oxides MAs2O6 (M = Mn, Co, Ni) containing high-spin M(2+) ions undergo a long-range antiferromagnetic ordering below 30 K, but their ordered magnetic structures are not identical. While CoAs2O6 and NiAs2O6 adopt the commensurate superstructure of q1 = (0, 0, 1/2), MnAs2O6 has the incommensurate superstructure of q2 = (0.055, 0.389, 0.136). The cause for this difference was examined by calculating their spin exchange and magnetic dipole-dipole interaction energies. In CoAs2O6 and NiAs2O6, the strongest M-O···O-M spin exchange, J1, dominates over other exchanges, hence leading to the q1 superstructure. For MnAs2O6, the spin exchanges are not a deciding factor leading to its magnetic superstructure, being all weak and comparable in strengths, but the magnetic dipole-dipole interactions are.

Special grain boundaries of anatase nanocondensates by oriented attachment

Anatase nanocondensates with a 1-D commensurate superstructure as prepared by the pulsed laser ablation of Ti in tetraethyl orthosilicate were identified by transmission electron microscopy to have special grain boundaries, i.e. symmetrical [100] tilt boundaries with (001) or (01) interfaces and an asymmetrical [10] tilt boundary with a (001)/(11) heterointerface. The [100] tilt boundaries are in fact due to basal twinning with a coherent (001) and a semi-coherent (01) interface, respectively, both following the Burgers vector = 1/2[00] + 1/4[00]. The (001)/(11) heterointerface is decorated with {101} ledges, both having a primitive coincidence site lattice for fair lattice coherency. These special grain boundaries can be rationalized by the (hkl)-specific rotation/coalescence of the crystalline nanocondensates.

Nickel aluminate oxides/hydroxides by pulsed laser ablation of NiAl2O4 powder in water

Submicron-sized NiAl2+X O4 fragments and nanocondensates of Ni-doped γ-Al2O3, Al-doped NiO and β-Ni(OH)2 were synthesized simultaneously by pulsed laser ablation of NiAl2O4 powder in water and characterized using X-ray/electron diffraction and optical spectroscopy. The NiAl2+X O4 is Al-enriched spinel with dislocations and subgrains. The Ni-doped γ-Al2O3 spinel has paracrystalline distribution (i.e., with fair constant longitudinal spacing, but variable relative lateral translations) of defect clusters and intimate intergrowth of θ-Al2O3 and 2x( $$ \overline{1} $$ 3 $$ \overline{1} $$ ) commensurate superstructure. The Al-doped NiO has perfect cubo-octahedron shape and as small as 5 nm in size. The β-Ni(OH)2 and 1-D turbostratic hydroxide lamellae occurred as a matrix of these oxide nanoparticles. The colloidal suspension containing the composite phases has a minimum band gap of 5.3 eV for potential photocatalytic applications.

High resolution scanning tunnelling microscopy and extended x-ray-absorption fine structure study of the (5 × 3) silicide structure on Cu(001)

Using low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), and x-ray absorption spectroscopy (XAS) techniques, we have studied the first steps of silicon adsorption onto Cu (001) single crystal substrate. For low coverage (∼0.5 ML) and after annealing at 100 °C, STM images and LEED patterns reveal the formation of an ordered quasi commensurate (5×3) superstructure. From a quantitative analysis of XAS data, we extract the Si-Cu distance and detail the local atomic arrangement of the (5×3) structure.

The composite modulated structure of cupropearceite and cupropolybasite and its behavior toward low temperature

Cupropearceite, [(Cu 3.51 Ag 2.50 Fe 0.01 ) ∑6.02 (As 1.72 Sb 0.24 ) ∑1.96 S 7 ][Ag 9 CuS 4 ], and cupropolybasite, [(Cu 3.82 Ag 2.42 Zn 0.02 Pb 0.01 ) ∑6.27 (Sb 1.19 As 0.73 ) ∑.92 S 7 ][Ag 9 CuS 4 ], both exhibit fast-ion conduction at very low temperatures. The structural relationship between the various phases is not fully understood as yet and is addressed in this study. Samples of these materials were studied by means of synchrotron radiation at room temperature and transmission electron microscopy at room temperature and low temperature (both liquid N 2 and liquid He) to have a better understanding of the stabilization of the fast-ion conducting form at low and ultra-low temperature in these minerals. The study at room temperature did not evidence any doubling of unit-cell parameters with respect to the basic Tac unit cell, of the type typically observed for minerals of the pearceite-polybasite group. On the other hand, relatively strong and well-defined satellite reflections relating to the pseudo-hexagonal arrangement of the Ag + ions at G ± ~1.39(1) * positions of the reciprocal space, where G represents the average structure Bragg reflections, were clearly observed. Although this seems to suggest that the Ag + ion distribution can adequately be described by a two-dimensional displacive modulation of the average P 3 m 1 structure ( Tac polytype) with the incommensurate modulation wave vectors of the satellite reflections q 1 = ~0.39(1)( a F * + b F *) and q 2 = ~0.39(1)( a F * − b F *), where the subscript F indicates the framework substructure, the structure observed is better described as a composite modulated structure because of the intensity asymmetry of the satellite reflections. Low-temperature TEM investigations show that the satellites are still present at both 90 and 4.2 K, with a remarkable temperature-dependent shift in their positions giving rise to a variation of the coefficient α of the modulation vectors from 0.39 at room temperature, trough ~0.40 at 90 K to ~0.5 at 4.2 K. Thus, the incommensurate modulation, strengthened by the very low temperature, approaches almost the α ~0.5 value, indicative of a commensurate modulation. The 4.2 K structure could thus be a low-temperature commensurate superstructure (“lock-in phase”), observed for the first time in the minerals of the pearceite-polybasite group.

Initial Growth of N,N′-Bis(1-ethylpropyl)perylene-3,4,9,10-tetracarboxdiimide Films on Cu(100)

We have studied the initial stages of the adsorption of N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxdiimide (EP-PTCDI) on Cu(100) using scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and density functional theory (DFT) calculations. STM images show that the EP-PTCDI molecules initially absorb on the surface, forming ordered islands, without preferential nucleation at the step-edges of the terraces as is the case for adsorption of the same molecule on Ag(111). The 2D islands grow upon the completion of the surface with a single-molecular layer. The experimental and simulated LEED patterns show that the molecular arrangement within the islands can be explained by a commensurate superstructure and its four rotational equivalents. The electron spectroscopies show net charge transfer from the surface to the molecule and a decrease of about 0.7 eV in the work function (from 4.59 to 3.9 eV) after a single-layer adsorption. The DFT ca...

To tilt or not to tilt: Correction of the distortion caused by inclined sample surfaces in low-energy electron diffraction

Low-energy electron diffraction (LEED) is a widely employed technique for the structural characterization of crystalline surfaces and epitaxial adsorbates. For technical reasons the accessible reciprocal space is limited at a given primary electron energy E. This limitation may be overcome by sweeping E to observe higher diffraction orders decisively enhancing the quantitative examination. Yet, in many cases, such as molecular films with rather large unit cells, the adsorbate reflexes become less pronounced at energies high enough to observe substrate reflexes. One possibility to overcome this problem is an intentional inclination of the sample surface during the measurement at the expense of the quantitative interpretability of then severely distorted diffraction patterns. Here, we introduce a correction method for the axially symmetric distortion in LEED images of tilted samples. We provide experimental confirmation for micro-channel plate LEED and spot-profile analysis LEED instruments using the (7×7) reconstructed surface of a Si(111) single crystal as a reference sample. Finally, we demonstrate that the correction of this distortion considerably improves the quantitative analysis of diffraction patterns of adsorbates since substrate and adsorbate reflexes can be evaluated simultaneously. As an illustrative example we have chosen an epitaxial monolayer of 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(111) that is known to form a commensurate superstructure.

On the densification and hydration of CaCO3 particles by Q-switched laser pulses in water

Calcite powders subjected to Q-switched laser pulses in water were characterized by X-ray/electron diffraction and optical spectroscopy to have a significant internal compressive stress (up to ca. 1.5GPa) with accompanied transformation into defective calcite II and hydrates. The defective calcite II particles were (010), (001), (01¯1), (013) and (01¯3) faceted with 2×(020)II commensurate superstructure and tended to hydrate epitaxially as monohydrocalcite co-existing with ikaite (CaCO3·6H2O) with extensive cleavages and amorphous calcium carbonate with porous structure. The colloidal suspension containing the densified calcite polymorphs and hydrates showed two UV–visible absorptions corresponding to a minimum band gap of ca. 5 and 3eV, respectively.

Structural diversity in iron oxide nanoparticle assemblies as directed by particle morphology and orientation

The mesostructure of ordered arrays of anisotropic nanoparticles is controlled by a combination of packing constraints and interparticle interactions, two factors that are strongly dependent on the particle morphology. We have investigated how the degree of truncation of iron oxide nanocubes controls the mesostructure and particle orientation in drop cast mesocrystal arrays. The combination of grazing incidence small-angle X-ray scattering and scanning electron microscopy shows that mesocrystals of highly truncated cubic nanoparticles assemble in an fcc-type mesostructure, similar to arrays formed by iron oxide nanospheres, but with a significantly reduced packing density and displaying two different growth orientations. Strong satellite reflections in the GISAXS pattern indicate a commensurate mesoscopic superstructure that is related to stacking faults in mesocrystals of the anisotropic nanocubes. Our results show how subtle variation in shape anisotropy can induce oriented arrangements of nanoparticles of different structures and also create mesoscopic superstructures of larger periodicity.

Growth and adsorption geometry of naphthalene overlayers on HOPG studied by low-temperature scanning tunneling microscopy

We have investigated the adsorption geometries of naphthalene on the basal plane of graphite using low-temperature scanning tunneling microscopy (STM). At sub-monolayer coverage, the commensurate superstructure of (2√3×2√3) R30° is observed. Molecules in ordered layers are imaged with sub-molecular spatial resolution where the shape of the molecular image reflects the shape of molecular orbitals in the vicinity of the Fermi level. The multilayer films grow in a layer-by-layer mode with a superstructure described by the matrix 3-123 up to several monolayers. The tilted configuration of the molecule is compared with other adsorption systems.

Electron–hole instability in 1T-TiSe2

In this paper, we address the question of the origin of the charge density wave instability in 1T-TiSe2. We develop a model considering the direct Coulomb interaction between electrons and holes in the valence and conduction bands near the Fermi energy. Using the Bethe–Salpeter equation, we calculate the electron–hole correlator, which reveals an instability at low temperature leading to a transition toward a commensurate superstructure mediated by the electron–phonon coupling, in agreement with experiments. On the basis of this correlator, the electron self-energies are then calculated and the corresponding photoemission spectra are compared with the experimental ones, revealing good agreement. The signature of electron–hole fluctuations in photoemission is emphasized. Furthermore, we calculate the spectral function of the phonon mode, whose softening is experimentally observed at the transition.

Phase behavior of Ni- and Co-doped SnO2 via reactive sintering and solution annealing

Abstract SnO2–NiO and SnO2–CoO powders mixed in a molar ratio of 92: 8 were reactively sintered for up to 200 h at 1400 °C to form composites having NiO and Co2SnO4 particles embedded in Ni- and Co-doped SnO2 grains, respectively. Analytical electron microscopic observations indicated the embedded particles remained nonepitaxial with respect to the host grains despite prolonged holding at a high homologous temperature, owing to a rather limited solid solubility and vague interfacial energy cusp. Upon cooling or solution annealing at 900 °C, the Niand Co-doped SnO2 formed {100}, {110}, {101} and/or (001)-specific Guinier–Preston zone plates and dislocations. The formation of additional commensurate superstructure in Ni- but not Co-doped SnO2 indicated a coupled atom clustering and ordering process for the former during solution annealing.

Structural Evolution of Perfluoro-Pentacene Films on Ag(111): Transition from 2D to 3D Growth

The structural evolution and thermal stability of perfluoro-pentacene (PF-PEN) thin films on Ag(111) have been studied by means of low-temperature scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). Well-defined monolayer films can be prepared by utilizing the different adsorption energy of mono- and multilayer films and selectively desorbing multilayers upon careful heating at 380 K, whereas at temperatures above 400 K, a dissociation occurs. In the first monolayer, the molecules adopt a planar adsorption geometry and form a well-ordered commensurate (6 × 3) superstructure where molecules are uniformly oriented with their long axis along the <110> azimuth. This molecular orientation is also maintained in the second layer, where molecules exhibit a staggered packing motif, whereas further deposition leads to the formation of isolated, tall islands. Moreover, on smooth silver surfaces with extended terraces, growth of PF-PEN onto beforehand prepared long-range ordered monolayer films at elevated temperature leads to needle-like islands that are uniformly aligned at substrate steps along <110> azimuth directions.

Surface Modification and Planar Defects of Calcium Carbonates by Magnetic Water Treatment

Powdery calcium carbonates, predominantly calcite and aragonite, with planar defects and cation–anion mixed surfaces as deposited on low-carbon steel by magnetic water treatment (MWT) were characterized by X-ray diffraction, electron microscopy, and vibration spectroscopy. Calcite were found to form faceted nanoparticles having 3x () commensurate superstructure and with well-developed {} and {} surfaces to exhibit preferred orientations. Aragonite occurred as laths having 3x () commensurate superstructure and with well-developed () surface extending along [100] direction up to micrometers in length. The (hkil)-specific coalescence of calcite and rapid lath growth of aragonite under the combined effects of Lorentz force and a precondensation event account for a beneficial larger particulate/colony size for the removal of the carbonate scale from the steel substrate. The coexisting magnetite particles have well-developed {011} surfaces regardless of MWT.

Transition from a phase-segregated state to single-phase incommensurate sodium ordering inγ-NaxCoO2(x≈0.53)

Synchrotron X-ray diffraction investigations of two single crystals of Na_xCoO_2 from different batches with composition x = 0.525-0.530 reveal homogeneous incommensurate sodium ordering with propagation vector (0.53 0.53 0) at room-temperature. The incommensurate (qq0) superstructure exists between 220 K and 430 K. The value of q varies between q = 0.514 and 0.529, showing a broad plateau at the latter value between 260 K and 360 K. On cooling, unusual reversible phase segregation into two volume fractions is observed. Below 220 K, one volume fraction shows the well-known commensurate orthorhombic x = 0.50 superstructure, while a second volume fraction with x = 0.55 exhibits another commensurate superstructure, presumably with a 6a x 6a x c hexagonal supercell. We argue that the commensurate-to-incommensurate transition is an intrinsic feature of samples with Na concentrations x = 0.5 + d with d ~ 0.03.

The photo-induced commensurate modulated structure in site-selective spin crossover complex trans-[Fe(abpt)2(NCS)2

The photo-induced superstructure of polymorph C of trans-[Fe(abpt)(2)(NCS)(2)] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) is discovered as a commensurate modulated structure by single-crystal X-ray diffraction under irradiation. The crystal structure at 25 K before the photo-irradiation composed of two crystallographically independent iron molecules, one of which exhibits high spin (HS) state and the other at low spin (LS) state. Under green laser light (λ = 532 nm) irradiation, the LS molecule (Fe1) is found to be excited to a metastable HS state and gives rise to a commensurate tripled superstructure along the c axis. In addition, it is confirmed that this modulation persists until the HS → LS relaxation temperature beyond 52 K. Our structural findings suggest that the structural modulation and the site-selective LS → HS excitation are highly correlated.

Structure and crystal chemistry of fluorite-related Bi 38Mo 7O 78 from single crystal X-ray diffraction and ab initio calculations

The floating-zone furnace method was used to synthesize single crystals of the fluorite-related δ-Bi 2O 3-type phase Bi 38Mo 7O 78 for the first time. Single crystal synchrotron X-ray diffraction data, in conjunction with ab initio (density functional theory) calculations, were used to solve, optimize, and refine the 5×3×3 commensurate superstructure of fluorite-type δ-Bi 2O 3 in Pbcn ( a=28.7058(11) Å, b=16.8493(7) Å and c=16.9376(6) Å, Z=4, R F =11.26%, w R I =21.67%). The structure contains stepped channels of Mo 6+ in tetrahedral environments along the b axis and chains of Mo 6+ in octahedral environments along the ac plane. The role of the stepped channels in oxide ion conduction is discussed. The simultaneous presence of both tetrahedral and octahedral coordination environments for Mo 6+, something not previously observed in Mo 6+-doped δ-Bi 2O 3-type phases, is supported by charge balance considerations in addition to the results of crystallographic and ab initio analysis.