Distortion Of Symmetry Research Articles

Antiferromagnetic–paramagnetic insulating transition in Cr-dopedV2O3 investigated by EXAFS analysis

We have performed extended x-ray absorption fine-structure (EXAFS) spectroscopy on a 2.8% Cr-dopedV2O3 sample, with the aim of studying its structural evolution in a wide temperaturerange across the paramagnetic–antiferromagnetic insulating phase transition atTc. The data were registered with two different set-ups in fluorescence and transmission geometries,for polarized and unpolarized spectra, respectively. Our idea, based on previous experimentsreported in the literature, is that extended structural modifications of the nominal trigonalsymmetry are present in the paramagnetic insulating phase for several tens of degrees aboveTc, involving further-nearest-neighbor vanadium ions. Our data confirm that the paramagneticinsulating phase is not structurally homogeneous in a temperature range of about 30 K aroundTc, where local distortions of monoclinic symmetry involving further-nearest neighbors arepresent. Moreover, the analysis of the absorption profile at Cr K-edge suggests that Cr ionsenter the lattice randomly. We finally analyze our findings in light of current theoreticalmodels.

Relaxation of radiation-induced defects in CuO

The relaxation of radiation-induced defects in the CuO polycrystal and nanoceramic with the particle size d = 15 nm irradiated by an electron dose of F = 5 × 1018 cm−2 has been studied. In the irradiated samples, a strong susceptibility increase with decreasing temperature T < 150 K is observed. This increase is due to the formation of ferromagnetic polarons in the antiferromagnetic matrix near the defects. The structural symmetry distortion makes the samples unstable. The time variations of the magnetic properties in the CuO samples prepared by different methods are compared.

EPR and vibrational studies of YVO 4:Tm 3+, Yb 3+ single crystal

A well oriented YVO 4 single crystal, with 5% Yb 3+ and 2% Tm 3+ nominal doping, was investigated using the Raman and EPR techniques. The EPR measurements suggest that Yb 3+ ions occupy eight-coordinated Y 3+ sites forming bisdisphenoids of the D 2d symmetry. An inhomogeneous distribution of rare-earth ions leads to a significant distortion of the local point symmetry ( C 1). It seems that strong dipole–dipole interactions between Yb 3+ ions are responsible for the distortion. As a result, two types of ytterbium magnetic centers appear. They correspond to paired magnetic centers and distorted isolated paramagnetic centers that are strongly sensitive to the magnetic field directions and some imperfections of the crystal. Pair centers can be recorded through the rotation around the c-crystal axis, whereas isolated centers can be measured when the crystal is rotated around the a-crystal axis. With the increasing temperature, the ytterbium signal disappeared at about 23 K and a group of narrow lines became visible. These lines, observed in the range of 240–550 mT, correspond to the Gd 3+ ( S = 7/2) ions, doped to the structure unintentionally from the basic materials.

Superconductivity in Hg-substituted BaPb 0.75Bi 0.25O 3

A series of Hg-doped BaPb 0.75Bi 0.25O 3 (BPBO) with a nominal composition of BaPb 0.75 − x Hg x Bi 0.25O 3 ( x = 0 – 0.40 with 0.05 intervals) has been synthesized by solid state reaction. The system shows a lattice parameter expansion and lattice symmetry distortion with Hg doping. Superconducting transition temperature T c and superconducting volume fraction of the system decrease with Hg doping level in the low doping level region ( 0 ⩽ x ⩽ 0.25 ) and are nearly fully suppressed at x = 0.25 . However, the superconductivity is recovered with further increasing Hg content at x > 0.3 . The possible mechanisms of the superconductivity in the low doping level region and the recovery of superconductivity in the high doping level region for Hg-doped BPBO system have been discussed.

IRMPD Spectroscopy of Anionic Group II Metal Nitrate Cluster Ions

Anionic group II metal nitrate clusters of the formula [M(2)(NO(3))(5)](-), where M(2) = Mg(2), MgCa, Ca(2), and Sr(2), are investigated by infrared multiple photon dissociation (IRMPD) spectroscopy to obtain vibrational spectra in the mid-IR region. The IR spectra are dominated by the symmetric and the antisymmetric nitrate stretches, with the latter split into high and low-frequency components due to the distortion of nitrate anion symmetry by interactions with the cation. Density functional theory (DFT) is used to predict geometries and vibrational spectra for comparison to the experimental spectra. Calculations yield two stable isomers: the first one contains two terminal nitrate anions on each cation and a single bridging nitrate ("mono-bridging"), while the second structure features a single terminal nitrate on each cation with three bridging nitrate ligands ("tri-bridging"). The tri-bridging isomer is calculated to be lower in energy than the mono-bridging one for all species. Theoretical spectra of the tri-bridging structure provide a better qualitative match to the experimental infrared spectra of [Mg(2)(NO(3))(5)](-) and [MgCa(NO(3))(5)](-). However, the profile of the low-frequency nu(3) band for the Mg(2) complex suggests a third possible isomer not predicted by theory. The IRMPD spectra of the Ca(2) and Sr(2) complexes are better reconciled by a weighted summation of the spectra of both isomers suggesting that a mixture of structures is present.

Dielectric spectra in molecular single crystals

Abstract A theoretical description of the frequency-dependent, complex dielectric permittivity of molecular single crystals is given within the framework of the relaxation polarization mechanism. The extended angular jump model, accounting for the local symmetry distortion, simulates the hindered molecular motion. Both the real and imaginary parts of dielectric permittivity are anisotropic in all cases of point symmetry groups of molecule motion, except for those of the cubic crystallographic system. The three-dimensional graphs of dielectric spectra, which can be helpful in practical application, illustrate the predicted spectra.

ENDOFULLERENES M@C60 WITH DIFFERENT MONOVALENT METALS

Endofullerenes M@ C 60 with M = Li , Na , Ag are considered theoretically at the ab initio level. The electronic structure is calculated to reveal effects of the nature of metal atoms inside C60 upon features of the minimum energy endostructures. The Hartree–Fock and DFT methods with all-electronic ( Li and Na ) and effective core potential ( Ag ) basis sets were used admitting an arbitrary symmetry distortion (down to the C 1 point group). All structures display the off-center position of the endoatoms. The charge transfer between metal atoms and the carbon cage is also strongly dependent on the nature of the metals. Ionization potentials and atomic radii are proposed as the basic factors providing properties of the endofullerenes.

Composition and structure refinement of superconducting crystals Y1−x Tb x Ba2Cu3O6+δ (x = 0.10; δ = 0.75)

Crystals of Y0.90Tb0.10Ba2Cu3O6.75 have been prepared by spontaneous crystallization from slowly cooled non-stoichiometric melt of the system Y-Tb-Ba-Cu-O. Average size of platelet crystals having mirror surface is 2×2, the largest — 8×9 mm with thickness 0.1–0.2 mm. The crystals have been characterized by powder X-ray diffraction and electron microprobe analysis. Tetragonal symmetry of the crystals has been determined by X-ray diffraction. Magnetic susceptibility measurements have revealed that the crystals manifest transition to superconducting state without additional annealing (Tc = 60 K). Structures and compositions — Y/Tb ratio (σ = 0.01) and oxygen content (σ = 0.04) — have been refined for two single crystals. Possibility of rhombic distortion of the tetragonal symmetry is discussed.

Role of oxygen atoms in CaF2 crystals doped with Eu atom

The optical transparency in function of wavelength of CaF2 crystals doped with Eu (denoted by CaF2:Eu) has been measured. The optical absorption occurs below λ=400 nm depending on the amount of Eu atoms doped. The short wavelength below 400 nm has been shifted to around λ=425 nm. As a result, the 425 nm wavelength increased and the light below 400 nm was cut through CaF2:Eu crystals. This scintillating effect has been found much increased by adding oxygen atoms in crystals. This is due to the distortion of electrical and structural symmetry in crystals by adding oxygen atoms in the form of oxide materials.

Enhanced upconverted photoluminescence in Er3+ and Yb3+ codoped ZnO nanocrystals with and without Li+ ions

The upconversion luminescence spectral intensity of Er3+ in Er3+ and Yb3+ codoped ZnO nanocrystals with and without Li+ are investigated. Yb3+ ions as a tradition sensibilizer have efficient energy transfer processes from Yb3+ (2F5/2) to Er3+ (4I13/2, 4I11/2, 4F9/2), which lead to the increment of upconversion luminescence intensity. Following by adding Li+ to the Er3+ and Yb3+ codoped ZnO nanocrystals, the upconversion intensity emitted by Er3+ ions is found greatly enhanced. The enhancement is attributed to the distortion of the local field symmetry of Er3+ ions, so increases various intra-4f transitions of Er3+ ions. Both Yb3+ and Li+ can disperse Er3+ ions in specimen, so reduced the interaction between neighboring Er3+ ions.

Electron delocalization patterns in models of distorted (D2d) mixed-valence cubanes

Low-symmetry distortions are present in cubanes such as Fe(4)S(4), but their effects on electron delocalization properties are not well-understood. Mixed-valence cubanes often exhibit experimentally measurable "pair delocalization" of a delocalizable electron. An important question is, what is the role of physical interactions (vibronic, electronic, exchange) and symmetry distortions in determining the electron delocalization pattern? Semiclassical models are used to explore the electron delocalization patterns of S=1/2 tetragonally (D(2d)) distorted mixed-valence cubanes comprising four metal centers with bridging ligands, a single delocalizable "excess" electron, and either closed-shell or open-shell ion cores. Phase diagrams show that distorted S=1/2 ground state cubanes with antiferromagnetic exchange (as found in nature) have delocalization patterns qualitatively similar to those of an S=1/2 model with no Heisenberg exchange, suggesting that exchange is not necessarily a dominant factor in determining electron delocalization properties. The open-shell model reveals two types of pair delocalization for the S=1/2 ground state, with differing dimer subunit spins for compressed and elongated geometries. Previous studies emphasize the importance of exchange interactions for pair delocalization. Here, it is shown that electron exchange is not always necessary for pair delocalization and that it can be achieved with relatively small tetragonal distortions from tetrahedral (T(d)) symmetry. The results contradict those of an earlier theoretical study of distorted Fe(4)S(4) clusters, which concluded that distortions of lower symmetry than D(2d) are necessary to induce a transition to pair delocalization.

Synthesis under pressure and characterizations through optical spectroscopy of jahn-teller cations (LS Ni3+, is Co3+) as probes diluted in a perovskite matrix

The objective is to explore through optical spectroscopy and magnetic measurements the coordination and electronic structures of transition-metal ions introduced as impurities with unusual valence states in the oxide perovskite LaAlO3. The selected transition-metal ions Ni3+(3d7) and Co3+(3d6) are characterized by an electronic configuration likely leading to an orbital degenerate E state in Oh symmetry, and thus electron-lattice coupling due to the Jahn-Teller effect may induce low symmetry distortion around the impurity oxygen octahedron. We show that a sol-gel process followed by high oxygen pressure treatments yields stabilization of trivalent state in oxide perovskite. Information about the coordination, electronic structure and aggregation around the magnetic impurity was obtained from X-ray diffraction, FTIR and optical spectroscopy. Finally, evidence on the possible existence of intermediate spin state in Co3+ is under consideration.

Site-selective time-resolved laser fluorescence spectroscopy of Eu 3+ in calcite

Three samples of calcite homogeneously doped with Eu 3+ were synthesized in a mixed-flow reactor. By means of selective excitation of the D 0 5 → F 0 7 transition at low temperatures ( T < 20 K ), three different Eu 3+ species (species A, B, and C, respectively) could be discriminated. For each one, the emission spectrum and lifetime were obtained after selective excitation of the single species. On the basis of these data, species C could be identified as Eu 3+ incorporated into the calcite lattice on the (nearly) octahedral Ca 2+ site. Species B was also identified as Eu 3+ incorporated into the calcite lattice, but the ligand field shows a much weaker symmetry. Species A, however, is not incorporated into the crystal's bulk, having 1–2 H 2O ligands left in its first coordination sphere and showing very little symmetry, and is considered as Eu 3+ adsorbed onto the calcite surface. The emission spectra of species C for Eu:calcite grown in the presence of Na + were found to differ from those of Eu:calcite synthesized in the presence of K +. The latter revealed a strong distortion in site symmetry, which was not observed in the samples grown in Na + solutions. This finding provides spectroscopic evidence in favor of an incorporation mechanism based on the charge–balanced coupled substitution of Na + + Eu 3 + ↔ 2 Ca 2 + .

External Electric Field Effects on Absorption, Fluorescence, and Phosphorescence Spectra of Diphenylpolyynes in a Polymer Film

External electric field effects on absorption, fluorescence, and phosphorescence spectra of a series of unsubstituted diphenylpolyynes have been examined in a PMMA film. The analysis of the electroabsorption spectra indicates that the shorter diphenylpolyynes exhibit only the change in molecular polarizability, whereas the longer ones exhibit the change both in dipole moment and in molecular polarizability following absorption. The finding of the change in dipole moment following absorption of centrosymmetric diphenylpolyynes is interpreted in terms of the symmetry distortion upon doping a polymer film. When the external electric field is applied, the fluorescence yield is reduced and enhanced, respectively, in diphenylacetylene and diphenyloctatetrayne, indicating that the rate of the nonradiative process from the fluorescence state is accelerated in diphenylacetylene and decelerated in diphenyloctatetrayne by an external electric field. All of the diphenylpolyynes used in the present study exhibit the change in molecular polarizability following the phosphorescence process.

Micromagnetic characterization of a rotation sensor based on the planar Hall effect

Thin films of Ni 80Fe 20 (Permalloy) and structures as Ni 80Fe 20/NM/Ni 80Fe 20 were used to build low cost rotation sensors. The structures were deposited onto oxidized Si wafers. NM denotes Cu or Al 2O 3 layers. Angular dependencies of the planar Hall effect (PHE) are investigated for different values of the magnetic field strength. A distortion of the angular dependence symmetry with respect to the abscissa axis was observed. This is due to contacts misalignment, hysteretic behaviour of the magnetic material and some coupling effects in the multilayered structure. We performed micromagnetic simulations to discuss the effect of the magnetic field strength on the shape of the angular dependence of PHE. The parameters used for micromagnetic simulations are inspired from the film structure.

Influence of magnetic field on the tensoresistive effect in iron-based amorphous alloys

The effect of an applied magnetic field H on the tensoresistive effect in Fe-T-B (T = Fe, Cr, and Co) and Fe-B-Si amorphous metallic alloys has been studied. The applied magnetic field H is shown to substantially affect the coefficient of tensoresistance (CTR) π determined upon a longitudinal deformation of samples. The principal cause of such an effect is a change in Young’s modulus under the effect of a magnetic field, i.e., the ΔE effect. Based on the performed analysis, an equation for the determination of the ΔE effect from the measurement of the π(H) dependences is suggested. The results obtained indirectly confirm the assumption on the fact that the anomalous value of the coefficient of tensoresistance in amorphous alloys is due to a distortion of the spherical symmetry of the structure factor.

Uniaxial Strain Controlling Magnetic Anisotropy in (Ga,Mn)As

A theoretical study of magnetocrystalline anisotropy controlled by in-plane lattice symmetry distortions is presented. The uniaxial strain, only presumed by earlier studies in order to model the observed uniaxial magnetic anisotropy, has recently become an experimentally accessible parameter. We show that the lithographically induced strain can compete with intrinsic symmetry breaking, easy axes can take general in-plane directions, and anisotropy flelds scale linearly with strain for typical experimental strain magnitudes. Our results are in qualitative agreement with experimental results.

Dipolar structures in magnetite ferrofluids studied with small-angle neutron scattering with and without applied magnetic field

Field-induced structure formation in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment was studied with small-angle neutron scattering (SANS) as a function of the magnetic interactions. The interactions were tuned by adjusting the size of the well-defined, single-magnetic-domain magnetite (Fe3O4) particles and by applying an external magnetic field. For decreasing particle dipole moments, the data show a progressive distortion of the hexagonal symmetry, resulting from the formation of magnetic sheets. The SANS data show qualitative agreement with recent cryogenic transmission electron microscopy results obtained in 2D [Klokkenburg, Phys. Rev. Lett. 97, 185702 (2006)] on the same ferrofluids.

Static Normal Coordinate Deformations of the Heme Group in Mutants of Ferrocytochrome c from Saccharomyces cerevisiae Probed by Resonance Raman Spectroscopy

The function of heme proteins is, to a significant extent, influenced by the ligand field probed by the heme iron, which itself can be affected by deformations of the heme macrocycle. The exploration of this field is difficult because the heme structure obtained from X-ray crystallography is not resolved enough to unambiguously identify structural changes on the scale of 10(-2) A. However, asymmetric deformations in this order of magnitude affect the depolarization ratio of the resonance Raman lines assignable to normal vibrations of the heme group. We have measured the dispersion of the depolarization ratios of four structure sensitive Raman bands (i.e., nu4, nu11, nu21, and nu28) in yeast iso-1-ferrocytochrome c and its mutants N52V, Y67F, and N52VY67F with B- and Q-band excitation. The DPR dispersion of all bands indicates the presence of asymmetric in-plane and out-of-plane deformations. The replacement of the polar tyrosine residue at position 67 by phenylalanine significantly increases the triclinic B2g deformation, which involves a distortion of the pyrrole symmetry. We relate this deformation to changes of the electronic structure of pyrrole A, which modulates the interaction between its propionate substituents and the protein environment. This specific heme deformation is eliminated in the double mutant N52VY67F. The additional substitution of N52 by valine induces a tetragonal B1g deformation which involves asymmetric changes of the Fe-N distances and increases the rhombicity of the ligand field probed by the heme iron. This heme deformation might be caused by the elimination of the water-protein hydrogen-bonding network in the heme cavity. The single mutation N52V does not significantly perturb the heme symmetry, but a small B1g deformation is consistent with our data and the heme structure obtained from a 1 ns molecular dynamics simulation of the protein.

Direct Contact versus Solvent-Shared Ion Pairs in NiCl2 Electrolytes Monitored by Multiplet Effects at Ni(II) L Edge X-ray Absorption

We investigate the local electronic structure in aqueous NiCl2 electrolytes by Ni L edge X-ray absorption spectroscopy. The experimental findings are interpreted in conjunction with multiplet calculations of the electronic structure and the resulting spectral shape. In contrast to the situation in the solid, the electronic structure in the electrolyte reflects the absence of direct contact Ni-Cl ion pairs. We observe a systematic change of the intensity ratio of singlet- and triplet-related spectral features as a function of electrolyte concentration. These changes can be described theoretically by a change in the weight of transition matrix contributions with different symmetries. We interpret these findings as being due to progressive distortions of the local symmetry induced by solvent-shared ion pairs.