Nearest-neighbour Shell Research Articles

Structural Studies of Vapour Phase Transport Reactions Using X-ray Absorption Spectroscopy and Matrix Isolation

The vapour above heated ferric chloride (Fe2Cl6) been isolated in nitrogen, argon and neon matrices. The XANES features are indicative of iron in a tetrahedral environment and the interatomic distances from the EXAFS data for the nearest neighbour shells are in good agreement with the vapour phase electron diffraction data. In the case of the nitrogen matrix isolated species the Fe...Fe distance has been determined, indicating the potential usefulness of matrix XAS for the study of vapour phase intermediates. The reasons for the non-observance of this shell in argon and neon matrices are considered.

Mössbauer analysis of Fe−N austenites

Analysis of transmission Mossbauer spectra for high nitrogen Fe−N austenites is done in terms of five iron environments dealing with first and second nearest neighbour shells. The similar analysis for Fe−C austenites which displays only three iron environments led to the conclusion that carbon atoms are ordered whereas nitrogen atoms are more randomly distributed.

A neutron diffraction study on amorphous Fe 0.25Zr 0.75

Neutron diffraction experiments on LAD at ISIS indicate that there is a difference in composition of the nearest and next nearest neighbour shells according to the cooling rate in amorphous Fe 0.25Zr 0.75.

Determinableness of the reduced frequencies from the observable diffusional self-correlation functions

It has been shown that the maximum number of the reduced frequencies determinable from the diffusional self-correlation function of the impurity equals twice the number of different nearest neighbour shells surrounding an impurity. The latter statement is valid in the encounter approximation applied to the unbounded diffusion of the impurity via the monovacancy mechanism in the lattices having all sites equivalent. Therefore, only two frequencies can be measured for cubic lattices and four frequencies for hexagonal lattices at most. The number of indeterminable frequencies increases rapidly with the increasing complexity of the diffusion model.

NMR study of Y 2Fe 17C x compounds

The NMR spin-echo spectra of 89Y in Y 2Fe 17C x (x = 0, 0.4 and 1) are presented. The well resolved satellite patterns are interpreted as being caused by different numbers of C nearest neighbours. The changes of the Y hyperfine field caused by the introduction of a variable number of carbon atoms into the nearest neighbour shell of yttrium are determined. From these data we conclude that the introduction of C atoms has a large influence on the local electronic structure at the rare earth site.

Mössbauer effect study of the hyperfine fields and isomer shift of ordered Fe−Co alloys

The influence of Co concentration on the87Fe hyperfine magnetic field and isomer shift in ordered and disordered FeCo alloys in the composition range (34–70) at .% is studied. It is shown that the model which assumes an additivity of the hyperfine field perturbation from solutes in the first two nearest neighbour shells of Fe nucleus very well describes the experimental data.

Mössbauer isomer shift and hyperfine fields of ordered and disordered Fe3Al alloys

The Mossbauer technique was used to measure the hyperfine magnetic field and isomer shift of57Fe nucleus in the ordered and disordered Fe3Al alloys. The Mossbauer spectra were analyzed to yeld the hyperfine magnetic field distribution curves. A linear correlation has been revealed between the average hyperfine field and the average number of Al atoms in the first two nearest neighbour shells of57Fe nucleus,N Al, for both ordered and disordered alloys. The measured values of the mean isomer shift agree very well with the values expected from the Miedema-Van der Woude model.

Derivation of many-body potentials for b.c.c. metals

Abstract A method of deriving the shape of pair potentials describing both the long- and short-wavelength behaviour of b.c.c. metals within the Finnis-Sinclair many-body-potential model is described. The method guarantees a fit to both the elastic constants and zone-boundary vibrational frequencies of b.c.c. metals using the five nearest-neighbour shells in the b.c.c. structure. The application of the method to niobium is briefly discussed and compared with previous work.

Surface structure determination by SEXAFS: The reliability of bond lengths and coordination numbers from multi-shell analyses

Any structural determination from measurements of surface extended X-ray absorption fine structure (SEXAFS) is dependent on deriving sufficient, reliable structural parameters from experimental spectra. The reliability with which a multi-shell analysis (based on computation of the EXAFS spectrum and comparison with experimental data) can produce bond lengths and coordination numbers has been tested. Furthermore, the limitations placed on such an analysis by superimposing typical experimental levels of random noise onto the input data set has been investigated, both in terms of the number of parameters which can be reproduced and their associated errors. Significance tests have also been applied to a multi-shell analysis of experimental SEXAFS measurements of chlorine adsorbed on silver {110}. These tests on both real and simulated data show that a number of (reproducible) bond lengths may be expected from a multi-shell analysis and with a sufficient accuracy to distinguish between shells separated by as little as ≈ 0.3 A ̊ , for a maximum k of 8.7 Å −1. The effective coordination number of the nearest-neighbour shell can be reproduced within an error limit of ±0.2. Large correlations between Debye-Waller factors, coordination numbers and inelastic terms in the EXAFS amplitude place a high degree of inaccuracy, in some cases as high as ±3, on the values obtained for the effective coordination number of higher shells, which cannot by themselves be used to infer the adsorption site geometry of an adatom.

Thermodynamic properties of molten LiMg alloy

Abstract The activity, the heat of mixing, the concentration fluctuation in the long wavelength limit (Scc(0)) and the chemical short range order parameter (CSRO) have been studied as a function of composition in LiMg molten alloys by using first order quasi-chemical (QC) theory. The interchange energy has been considered a function of temperature and thus various thermodynamic quantities are calculated at elevated temperatures. The study reveals that though LiMg is considered a simple regular alloy, its various thermodynamic properties deviate from the ideal solution behaviour. Scc(0) computed at different temperatures from QC-theory are found to be smaller than the ideal values and heterocoordination in the nearest neighbour shell is found to exist through the entire concentration range. On the other hand, Scc(0) computed directly from the measured activity data indicates that the LiMg liquid alloy behaves as a segregating system for smaller content of Li (CLi⩽0.3). The building of heterocoordination starts around CLi⩾0.3 and remains for the rest of the compositions.

INVESTIGATION OF THE DOUBLE-PHASE TRANSITION OF Tm3Fe5O12, BY EXAFS SPECTROSCOPY

Coprecipitated Thulium-iron hydroxides with garnet stoichiometry (TmIG) have been investigated at several annealing temperatures (from 333 K to 1453 K) using differential thermal analysis (DTA), x-ray diffraction (XRD) and EXAFS at the LIII edge of Tm. Two phase transitions at 1043 K and 1091 K are observed by DTA. XRD proves that the crystallization of TmIG is completed at 1091 K. The analysis of the nearest neighbour shell of the EXAFS data shows the presence of three temperature regions separated by the two phase transitions. The different oxide geometries in these regions are elucidated by EXAFS fits.

Hydrogen distribution around Eu 2+ and Gd 3+ probes in samarium hydrides

Dilute 153Eu and 155Gd probe atoms formed by radioactive decay of 153Sm and 155Eu, respectively, in SmH 2+ x (0 ⩽ x ⩽ 1) between 4.1 and 20 K have been studied by Mössbauer spectroscopy. The results demonstrate that the local hydrogen environment in SmH 2+ x determines the europium valency. Eu 2+ formed in stoichiometric SmH 2 at low temperatures experiences local cubic symmetry. In the SmH 2+ x series, more than three octahedral hydrogen neighbours force europium into the trivalent state. At elevated temperatures, however, Eu 2+ repels a hydrogen from its nearestneighbour shell. This is evidenced by the observation of a well-defined quadrupole interaction at Gd 3+ formed from Eu 2+ in SmH 2 at low temperatures. SmH 2 is found to order magnetically at 13 ± 2 K.

Magnetic anisotropy, magnetostriction and intermediate range order in Co-P alloys

Coercivity, magnetostriction and saturation field as a measure of magnetic ansitropy energy have been investigated in Co 100- x P x with 5 ⩽ x ⩽ 26. According to their magnetization curves and coercivities crystalline alloys (5 ⩽ x ⩽ 11) contain hcp-inclusions whereas amorphous material (12 ⩽ x ⩽ 26) is magnetically soft. The saturation magnetization is mainly determined by the nearest neighbour shell but magnetostriction is also correlated with the further environment of the cobalt atoms. The abrupt breakdown of the value |λ s| of the saturation magnetostriction with increasing phosphorus content of the crystalline alloys points to a growing structural disorder. λ s of the amorphous material depends on the saturation magnetization alone (λ s ≈ σ 0 2) which decreases with increasing phosphorus content. The small value of λ s suggests that angular correlations do not exceed the second coordination shell.

Photocapacitance and luminescence studies of charge-transfer and internal transitions of Cu in CdSxSe1-x

The dominant deep acceptor in as-grown CdSxSe1-x, attributed to copper, has been studied using both photocapacitance and photoluminescence techniques for x=0.5-1. Photocapacitance results on internal transitions and transitions to the bands together with luminescence data on donor-acceptor pairs and intra-centre transitions give a consistent picture of the system. It is found that the reduction of the site symmetry due to the mixed S and Se nearest-neighbour shell results in a further splitting of the energy levels of CdSxSe1-x:Cu, and that the energy distance from the acceptor to the valence band decreases with increasing Se content while the distance to the conduction band is comparatively little affected.

Influence of structural anisotropy on the extended X-ray absorption fine structure at the K edge of polycrystalline cadmium

The extended X-ray absorption fine structure at the K edge of polycrystalline cadmium at liquid nitrogen temperature has been measured with X-radiation of a conventional X-ray source. The magnitude of the Fourier transform of the fine structure yields a single maximum which is attributed to the first of two close-lying nearest-neighbour shells alone. The absence of a second-shell contribution due to thermal damping is in good agreement with data obtained from measurements on a cadmium single crystal.

Magnetic properties of solute 57Fe in β phase palladium hydride

The Mössbauer emission spectra of sources of 57Co in β-PdH x were studied at hydrogen concentrations x between 0.62 and 0.96. The spins of 57Fe formed in the decay of 57Co were polarized at temperatures between 1.3 and 9 K in external magnetic fields up to 6 T. From the resulting hyperfine patterns the saturation magnetic moments and hyperfine fields were obtained. At x = 0.62 μ s = 3.1(1) μ B and B s = −28.9(5) T are found for substitutional iron solutes without nearest hydrogen neighbours. At high hydrogen concentrations ( x ⩾ 0.9) the magnetic parameters of iron species with up to six nearest hydrogen neighbours could be obtained separately. The saturation hyperfine fields and magnetic moments were found to decrease with the number of nearest neighbours to about 18 T and 2.2 μ B for iron impurities with six nearest hydrogen neighbours. The electron density at the iron nuclei also decreases with the number of neighbouring hydrogen atoms. From the probabilities of finding specific solute-hydrogen configurations, the excess energies needed to bring a specific number of hydrogen atoms into the nearest-neighbour shell around an iron solute can be derived.

Vacancy trapping at111In in tungsten

TDPAC measurements were carried out in tungsten foils and single crystals using implanted111In as a probe nucleus. After damaging the bcc tungsten lattice by heavy ion implantation, electron- and proton-irradiation the trapping of three different defects in the temperature range 293–1,000 K was observed. They were characterized by quadrupole interaction frequencies of vQ1 = 142(2) MHz, vQ2 = 181(5) MHz and vQ3 = 263(5) MHz. The largest fraction of In nuclei experiencing electric field gradients corresponding to vQ1 and vQ2 was reached at app. 630 K whereas for vQ3 this maximum appeared at app. 850 K. The defect configurations producing the different electric field gradients could be identified as a monovacancy trapped in a nearest neighbour 〈111〉 position (vQ1), a double vacancy consisting of two adjacent vacancies in the nearest neighbour shell of the probe atom (vQ2) and possibly a larger cluster (vQ3).

Short Range Order and Concentration Fluctuations in Regular and Compound Forming Molten Alloys

Abstract First, a statistical mechanical model is developed for compound forming molten alloys with the aim that it explains both the concentration dependence of the thermodynamic quantities-free energy of mixing GM, concentration fluctuations Scc (0), etc.-and the short range order (SRO) parameter x 1 for the nearest neighbour shell obtained from neutron and X-ray scattering. The model assumes the existence of appropriate privileged groups or chemical complexes A μ B v(μ, v small integers) and that the energy of an AB, AA or BB bound depend on whether that bond is part of the complex or not. The expression for G M reduces to the well-known quasi-chemical expression for regular alloys when no complexes are formed. The theory is next applied to discuss G M, S cc(0), etc., for three compound forming alloys, MgBi, LiPb and AgAl. The bond energies once chosen to explain thermodynamic data enable one to evaluate SRO α1. The calculated α1 are, in general, good agreement with the available data in both the compound forming and regular alloys. The theory also provides a natural explanation of change of α1 on melting in such alloys as Cd-Mg.

Quadrupole effects and chemical bonding in vanadium oxycarbides

The 51V NMR spectra for polycrystalline refractory vanadium carbides and oxycarbides have been recorded in the frequency range from 4 to 15 MHz. The resonance line shape analysis carried outallows the separate 51V spectral features to be related to different types of crystal surrounding for the vanadium atoms and the determination of quadrupole coupling constants. Using the results of electronic structure calculcations for the clusters in oxycarbide crystal lattice (carried out by the MO LCAO method) we have studied the relative contributions to electric field gradient at 51V in the compounds under consideration. The conclusion is that the appearance of different types of defects in the first nearest neighbour shell (vacancies or impurity atoms) results in the pertrubations of electron density at the Fermi level which tends to compensate lacking or excessive defect charge.

Elastic core effect and clustering tendency by EXAFS in as-quenched Al-Zn alloys

Random distributions of solute atoms are observed for as-quenched solid solutions for concentrations of zinc up to 2.6 at.%. For Al-4.4 at.% Zn or Al-6.8 at.% Zn, evidence of clustering effects is observed: the first peak of the Fourier transform of the spectrum is reduced to 73% and 47% respectively by the mixing of Al and Zn backscatterers in the nearest-neighbour shell. For the as-quenched Al-4.4 at.% Zn, 1.5 (+or-0.25) zinc atoms are present among the twelve atoms of the first shell and 3.25 (+or-0.25) zinc atoms for Al-6.8 at.% Zn. As for Al-Cu, it has been possible to determine the magnitude of the 'elastic core effect' (strain on the first shell) in dilute solutions. As expected, this effect is weak, db approximately=-0.02+or-0.01 AA, where 'b' is the radius of the first shell in aluminium. Nevertheless, it is about three times higher than the calculated value derived from the slope of Vegard's law (da/ac=-0.020) throughout an elastic theory of a continuous medium.