Planar Clusters Research Articles

Structure defects, phase transitions, magnetic resonance and magneto-transport properties of La0.6–xEuxSr0.3Mn1.1O3–δ ceramics

Structure and its defects, magnetic resonance and magneto-transport properties of La0.6–xEuxSr0.3Mn1.1O3–δ magnetoresistive ceramics were investigated by x-ray diffraction, thermogravimetric, resistance, magnetic, 55Mn NMR and magnetoresistance methods. It was found that isovalent substitution of lanthanum by europium A-cation of a smaller ionic radius increases the structural imperfection and leads to a symmetry change from the rhombohedrally distorted perovskite structure of R3¯c symmetry to the pseudocubic type. It was shown that the real structure contains anionic and cationic vacancies, the concentrations of which increases with the Eu concentration and the sintering temperature tann. A decrease in the temperature of the metal–semiconductor Tms and ferromagnetic–paramagnetic TC phase transitions as well as an increase in the resistivity ρ and the activation energy Ea with increasing x are due to an increase in vacancy concentration, which weakens the high-frequency electron double exchange Mn3+ ↔ O2– ↔ Mn4+. The crystal structure of the compositions x = 0 and 0.1 contains nanostructured planar clusters, causing anomalous magnetic hysteresis at T = 77 K. Broad asymmetric 55Mn NMR spectra confirm high-frequency electron double exchange Mn3+(3d4) ↔ O2–(2p6) ↔ Mn4+(3d3) and indicate inhomogeneity of the manganese environment due to the surrounding ions and vacancies. The effective local fields of the hyperfine interaction HHF at 55Mn nuclei have been calculated by decomposing asymmetric NMR spectra into three Gaussian components. The constructed experimental phase diagram of the magnetic and conducting states of the La0.6–xEuxSr0.3Mn1.1O3–δ ceramics revealed strong correlation between the composition, structural imperfection, phase state, and magnetotransport properties of rare-earth manganites.

Comments on “Geometry and fast diffusion of AlCl4 cluster intercalated in graphite [Electrochim. Acta 195 (2016) 158–165

We report that the planar AlCl4 clusters in graphite reported in [Electrochim. Acta 195 (2016) 158-165] are not justified from the point of view of theory and their fast diffusion in graphite has nothing to do with the ultrafast rate capability of Al-ion battery in experiments.

Benchmarking Quantum Chemical Methods for Optical Absorption in Boron Wheels.

We benchmark various quantum chemical methods for calculating the optical absorption in planar boron wheel clusters. The geometries of neutral planar boron wheels B7, B8, and B9 clusters are optimized at the coupled-cluster singles doubles level of theory. The optical absorption spectra of these clusters are calculated using three wave-function-based methods, namely, configuration interaction singles, random phase approximation, and equation-of-motion coupled-cluster singles doubles (EOM-CCSD) as well as using a time-dependent density-functional-theory-based method using various hybrid and long-range-corrected exchange and correlation functionals. There is an ample variation in the optical absorption spectra computed using different density functionals. When compared to the EOM-CCSD spectrum, an excellent agreement is provided by CAM-B3LYP functional, followed by ωB97xD functional. PBE0, B3LYP, and B3PW91 functionals agree among each other. However, their spectra are red-shifted with respect to the EOM-CCSD counterpart. On the basis of the natural transition orbital analysis, the nature of optical excitation is also discussed.

Density functional theory study of CO oxidation by O2 on Aun (n = 11, 13 and 14) clusters as catalysis: From a comparative review

The CO oxidation mechanisms on the selected planar and spatial Au11, Au13 and Au14 clusters were investigated systematically with G09 program package. Carbon monoxide adsorbs to the Au atoms with the lowest coordination numbers, and after CO oxidation occurring at the peaks and edges of planar Au clusters, the clusters continue to maintain planarity. However, the energy barrier of the CO oxidation at the interior Au atoms of planar Au11 cluster is much higher, and the Au plane is destroyed after the first-step oxidation. The cluster structures of nonplanar Au clusters will be destroyed after CO oxidation.

Inelastic light scattering spectroscopy in Si/SiGe nanostructures: Strain, chemical composition and thermal properties

Abstract We present a review of recent studies of inelastic light scattering spectroscopy in two types of Si/SiGe nanostructures: planar superlattices and cluster (dot) multilayers including first- and second-order Raman scattering, polarized Raman scattering and low-frequency inelastic light scattering associated with folded acoustic phonons. The results are used in semi-quantitative analysis of chemical composition, strain and thermal conductivity in these technologically important materials for electronic and optoelectronic devices.

ChemInform Abstract: Beyond Organic Chemistry: Aromaticity in Atomic Clusters

AbstractReview: 133 refs.

The role of structural and magnetic inhomogeneities in the formation of magneto-transport properties of the La0.6−xSmxSr0.3Mn1.1O3−δ ceramics

X-ray diffraction, magnetic, transport and SEM measurements were used to investigate the role of structural and magnetic inhomogeneities in the formation of magneto-transport properties of the La0.6−xSmxSr0.3Mn1.1O3−δ (x=0–0.6) ceramics. Reduction of the a parameter of perovskite structure and change of its symmetry are shown to be due to both replacement of La3+ by Sm3+ and an increase concentration of anion and cation vacancies. Broad asymmetric 55Mn NMR spectra are indicative of the high frequency electron double exchange Mn3+↔Mn4+ and demonstrate the heterogeneity of magnetic and valence Mn-states. An excess manganese is dissolved in perovskite structure statistically without formation of planar antiferromagnetic clusters of Mn2+ ions in the deformed A positions. Constructing phase diagram of “composition–structure–properties” describes a strong correlation between structural, resistive and magnetic properties as well as reflects a special role of structural defects in the formation of magneto-transport properties of the rare-earth manganites.

The Planar CoB18− Cluster as a Motif for Metallo‐Borophenes

AbstractMonolayer‐boron (borophene) has been predicted with various atomic arrangements consisting of a triangular boron lattice with hexagonal vacancies. Its viability was confirmed by the observation of a planar hexagonal B36 cluster with a central six‐membered ring. Here we report a planar boron cluster doped with a transition‐metal atom in the boron network (CoB18−), suggesting the prospect of forming stable hetero‐borophenes. The CoB18− cluster was characterized by photoelectron spectroscopy and quantum chemistry calculations, showing that its most stable structure is planar with the Co atom as an integral part of a triangular boron lattice. Chemical bonding analyses show that the planar CoB18− is aromatic with ten π‐electrons and the Co atom has strong covalent interactions with the surrounding boron atoms. The current result suggests that transition metals can be doped into the planes of borophenes to create metallo‐borophenes, opening vast opportunities to design hetero‐borophenes with tunable chemical, magnetic, and optical properties.

The Planar CoB18− Cluster as a Motif for Metallo‐Borophenes

Monolayer-boron (borophene) has been predicted with various atomic arrangements consisting of a triangular boron lattice with hexagonal vacancies. Its viability was confirmed by the observation of a planar hexagonal B36 cluster with a central six-membered ring. Here we report a planar boron cluster doped with a transition-metal atom in the boron network (CoB18 (-) ), suggesting the prospect of forming stable hetero-borophenes. The CoB18 (-) cluster was characterized by photoelectron spectroscopy and quantum chemistry calculations, showing that its most stable structure is planar with the Co atom as an integral part of a triangular boron lattice. Chemical bonding analyses show that the planar CoB18 (-) is aromatic with ten π-electrons and the Co atom has strong covalent interactions with the surrounding boron atoms. The current result suggests that transition metals can be doped into the planes of borophenes to create metallo-borophenes, opening vast opportunities to design hetero-borophenes with tunable chemical, magnetic, and optical properties.

Electronic route to stabilize nanoscale spin textures in itinerant frustrated magnets

We unveil novel spin textures in an itinerant fermion model on a frustrated triangular lattice in the limit of low electronic density. Using hybrid Monte Carlo simulations on finite clusters we identify two type of nanoscale spin textures in the background of 120 degree order : (i) a planar ferromagnetic cluster, and (ii) and a non-coplanar cluster with spins oriented perpendicular to the 120 degree plane. Both these textures lead to localization of the electronic wavefunctions and are in-turn stabilized by the concomitant charge modulations. The non-coplanar spin texture is accompanied by an unusual scalar chirality pattern. A well defined electric charge and magnetic moment associated with these textures allow for their easy manipulation by external electric and magnetic fields - a desirable feature for data storage. We identify a localization-delocalization behavior for electronic wavefunctions which is unique to frustrated magnets, and propose a general framework for stabilizing similar spin textures in spin-charge coupled systems.

A sharp quantitative version of Hales' isoperimetric honeycomb theorem

We prove a sharp quantitative version of Hales' isoperimetric honeycomb theorem by exploiting a quantitative isoperimetric inequality for polygons and an improved convergence theorem for planar bubble clusters. Further applications include the description of isoperimetric tilings of the torus with respect to almost unit-area constraints or with respect to almost flat Riemannian metrics.

Magnetic linear and circular dichroism of planar sets of hollow silver nanoparticles: engineering the spectra by hollow size

We study the magneto-optical response of planar clusters of hollow silver nanoparticles. We use the coupled-dipole equations to describe the interaction of the electromagnetic wave with particles. The simplest achiral and anisotropic cluster, a dimer, exhibits magnetic-field-induced linear and circular dichroism. For particular directions of the magnetic field and light wave vector, the magnetic circular dichroism of a planar cluster is considerable. We find that the hollow size can be invoked to tailor the overall shape of the spectrum, in particular its peak position, bandwidth, and FWHM.

Effects of uniaxial strain on stability and structural evolution of vacancy clusters in copper

The effects of [001] uniaxial strain on the energetics, stable structures, and structural evolution of vacancy clusters with different structure and orientation characteristics in copper have been studied by molecular statics simulation. The dependence of binding energies as functions of strain for different cluster types, including linear, planar, and body types, shows complicated behavior. The binding energies of both linear and planar clusters monotonously vary with the strain from −10% to 10%, while those of body clusters decrease with increasing both tensile and compressive strain. According to the variation of the binding energies, it has been suggested that the linear and planar clusters tend to align parallel (perpendicular) to the strain axis under tensile (compressive) strain. Moreover, both the {001} planar cluster and body cluster become the dominant types when the clusters grow under high strain. Then, a mechanism that the local structure around a vacancy cluster tends to approach the ideal lattice structure without defects and strain has been applied to explain the effects of the uniaxial strain on the relative stability of the vacancy clusters. This tendency is closely tied to the level of the atomic relaxation which can be measured by the average atomic displacement of the nearest-neighbor atoms surrounding the vacancy cluster.

Aromatic character of planar boron-based clusters revisited by ring current calculations.

The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability. These compounds contain two different bonding patterns involving both σ and π delocalized bonds, and up to now, their aromaticity has been assigned mainly using the classical (4N + 2) electron count for both types of electrons. In the present study, we reexplored the aromatic feature of different types of planar boron-based clusters making use of the ring current approach. B3(+/-), B4(2-), B5(+/-), B6, B7(-), B8(2-), B9(-), B10(2-), B11(-), B12, B13(+), B14(2-) and B16(2-) are characterized by magnetic responses to be doubly σ and π aromatic species in which the π aromaticity can be predicted using the (4N + 2) electron count. The triply aromatic character of B12 and B13(+) is confirmed. The π electrons of B18(2-), B19(-) and B20(2-) obey the disk aromaticity rule with an electronic configuration of [1σ(2)1π(4)1δ(4)2σ(2)] rather than the (4N + 2) count. The double aromaticity feature is observed for boron hydride cycles including B@B5H5(+), Li7B5H5 and M@BnHn(q) clusters from both the (4N + 2) rule and ring current maps. The double π and σ aromaticity in carbon-boron planar cycles B7C(-), B8C, B6C2, B9C(-), B8C2 and B7C3(-) is in conflict with the Hückel electron count. This is also the case for the ions B11C5(+/-) whose ring current indicators suggest that they belong to the class of double aromaticity, in which the π electrons obey the disk aromaticity characteristics. In many clusters, the classical electron count cannot be applied, and the magnetic responses of the electron density expressed in terms of the ring current provide us with a more consistent criterion for determining their aromatic character.

A novel MOF showing a ring-like planar Zn6cluster and the coexistence of a single, double, and triple wall

Herein, we report a novel metal–organic framework, Zn6(L)3(4,4′-bpy)3(H2O)4·4DMF (1), which was synthesized using Zn(NO3)2·6H2O, 4,4′-bipyridine (4,4′-bpy) and 5,5′-(diazene-1,2-diyl)bis(2-hydroxybenzoic) acid (H4L). Single crystal X-ray diffraction reveals that the backbone of 1 is highly interesting due to the exceptional coexistence of single, double, triple walls. Furthermore, in 1 a rare ring-like planar Zn6 cluster is observed.

DFT insights into the cycloisomerization of ω-alkynylfuran catalyzed by planar gold clusters: mechanism and selectivity, as compared to Au(i)-catalysis

A detailed reaction mechanism of Au3–10-catalyzed cycloisomerization of ω-alkynylfuran was systemically investigated at the TPSSh/def2-TZVP levels.

A comparative study of AumRhn (4 ≤ m + n ≤ 6) clusters in the gas phase versus those deposited on (100) MgO.

A comparative theoretical study has been performed of the gas phase and deposited AumRhn (4 ≤ m + n ≤ 6) clusters. The combined use of a genetic algorithm and Density Functional Theory (DFT) calculations allows us to explore the potential energy surface and, therefore, find efficiently and automatically the global minimum configuration for each composition. Our results show interesting effects on the geometries of the clusters on deposition. This occurs because the rhodium atoms (electronically) prefer to be in contact with the MgO surface, sometimes promoting planar clusters to become three-dimensional when deposited, and three-dimensional clusters in the gas phase to become two-dimensional. Together with the change in geometries, the magnetic moment is reduced from the gas phase, as the electrons rearrange themselves when the cluster interacts with the substrate.

Circular dichroism induced by Fano resonances in planar chiral oligomers

AbstractWe present a general theory of circular dichroism in planar chiral nanostructures with rotational symmetry. It is demonstrated, analytically, that the handedness of the incident field's polarization can control whether a nanostructure induces either absorption or scattering losses, even when the total optical loss (extinction) is polarization‐independent. We show that this effect is a consequence of modal interference so that strong circular dichroism in absorption and scattering can be engineered by combining Fano resonances with planar chiral nanoparticle clusters.image

Phase transformations inLn2O3materials irradiated with swift heavy ions

Phase transformations induced in the cubic C-type lanthanide sesquioxides, ${\mathrm{Ln}}_{2}{\mathrm{O}}_{3}$ (Ln = Sm, Gd, Ho, Tm, and Lu), by dense electronic excitation are investigated. The structural modifications resulting from exposure to beams of 185 MeV Xe and 2246 MeV Au ions are characterized using synchrotron x-ray diffraction and Raman spectroscopy. The formation of a B-type polymorph, an X-type nonequilibrium phase, and an amorphous phase are observed. The specific phase formed and the transformation rate show dependence on the material composition, as well as the ion beam mass and energy. Atomistic mechanisms for these transformations are determined, indicating that formation of the B-type phase results from the production of anti-Frenkel defects and the aggregation of anion vacancies into planar clusters, whereas formation of the X-type and amorphous phases requires extensive displacement of both anions and cations. The observed variations in phase behavior with changing lanthanide ionic radius and deposited electronic energy density are related to the energetics of these transformation mechanisms.

A novel approach simulating pressure on square ice clusters

ABSTRACTThe recent experimental discovery of square ice under pressure has motivated the present study of some purely planar cluster of N water molecules by quantum chemical methods. As a starting point, four H2O molecules in a square bonded by H-bonds were examined. In order to assess how such a low-lying planar isomer might tend to ‘square ice’ for an infinite sheet, we follow with N = 16, 36 and 64 H2O clusters. Subsequently, the effect of pressure, simulated by the application of a quadrupolar field, on the structures of the different flakes as well as on the vibrational frequencies of the four-molecule cluster is studied.