Determination of the c(2 × 2) structure of sulfur on Fe{001} by low-energy electron diffraction

In the early stages of reaction with sulfur, a clean Fe{001} surface develops a c(2 × 2) superstructure. A low-energy electron diffraction analysis of this structure leads to a model in which the S atoms lie in the four-fold symmetrical sites on the Fe{001} surface, the S-Fe interplanar spacing being $\text{1.09 ± 0.05}\text{A}\text{?}$ and corresponding to an effective radius of 1.06 Å for the chemisorbed S atoms. In contrast to Fe{001} 1 × 1-O, the first interlayer spacing of the substrate here is not significantly expanded.     

Nucleic acid conformation: Crystal and molecular structure of deoxyguanosine (5′) phosphate (disodium salt)—Agauche-trans (gt) conformation about the C(4′)-C(5′) bond and O(1′) puckering of the furanose ring

The crystal structure of deoxyguanosine (5′) phosphate, disodium salt, (5′-dGMP Na₂ 4H₂O) has been determined from three dimensional single crystal x-ray data collected by multiple film, equi-inclination, Weissenberg method using CuK_a radiation. The crystal belongs to the monoclinic space groupP2₁ witha=16.002±0.003 Å,b=10.730±0.003 Å,c=5.575±0.005 Å andβ=101.9°. The structure was solved by symbolic addition method using the program Multan, the reliability index being 0.090. The guanine base is in the usualanti conformation about the C (1′)-N (9) bond withx _CN=52.3°. The structure shows two unique conformational features not observed in any nucleotide structure reported so far. The deoxyribose moiety shows O (1′)endo puckering with respect to the best four atom plane defined by C(1′)-C(2′)-C(3′)-C(4′). The conformation about the C(4′)-C(5′) bond isgauche-trans with ø₀₀=62.5° and ø_oc=174.8°. This is the first nucleotide structure where agt conformer similar to that found in the Watson-Crick double helical DNA model has been experimentally observed. These two conformational features have also direct relevance to the concept of ‘a conformationally rigid nucleotide unit’ developed by Sundaralingam. The nine membered guanine ring is essentially planar. Bases of molecules related by a ‘c’ cell translation tend to overlap, the shortest distance being 3.51 Å between the atoms N (3) and C(8). One of the sodium atoms Na(1) has an octahedral coordination with four water oxygens and O(6) and O(3′) atoms occupying the corners at distances ranging from 2.35 Å to 2.55 Å.     

Theoretical studies of the bonding of sulfur to models of the (100) surface of nickel

Electronic wavefunctions have been obtained as a function of geometry for a S atom bonded to Ni clusters consisting of 1 to 4 atoms designed to model bonding to the Ni(100) surface. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. Modeling the (100) surface with four Ni atoms, we find the optimum S position to be 1.33 Å above the surface, in good agreement with the value (1.30 ± 0.10 Å) from dynamic LEED intensity calculations. The bonding is qualitatively like that in H₂S with two covalent bonds to one diagonal pair of Ni atoms. There is a S pπ pair overlapping the other diagonal pair of Ni atoms. [Deleting this pair the S moves in to a position 1.04 Å from the surface.] There are two equivalent such structures, the resonance leading to equivalent S atoms and a c(2 × 2) structure for the S overlayer. The Ni in the layer beneath the surface seems to have little effect (~0.03 Å) on the calculated geometry. Bonding the S directly above a single Ni atom leads to a much weaker bond (D_e = 3.32 eV) than does bonding in a bridge position (D_e = 5.37 eV).     

A Study of Antiferromagnetic Ordering in β-FeF3·3H2O by the Use of the NMR Method

The present work cites the investigation results of local magnetic fields on ¹H and ¹⁹F nuclei and spin ordering in β-FeF₃·3H₂O. In the structure of this compound (space group P4₂/n, a=7.846 Å, c=7.754 Å, z=4^1,2) Fe atoms are bonded via bridged F atoms (1) in infinite chains along the /001/ axis. With such positioning, in which two paramagnetic atoms are separated by a diamagnetic atom (F, O, C1 and others) there is a possibility for indirect (superexchange) interaction. Linear chains … Fe - F - Fe - F - Fe - F … are separated from each other by F(2) atoms and water molecules H₂O (1) and H₂O (2), so that exchange between neighboring Fe atoms from various chains is ostensibly much encumbered.     

Influence of the radiationless transitions on an intensity of the optical radiation emitted by excited atoms ejected from solids by an ion beam

The excited atoms of the target material are ejected during an ion bombardment of solids. These atoms belong to one of two velocity groups — fast or slow. The fast atoms arise in binary collisions of bombarding ions with target atoms and the slow ones are knocked out as a result of a sputtering process. Excited atoms flying off the surface intersect the solid-vacuum boundary and can transfer their excitation energy in the radiationless transitions mainly of the resonance ionization type. The probability of this process depends strongly on the electronic energy level structure of solids and a velocity of ejected atoms. On this statement our method of the electronic energy level structure of solids study is based by means of investigation of excited atoms velocity spectrum. On the results of paper [4] we remark that in their experimental conditions the surface of the lithium target was apparently strongly oxidized. Using our method and results of paper [4] we can estimate the energy width of the conduction band of Li₂O to 0.4 eV. In general the cascade corrections to the mean life times of excited atoms may be important and one can take them into account. The detailed analysis of the influence of the cascade corrections to the mean life times of upper excited states of some TiI lines (λλ 5210 Å, 5064 Å, 4682 Å, 3981 Å, 4533 Å, 4856 Å) was carried out. It was found that in the case when our method was applied to determination of the work function of metallic titanium the cascade corrections either are negligible or not necessary.     

Synthesis,crystal structure and electrical properties of (C5H13NCl)2 SnCl6

In this work, a novel compound Bis(2-chloropropyl-N,N-dimethyl-1-ammonium) hexachloridostannate(IV) was synthesized and characterized by; single X-ray diffraction, Hirshfeld surface analysis, differential scanning calorimetric and dielectric measurement. The crystal structure refinement at room temperature reveled that this later belongs to the monoclinic compound with P2₁/n space group with the following unit cell parameters a = 7.2894(7) Å, b = 12.9351(12) Å, c = 12.2302(13) Å and β = 93.423 (6) °. The structure consists of isolated (SnCl₆)^2? octahedral anions connected together into layers via hydrogen bonds N–H….Cl between the chlorine atoms of the anions and the hydrogen atoms of the NH groups of the [C₅H₁₃NCl]⁺ cations. Hirschfeld surface analysis has been performed to gain insight into the behavior of these interactions. The differential scanning calorimetry spectrum discloses phase transitions at 367 and 376.7 K. The electrical properties of this compound have been measured in the temperature range 300–420 K and the frequency range 209 Hz–5 MHz. The Cole–Cole (Z′ versus Z″) plots are well fitted to an equivalent circuit model. The transition phase observed in the calorimetric study is confirmed by the change as function of temperature of electrical parameter such as the conductivity of grain (σ_g) and the σ_dc.     

Mössbauer spectroscopical investigation of the exchange biased Fe/MnF2 interface

Two different Fe/MnF₂ samples have been prepared by e-beam evaporation on MgO(001) substrates. The Fe layer in the samples includes a 10 Å thick ⁵⁷Fe probe layer either at the Fe/MnF₂ interface (interface sample) or 35 Å away from the interface (center sample). The samples are characterized by X-ray diffraction, conversion electron Mössbauer spectroscopy (CEMS) and SQUID magnetometry. ⁵⁷Fe CEMS has been employed to study the depth dependent hyperfine interactions in Fe/MnF₂ as a function of temperature between 18 K to 300 K. The hyperfine field B _hf has been obtained for the interfacial and off-interfacial ⁵⁷Fe layers. At the interface, besides B _hf of bcc-Fe, the presence of a component with a distribution P(B _hf ) is observed. The latter is assigned to interfacial ⁵⁷Fe atoms, indicating some (～15%, equivalent to ～1 Fe atomic layer) intermixing at the Fe/MnF₂ interface and a decrease of the average hf > by 21%. The influence of the interface disappears as the ⁵⁷Fe probe layer is placed away from the interface. The temperature dependence of the average hf > of the interface has been measured. The Fe spins, at remanence, are found to lie in the film plane.     

Magnetic anisotropy in multilayers

Magnetic anisotropy between in-plane and out of plane magnetic alignments is studied in a variety of multilayer systems using Mössbauer spectrosopy to observe the (Fe) magnetic orientation. The surface anisotropy in Fe/Au (1 1 1) multilayers is measured as K _s = 0.9 × 10^?3 Jm^?2. In Fe/Ni multilayers the dependence of magnetic orientation on external field applied normal to the layers enables volume and interface anisotropies K _v = (?5 ± 1) × 10⁴ Jm^?3 and K _s = (?0.6 ± 0.4)× 10^?3 Jm^?2 to be evaluated. In similar applied field experiments coherent rotation of the magnetic Fe and NiFe layers in Fe/Cu/NiFe/Cu multilayers was observed for intervening Cu layer thickness x = 5 Å but independent rotation for x = 50 Å. Out of plane magnetic components are observed for DyFe₂, YFe₂ thin films and DyFe₂/YFe₂ multilayers. In fields of up to 0.25 T applied inplane only the moments of the YFe₂ film showed significant rotation.     

Magnetic configurations of hexagonal iron rhenium multilayers

We present ab initio calculation of the electronic structure and magnetic properties of Fe/Re multilayers assuming hcp crystallographic phase. The system is modeled according to recent structural analysis which shows that Fe/Re has bct(0 0 1) ordering for 0⩽t_Re⩽8 Å after which a structural phase transition occurs to an hcp phase. Antiferromagnetic and ferromagnetic orders appear more stable than the nonmagnetic (NM) state, only when the Fe atoms begin to adopt an expanded volume of about 27% accompanied by an axial distortion (c/a=1.42). The sizeable magnetic moments calculated on Fe atoms contrast with the paramagnetic bulk hcp. These results are discussed and compared with recent experimental data on hcp Fe/Re superlattices and also with magnetic properties of Fe/Ru multilayers.     

Use of grazing incidence X-ray diffraction for the study of nitrogen implanted stainless steels

The identification of Fe, Ni and Cr nitrides formed by nitrogen-ion implantation (4×10¹⁷ N⁺ cm^-2, 50 keV) in austenitic steels is performed by X-ray diffraction under very low glancing angles (0.00 ⩽ i ⩽ 1.5°). Spectra obtained with increasing angles i permit the investigation of layers with depths varying from 20 to more than 1000 Å. This non-destructive technique allows the surface to be controlled at each step of the treatments. Spectra were recorded on polished steel prior to and after implantation, with or without electrochemical attack. A 100 Å martensitic layer formed during the mechanical polishing is observed on the austenitic substrate. This layer is destroyed by an anodic attack before implantation of the samples. After implantation a CrN or carbonitride overlayer of a few tens of Å in thickness, may be observed. In the subjacent layers several iron and nickel nitrides are present, mainly ϵ-Fe₂N-Fe₃N, ς-Fe₂N and Ni₃N.     

Deposition of iron on MgO(100): Reaction of CO and H2

Ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (EELS) and surface extended energy loss fine structure (SEELFS) were used to study the deposition of Fe on MgO(100) and to identify the surface compounds formed after reaction of CO/H₂ (1:1). The clean MgO(100) surface was characterized using the above techniques and the effect of argon ion bombardment damage to the surface was investigated. With the deposition of iron, metallic characteristics appear in the photoemission spectrum; the electron energy loss peaks of the MgO(100) substrate diminish in intensity with no significant shifts in loss energies. Fine structure analysis of the oxygen K-edge of the MgO(100) surface with less than 2 monolayers (ML) of iron suggests that the iron atoms bond with the oxygen at the surface of the MgO(100) lattice. For less than 4 ML of iron, the EEL spectra show that the deposited iron is oxidized after reaction of CO/H₂. Higher iron coverages result in carburization of the surface. Carbon deposition was observed with CO for all Fe coverages. Measurement of the fine structure above the carbon K-edge suggests that the types of carbide formed depend on the iron coverage; one carbide has a short CFe distance of 1.78 Å and the other a distance of 2.06 Å (high metal coverage).     

Atomic structure of Fe {111}

A clean Fe {111} surface was prepared and studied with LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). A LEED intensity analysis was carried out with a new computational scheme (THIN) specially designed for short interlayer spacings. The results are, for the fust interlayer spacing, d₁₂ = 0.70 ± 0.03 Å and for the inner potential V₀ = 11.1 ± 1.1 eV, the confidence intervals referring to 95% confidence level. Thus, the Fe {111} surface is contracted 15.4% with respect to the bulk (0.827 Å).     

Magnetic structure and preferential occupation of Fe in the composite compound Nd2Co6Fe

The crystal and magnetic structures of the composite compound Nd₂Co₆Fe have been investigated by high-resolution neutron powder diffraction and X-ray powder diffraction. The compound crystallizes in the hexagonal Ce₂Ni₇-type structure consisting of Nd(Co,Fe)₂ and Nd(Co,Fe)₅ structural blocks alternately stacked along the c-axis. Multi-pattern Rietveld refinement of neutron diffraction and X-ray diffraction data at room temperature reveal that substitution of Fe for Co occurs exclusively in the Nd(Co,Fe)₅ structural blocks. The preferential occupation of the Fe atoms in the structure is discussed based on the mixing enthalpy between Nd and Fe atoms and on the lattice distortions. In agreement with the reported magnetic phase diagram of the Nd₂Co_7−xFe_x compounds, magnetic structure models with the moments of all atoms in the ab plane at 300 K and along the c-axis at 450 K provide a satisfactory fitting to the experimental neutron diffraction data. The refinement results show that the atomic moments of (Co,Fe) atoms within the Nd(Co,Fe)₅ blocks decrease slightly with temperature, whereas the atomic moments of Nd in the compound and of (Co,Fe) atoms at the interface between the Nd(Co,Fe)₂ and Nd(Co,Fe)₅ blocks are reduced significantly.     

Polarized neutron diffuse scattering experiments on disordered Cr-rich Cr-Fe alloys

The results of polarized neutron elastic diffuse scattering measurements on disordered Cr-Fe alloys with 1.5 2.4, and 12 and 14.2 at % of Fe are reported and discussed. The measurements have been done using neutron wavelength of 4.25 A in the scattering vector range 0.2Å^?1 < K < 2Å^?1 and in function of temperature. The data show clustering effects in the alloys with iron concentration starting from 2.4 at%. At the iron concentration of 14.2 at% the clusters contain about 110 atoms and extend up to about 6.3 Å from a central atom. Above a characteristic temperature the iron-rich clusters behave as superparamagnetic entities and below this temperature they are magnetically frozen.At 4.6K the average iron moment within these clusters is found to be 2.0(2)μ_B in the alloys with 12 and 14.2 at % of Fe.     

Structure,electrical transport,and optical properties of a new ordered iron intercalated dichalcogenide,Fe0.34TiSe2

The compound Fe_0.34TiSe₂ crystallizes in space group D³_3d?P3&#x0304;ml of the trigonal system with 8 formula units in a cell of dimensions a = 7.148(4), c = 11.880(7) Å at 117 K. The structure of Fe_0.34TiSe₂ can be derived from that of TiSe₂ by insertion of Fe atoms into some of the octahedral holes between the layers. No two adjacent octahedral holes along c contain Fe atoms, that is no two Fe atoms are related by a translation of $\text{c}\text{2}$. The compound Fe_xTiSe₂, x = 0.34, exhibits physical properties remarkably different from those found in either the host material, TiSe₂, or in low iron concentration (x < 0.2) intercalated materials. The electrical resistivity of this compound exhibits a sharp decrease below 120 K which is coincident with a well defined peak in the Hall coefficient. These data have been interpreted as the onset of antiferromagnetic ordering as suggested by results of magnetic susceptibility measurements. Optical reflectance measurements indicate a simple Drude-like free carrier behavior and are consistent with the interpretation of iron as an electron donor in this material.     

Microwave substitution structure of the amine group in meta-chloroaniline C6H4CINH2

Microwave spectra of amine-deuterated species of 3-³⁵Cl-aniline, C₆H₄CINH₂, have been observed. The two amino hydrogen atoms are found to be out of the plane of the aromatic ring, and appear to be farther out of the plane when the amine group is deuterated. It has also been shown that the H_trans, c coordinate (0.300 Å) is larger than the cis one (0.281 Å).     

Structural transformation in Mg2NiH4

X-Ray diffraction measurements show that on heating Mg₂NiH₄ in a 1 atm pressure H₂ atmosphere, above ～250°C it transforms into a cubic structure, metal atoms in CaF₂ arrangement, a = 6.525 Å. It is concluded that the H atoms are in tetrahedral clusters, and that the structure is only weakly ionic. This conclusion is also supported by NMR measurements. The 20°C structure of Mg₂NiH₄ is shown to be describable primarily as a slight monoclinic distortion of the cubic unit cell; a = 6.594 Å, b = 6.412 Å, c = 6.490 Å and β = 93.1°. However, weak small angle lines show that a longer range order exists and that the true unit cell, which we have not determined, must be very large. To what extent the cubic phase should be considered a high temperature and/or low concentration (Mg₂NiH_4??) phase is not resolved.     

Structural and electrical properties of LiCo3/5Cu2/5VO4 ceramics

The LiCo_3/5Cu_2/5VO₄ compound is prepared by a solution-based chemical method and characterized by the techniques of X-ray diffraction, scanning electron microscopy and complex impedance spectroscopy. The X-ray diffraction study shows an orthorhombic unit cell structure of the material with lattice parameters a=13.8263 (30) Å, b=8.7051 (30) Å and c=3.1127 (30) Å. The nature of scanning electron micrographs of a sintered pellet of the material reveals that grains of unequal sizes (～0.2–3 μm) present an average grain size with a polydisperse distribution on the surface of the sample. Complex plane diagrams indicate grain interior and grain boundary contributions to the electrical response in the material. The electrical conductivity study reveals that electrical conduction in the material is a thermally activated process. The frequency dependence of the a.c. conductivity obeys Jonscher’s universal law.     

Surface sensitivity of low energy electron Mössbauer spectroscopy (LEEMS) using57Fe

Very low energy electrons (LEE) (E _e ≤15 eV) are produced with high intensity directly by Mössbauerabsorption and conversion in the case of⁵⁷Fe [1, 4, 5]. These electrons should be very surface sensitive due to their very low attenuation length compared to the 7.3 keV K-Conversion electrons of⁵⁷Fe [5, 11]. We have examined the surface sensitivity of these resonant LEE, using nonresonant⁵⁶Fe metal and⁵⁶Fe stainless steel foils coated with about 20 Å and 50 Å⁵⁷Fe, respectively. They were exposed to air after evaporation: The 20 Å samples are found to be fully oxidized [5]. Depth Selective Conversion Electron Mössbauer Spectroscopy (DCEMS), performed with a high transmission orange type magnetic spectrometer [5, 6, 13] reveals a two layer structure of the 50 Å samples. Low Energy Electron Mössbauer Spectroscopy (LEEMS) [5] is found to be significantly more surface sensitive than conventional DCEMS, but not as surface sensitive as Auger Electron Mössbauer Spectroscopy (AEMS) using LMM-Auger electrons of 500–600 eV, as expected due to the different mean free path. But because of the very low intensity of these Auger electrons this mode appears to be not very useful for practical application.     

Atomic structure of a 1<Emphasis Type="Italic">T</Emphasis>-TiSe<Subscript>2</Subscript> surface layer from photoelectron and Auger electron holography data

A three-dimensional (3D) reconstruction of the atomic structure of the (100) surface of a 1T-TiSe₂ layered dichalcogenide crystal has been performed from X-ray photoelectron and Auger electron diffraction data. The diffraction patterns of the emission of Auger electrons of Se(LMM) selenium and photoelectrons of Ti2p titanium have been considered as holographic diagrams. Being processed with the scattering pattern extraction algorithm using the maximum entropy method (SPEA-MEM), they provide individual 3D images of the nearest environment of selenium and titanium atoms in the TiSe₂ lattice. Using reconstructed 3D images, the positions of 128 atoms in the 2 × 2 × 1.5-nm region of the surface layer of TiSe₂ have been determined. The structure of the surface has a 1T polytype. Interatomic distances in the layer and van der Waals gap are larger than the respective parameters in the bulk of the crystal. It is assumed that titanium layers in two Se-Ti-Se upper surface structural units are displaced along the [001] axis. The structure of the surface layer can be described by a unit cell of the P3 space group with the parameters a = 3.85 Å and c = 14.4 Å.