Local Atomic Coordination Research Articles

Thermally induced fragmentation in an ice lattice

Molecular dynamics simulation of isobaric heating of an H2O lattice with diamond cubic structure reveals a distinct two-step sequence of hydrogen-bond breaking in a temperature range clearly below the melting point. Partial breakage, signaled by an abrupt increase in local atomic coordination and the coupling of intermolecular torsions with intramolecular angle bending, first occurs and leaves the lattice in a fragmented but still ordered state. This is then followed by full breakage at a temperature about 20–30 K higher, bringing the lattice to an orientationally disordered state characterized by a vibrational spectrum similar to that of water.

Sulfur K- and L-edge XANES and electronic structure of zinc, cadmium and mercury monosulfides: a comparative study

S K- and L-edge X-ray absorption near-edge structure (XANES) spectra of sphalerite (cubic ZnS), wurtzite (hexagonal ZnS), greenockite (hexagonal CdS), metacinnabar (cubic HgS) and cinnabar (trigonal HgS) have been obtained using synchrotron radiation. The near-edge features of the S K- and L-edge XANES are qualitatively interpreted based on the MO/energy band structures of these Zn, Cd and Hg monosulfides. The near-edge features reflect the density of the states (DOS) of unoccupied S 3 s, 3 p and 3 d states in the conduction band (CB). From ZnS to CdS and HgS, the S K- and L-edges shift toward lower energy by about 2.3 and 1.8 eV, respectively, and the energy gap decreases by about 1.9 eV, indicating that the unoccupied S 3 s and 3 p states, and the CB minimum move down to lower energy. In transition metal sulfides, the bonding behaviour of Zn 3 d 10, Cd 4 d 10 and Hg 5 d 10 electrons is distinct, and also differs significantly from that of Fe 3+ 3 d and Cu + 3 d electrons, which have a stronger bonding interaction with sulfur. The Fe 2+ 3 d crystal field band in the fundamental gap of (Zn, Fe)S has been confirmed to have little DOS involving S 3 s- and 3 P-like states. For tetrahedrally coordinated sphalerite, wurtzite, greenockite and metacinnabar, the post-edge features of the S K- and L-edge XANES are linearly correlated with the reciprocal interatomic distances and lattice plane distances. Hence, the S K- and L-edge XANES of Zn, Cd and Hg monsulfides provide information on the local structure and coordination of sulfur atoms. The K-edge XANES of different anions in ZnS-structure compounds are qualitatively similar, and may be used as structural fingerprints for studying local structures of unknown materials, such as chalcogenides glasses and metalloproteins.

Oxidation state of a buried interface: Near-edge x-ray fine structure of a crystal truncation rod.

We observe that the crystal-truncation-rod (CTR) diffraction from the ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ substrate of an ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$(001) interface has fine structure when the x-ray energy is tuned through the Cr K absorption edge. This fine structure is attributed to interference between scattering from the substrate and Cr atoms at the interface, which are coherent with the substrate. Comparisons with x-ray-absorption spectra of standard samples demonstrate that the Cr at the interface is predominantly in the ${\mathrm{Cr}}^{3+}$ oxidation state. We demonstrate how the near-edge energy dependence of the CTR diffracted intensity may be used to deduce the amplitude and phase of the interface scattering. This phase information is used in conjunction with reciprocal-lattice-vector-dependent measurements of diffracted power due to CTR's to deduce the structure of the ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ interface. While the local atomic coordination at the interface is the same as that of ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ and ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$, there is a stacking fault at the interface covering half of the interface area.

Calculated stacking-fault energies of elemental metals.

We have performed ab initio calculations of twin, intrinsic, and extrinsic face-centered-cubic stacking faults for all the 3d, 4d, and 5d transition metals by means of a Green's-function technique, based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations. The results are in excellent agreement with recent layer Korringa-Kohn-Rostoker Green's-function calculations where stacking-fault energies for Ni, Cu, Rh, Pd, Ag, Ir, and Au were found by means of the so-called force theorem. We find that the self-consistent fault energies for all the metals in the three transition series vary with atomic number essentially as the calculated structural energy differences between the face-centered-cubic and the hexagonal-close-packed phases. In addition we find that the simple relations between the different types of fault energies predicted by models based on the local atomic coordination are obeyed to a high degree of accuracy.

Real-Time Probe of Reaction Centers in Solid Combustions by QEXAFS on the Sub-Second Time Scale

A QEXAFS (quick-scanning EXAFS) technique has been devised to follow the local atomic coordination changes about selected reactants in a class of highly exothermic solid combustion reactions. Real-time EXAFS measurements during the combustion were made in the time frame of a few seconds. By tuning the monochromator to a specific energy, at which maximum changes occur in an EXAFS feature of an element transforming from the reactant to the product phase, a time resolution of 20 ms was achieved. The Ni + Al → NiAl reaction has been investigated in some detail in light of a possible intermediate phase in the so-called after-burn region. The present QEXAFS findings together with the recent time-resolved diffraction data on the same system lends, for the first time, experimental insights into the structural macrokinetics of this class of combustion systems.

A critical discussion of some models for the concentration dependence of magnetic moments in amorphous and microcrystalline alloys of Fe, Co or Ni with P

The concept of localized moments as well as the opposite picture of itinerant band-magnetism have been applied up to the present day to explain the magnetism of alloys, leading to two different concepts for the concentration dependence of magnetic moments in 3d-TM-M alloys: The coordination bond model of Corb et al. considering the role of the local atomic coordination around the M-atoms and the band-gap theory of Malozemoff et al. which relates the magnetization of alloys with an averaged magnetic valence. A comparison of the magnetic moments of amorphous and microcrystalline Fe 100− x -, Co 100− x - and Ni 100− x P x (0 ≤ x ≤ 28) shows that the Fe-P system is excellent qualified for a critical discussion of the models. In addition to strong differences in the short range order between the microcrystalline and the amorphous phase the magnetic behaviour continuously changes from the weak ferromagnetism of pure bcc-Fe to the strong ferromagnetism of amorphous Fe-P. This offers the special opportunity to discuss the important influence of changes in the short range order on magnetic moments.

Real-time probe of reaction centers in solid combustions on the subsecond time scale.

A quick-scanning extended x-ray-absorption fine-structure (QEXAFS) technique has been devised to follow the local atomic coordination changes about selected reactants in a class of highly exothermic solid-combustion reactions. Real-time EXAFS measurements during the combustion were made in the time frame of a few seconds. By tuning the monochromator to a specific energy, at which maximum changes occur in an EXAFS feature of an element transforming from the reactant to the product phase, a time resolution of 20 ms was achieved. The Ni+Al\ensuremath{\rightarrow}NiAl reaction has been investigated in some detail in light of a possible intermediate phase in the so-called ``after-burn'' region. The present QEXAFS findings together with the recent time-resolved diffraction data on the same system lends experimental insights into the structural macrokinetics of this class of combustion systems.

Quantification of atomic-scale grain boundary parameters by high-resolution electron microscopy

Recent grain boundary modeling studies in fcc metals have revealed global correlations between GB energy and (i) the rigid-body translation normal to the grain boundary (the so-called volume expansion) and (ii) the degree of deviation of the local atomic coordination across the interface relative to the ideal lattice. This communication describes an approach towards evaluation of such parameters by analyzing images obtained by axial high-resolution electron microscopy, utilizing digital image-processing techniques. Observations on several 〈1 1 0〉 tilt GB's in Au are presented.

Local coordination of carbon atoms in amorphous carbon.

Amorphous carbon has been studied by $^{13}\mathrm{C}$ and $^{1}\mathrm{H}$ NMR with the techniques of static single-pulse excitation, magic-angle spinning, and cross polarization with magic-angle spinning. A spin-lattice relaxation time of 0.7 s was obtained for $^{13}\mathrm{C}$ by progressive saturation. Two different components are seen in the NMR spectrum of $^{13}\mathrm{C}$ in the static sample. These were resolved using a tensor fitting routine with Gaussian broadening functions. The downfield component corresponds to ``${\mathit{sp}}^{2}$-like'' carbon atoms, which comprise 93.6% of the total signal. This component was fit to an axially symmetric shift tensor with ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{?}}$=-28 ppm, ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{\ensuremath{\perp}}}$=209 ppm, and \ensuremath{\sigma}\ifmmode\bar\else\textasciimacron\fi{}=130 ppm. The remaining upfield component is assigned to ``${\mathit{sp}}^{3}$-like'' carbon atoms. These were fit to a symmetric shift tensor with \ensuremath{\sigma}\ifmmode\bar\else\textasciimacron\fi{}=62 ppm. The concentration of the dangling bonds, as inferred from spin counting by ESR, is about 2\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. About two thirds of the carbon atoms are not seen in the static NMR measurement because of the high concentration of the unpaired electrons, which leads to severe inhomogeneous line broadening. These carbon atoms are detected under magic-angle spinning by the sidebands, which spread over a range of 2000 ppm. The first moment is located at 130 (\ifmmode\pm\else\textpm\fi{}5) ppm. The upper limit of the fraction of hydrogenated carbon is estimated to be 1.5%.

High-resolution electron microscopy of interfaces in fcc materials

Modern high-resolution electron microscopy (HREM) instruments, which are capable of a point-to-point resolution of better than 0.2 nm, have allowed atomic-scale observations of a variety of internal interfaces. The application of the HREM technique to fcc model systems for the purpose of addressing a number of interface issues will be examined in this paper. Atomic structure observations for heterophase interfaces of metal/metal and metal/metal-oxide systems as well as HREM studies of grain boundaries in NiO and Au will be discussed with emphasis on generic structural features and the role of the interface plane. Comparisons between observed interface structures and atomistic computer modeling results have shown agreements for some interfaces, and certain differences in others. A number of structural features are common to both metal and oxide grain boundaries, as well as certain heterophase boundaries. Of particular importance in close-packed solids appears to be the tendency to preserve, as much as possible, local atomic coordination, giving rise to atomically well-matched regions that alternate along the interface with regions of misfit. It is commonly observed that heterophase interfaces are preferentially formed on dense-packed planes. Low-index planes are also frequently observed in asymmetric grain boundaries. In addition to the observation of misfit dislocations in heterophase boundaries, misfit-dislocation-like defects have also been found in asymmetric, incommensurate grain boundaries. The tendency for maintaining coherence between dense-packed planes across the interface has resulted in the formation of novel three-dimensional grain boundary structures. HREM observations have brought new insights into the correlations between macroscopic geometry, interfacial energy, and microscopic atomic relaxations.

Bond particle model for semiconductor melts and its application to the liquid structure of germanium

A simple type of liquid-state model is proposed to describe on a primitive level the melt of an elemental group IV semiconductor as a mixture of atoms and bond particles. The latter, on increase of a coupling strength parameter become increasingly localised between pairs of atoms up to local tetrahedral coordination of atoms by bond particles. Angular interatomic correlations are built into the model as bond particle localisation grows, even though the bare interactions between the components of the liquid are formally described solely in terms of central pair potentials. The model is solved for liquid structure by standard integral equation techniques of liquid-state theory and by Monte Carlo simulation, for values of the parameters which are appropriate to liquid germanium down to strongly supercooled states. The calculated liquid structure is compared with the results of diffraction experiments on liquid germanium near freezing and discussed in relation to diffraction data on amorphous germanium. The model suggests simple melting criteria for elemental and polar semiconductors, which are empirically verified.

The self-consistent band structure of PtS2obtained by the linear muffin-tin orbital method in the atomic sphere approximation

The self-consistent band structure and the density of states of the layer compound PtS2 have been calculated using the linear muffin-tin orbital method in the atomic sphere approximation. This calculation includes the relativistic mass-velocity and Darwin terms but excludes the spin-orbit coupling. The Barth-Hedin local exchange-correlation potential was used. The results confirm the semiconducting nature of PtS2, and the band structure obtained is in good agreement with the authors' optical experimental measurements. The results are discussed in terms of charge transfer and local coordination of the constituent atoms.

Ion-beam mixing at Fe:metallic-glass (Fe67Co18B14Si1) interface: A conversion-electron Mössbauer spectroscopic study

Ion-beam mixing at an Fe:metallic glass (Fe67Co18B14Si1) interface is studied by employing the technique of conversion electron Mössbauer spectroscopy (CEMS). A 230-Å-thick overlayer of iron (enriched to 33% in the concentration of 57Fe Mössbauer isotope) was deposited on the shiny surface of metallic glass and such composites were bombarded with 100-keV Kr+ ions at dose values in the range between 1×1015 and 2×1016 ions/cm2. The transformations in the local atomic arrangements across the interface were investigated by monitoring the changes in the hyperfine-interaction parameters. It is shown that mixing leads to significant changes in the composition, in the vicinity of the interface as a function of the ion dose. At low dose (1×1015 ions/cm2) the local atomic coordination is found to be rich in the transition-metal concentration, while at a higher dose (2×1016 ions/cm2) it is observed to be rich in the boron concentration. Interestingly, at an intermediate dose 1×1016 ions/cm2 the composite near the interface region partially crystallizes and this structural state is found to revert back to the amorphous state upon thermal annealing at 300 °C. The observations made on the basis of CEMS are well supported by x-ray diffraction measurements.

A surface EXAFS study of the local coordination of barium atoms at the active-sites of thermionic cathodes

The local coordination of barium atoms at the active-sites of thermionic cathodes has been studied using Surface Extended X-ray Absorption Fine Structure (SEXAFS). On the (polycrystalline) surfaces of tungsten and tungsten/osmium alloy dispenser cathodes Ba is bonded to oxygen with a well-defined short-range order. The Ba-O distance is almost the same (2.62±0.04 Å) for both cathodes, with oxygen atoms occupying hollow sites of the substrate. However, the alloy cathode has Ba bonded to two oxygen near neighbours (compared to one for the tungsten cathode), which will enhance the surface dipole. This provides an explanation of the lower work function observed for the alloy cathode. Together with the results of surface-enhanced X-ray diffraction measurements, the data suggest that surface morphology plays an important role in determining the active-site structures.

Neutron and X-ray diffraction patterns of aqueous sulphuric acid solutions

The total X-ray and neutron diffraction patterns from aqueous sulphuric acid solutions of different concentrations are presented. They are related to an average of all pair correlation functions for the system. Although such complicated quantities are difficult to interpret in terms of local order, by combining neutron and X-ray diffraction it is possible to isolate particular structural features. Following a procedure already employed in a previous study we use both techniques to identify local coordination of both heavy and light atoms. We find that tetrahedral coordination of the SO4 groups is maintained in the liquid state, for all the molar composition analysed, and the bond lengths are in good agreement with those of solid sulphuric acid. In addition we found that the experimental O-D coordination numbers are higher than the calculated ones with increasing sulphuric acid concentration.

The local coordination of barium atoms at the active-sites of thermionic cathodes

The local coordination of barium atoms at the active-sites of thermionic cathodes has been studied using Surface Extended X-ray Absorption Fine Structure (SEXAFS). On the (polyerystalline) surfaces of tungsten and tungsten/osmium alloy dispenser cathodes Ba is bonded to oxygen with a wall- defined short-range order. The Ba-O distance is almost the same (2.62 ± 0.04 Å) for both cathodes, with oxygen atoms occupying hollow sites of the substrate. However, the alloy cathode has Ba bonded to two oxygen near neighbours (compared to one for the tungsten cathode), which will enhance the surface dipole. This provides an explanation of the lower work function observed for the alloy cathode. Results of surface- enhanced X-ray diffraction measurements of the cathodes suggest that surface morphology plays an important role in determining the active site structures.

Study of the electronic structures of Ni-group metal ditellurides: NiTe2, PdTe2and PtTe2by the self-consistent LMTO-ASA method

The self-consistent band structures and the density-of-states functions of Ni-group metal ditellurides (NiTe2, PdTe2 and PtTe2) have been calculated using the LMTO-ASA method. The results indicate that all three compounds are metals with a complex Fermi surface and a relatively large electron density of states at the Fermi level. They also show that d orbitals of the metal atoms are strongly hybridised with the 5p orbitals of tellurium atoms forming mainly covalent bonds. The calculated electron band structures are discussed in terms of atomic orbital binding energies and the local coordination of metal atoms as well as the effect of any charge transfer. A detailed comparison of the results with previously measured photo-emission spectra and dHvA measurements show that there is a substantial agreement between the calculated band structures and the experimental data. It is also found that such an agreement is achieved only after self-consistency in the calculations has been reached.

Chemical analyses and studies of electronic structure of solids by electron energy loss spectroscopy with an electron microscope

Both the thickness and the composition of a specimen can be deduced from plasmon spectroscopy, which is readily recorded with an electron microscope to which has been attached an electron spectrometer of resolution 1—3 eV. This is illustrated in the study of alkali metals and sp metals such as magnesium and aluminium , which are produced by decomposing ternary or binary hydrides (for example, NaAlH 4 or MgH 2 ). But other m aterials, including silicon, are also am enable to studies of this kind. The elemental composition, electronic structure and inter-atomic distances of a sample can be derived from the core-electron (K, L, M edge) loss peaks and their fine structure. And from the near-edge energy loss structure, and in particular from the ‘white line’ intensity ratios (for example, L 2 /L 3 ), the number of d-electrons and hence the oxidation state of transition elements in com pounds of first-row transition metal series may be determined. In the region beyond the near-edge structure, extended electron energy loss fine structure (e.x.e.l.f.s.) is observed and this may be used to obtain information about the local co-ordination of different atoms in the sample. Finally, by monitoring the Doppler broadening of the scattered electrons liberated from the sample as a consequence of the ‘electron' equivalent of the Compton-process, we can probe the electronic properties of the solid in m omentum space. This reveals the nature of the bonding in simple solids; and, in particular, reveals whether an amorphous sample of carbon is more nearly graphitic or adamantine.

The electronic structures of platinum dichalcogenides: PtS2, PtSe2and PtTe2

The band structures and density of states functions of platinum dichalcogenides, calculated using the LMTO-ASA method in conjunction with muffin-tin potentials, are reported for the first time. The results predict that PtS2 is a semiconductor with an energy gap of about 1.2 eV, PtSe2 is a semimetal with a slight indirect overlap of the conduction and valence bands, the PtTe2 is a metal with a complex Fermi surface. The electronic structures of these compounds are found to be substantially different from those of group IVb, Vb, and VIb TMDC having the same crystal structure. The results are discussed in terms of atomic orbital binding energies and the local coordination of the constituent atoms. The band structures obtained are generally in good agreement with the available experimental measurements.

Optical and electronic properties of Fe doped bulk a-As 2S 3

Several glasses in the (As 2S 3) 100−xFe x system have been prepared from the melt under a variety of preparation conditions. The AC conductivity and optical properties of the homogeneous samples have been determined and compared with previous published results. The role of the impurity atoms is discussed with reference to spectroscopic experiments aimed at determining the local coordination of the Fe atoms.