Long-range Atomic Order Research Articles

Carbon nanocones: wall structure and morphology

Large-scale production of conical carbon nanostructures is possible through pyrolysis of hydrocarbons in a plasma torch process. The resulting carbon cones occur in five distinctly different forms, and disc-shaped particles are produced as well. The structure and properties of these carbon cones and discs have been relatively little explored until now. Here we characterize the structure of these particles using transmission electron microscopy, synchrotron x-ray and electron diffraction. The carbon nanocones are found to exhibit several interesting structural features; instead of having a uniform cross-section, the walls consist of a relatively thin inner graphite-like layer with a non-crystalline envelope, where the amount of the latter can be modified significantly by annealing. The cones appear with a well-defined faceting along the cone edge, demonstrating strict long-range atomic ordering; they also present occasional examples of symmetry breaking, such as two apexes appearing in the same carbon nanocone.

Off-diagonal long-range order and supersolidity in a quantum solid with vacancies

We consider a lattice of bosonic atoms, whose number $N$ may be smaller than the number of lattice sites $M$. We study the Hartree-Fock wave function built up from localized wave functions $w(\mathbf{r})$ of single atoms, with nearest-neighboring overlap. The zero-momentum particle number is expressed in terms of permanents of matrices. In one dimension, it is analytically calculated to be $\ensuremath{\alpha}N(M\ensuremath{-}N+1)/M$, with $\ensuremath{\alpha}={|\ensuremath{\int}w(\mathbf{r})d\ensuremath{\Omega}|}^{2}/[(1+2a)l]$, where $a$ is the nearest-neighboring overlap and $l$ is the lattice constant. $\ensuremath{\alpha}$ is on the order of 1. The result indicates that the condensate fraction is proportional to and of the same order of magnitude as that of the vacancy concentration, hence there is off-diagonal long-range order or Bose-Einstein condensation of atoms when the number of vacancies $M\ensuremath{-}N$ is a finite fraction of the number of the lattice sites $M$.

Electron microscopy study of L10‐FePtCu nanoparticles synthesized at 613K

Finely dispersed hard magnetic L1o-FePtCu nanoparticles with 100 orientation were directly synthesized by RF-sputtering on NaCl substrate at a temperature of 613 K. The maximum coercivity of the particles was 1.4 kOe (at RT). Degrees of atomic long-range order (LRO) for the L1o-FePtCu nanoparticles with different sizes were obtained using nanobeam electron diffraction technique. The decrease of LRO parameter became remarkable when the size became below 8 nm. The coercivity value also decreased with decreasing the particle size. The relation between the LRO parameter decrease and the coercivity decrease with particle size was discussed.

Entropy change and effect of magnetic field on martensitic transformation in a metamagnetic Ni–Co–Mn–In shape memory alloy

We studied the entropy change and the shift of the martensitic transformation temperatures with magnetic field in samples of a polycrystalline Ni–Co–Mn–In alloy having different degrees of long-range atomic order due to different heat treatments. We found, for the samples of the same composition, strong variations of the entropy change with the degree of atomic order, mediated by the difference between the Curie and martensitic transformation temperatures. Calculations of the field-induced shift of the transformation using data of entropy variations show good agreement with experimental results.

Order-disorder phase transition and stress-induced diffusion in Au-Cu

Isothermal mechanical spectroscopy by means of a forced torsion pendulum (measuring internal friction/mechanical loss) was used to study the interplay of long-range atomic order and stress-induced diffusion (Zener relaxation) in ${\text{Au}}_{57%}{\text{Cu}}_{43%}$. Our results show that the relaxation strength of stress-induced diffusion exhibits the typical Curie-Weiss-type behavior in the disordered solid solution and then gradually goes to zero below the critical temperature marking the phase transition to the long-range-ordered AuCu II phase. The breakdown of the relaxation peak reflects the kinetics of the ordering process. The diffusion data were used to establish the transformation time vs temperature (TTT) diagram of the phase transformation.

Composition dependent properties of Fe3Si films grown on GaAs(113)A substrates

Structural, electrical, and magnetic properties of Fe3Si/GaAs(113)A hybrid structures are studied, dependent on the layer composition varying from 15 to 26 at. % Si. The presence of superlattice reflections in x-ray diffraction and lower resistivity confirms the long-range atomic ordering in the stoichiometric Fe3Si films, reflecting the D03 crystal structure. The observed atomic ordering is also found to influence the sign and magnitude of the antisymmetric component of the planar Hall effect observed in this orientation. However a finite disorder is observed even in nearly stoichiometric samples.

Ensembles of indium phosphide nanowires: physical properties and functional devices integrated on non-single crystal platforms

A new route to grow an ensemble of indium phosphide single-crystal semiconductor nanowires is described. Unlike conventional epitaxial growth of single-crystal semiconductor films, the proposed route for growing semiconductor nanowires does not require a single-crystal semiconductor substrate. In the proposed route, instead of using single-crystal semiconductor substrates that are characterized by their long-range atomic ordering, a template layer that possesses short-range atomic ordering prepared on a non-single-crystal substrate is employed. On the template layer, epitaxial information associated with its short-range atomic ordering is available within an area that is comparable to that of a nanowire root. Thus the template layer locally provides epitaxial information required for the growth of semiconductor nanowires. In the particular demonstration described in this paper, hydrogenated silicon was used as a template layer for epitaxial growth of indium phosphide nanowires. The indium phosphide nanowires grown on the hydrogenerated silicon template layer were found to be single crystal and optically active. Simple photoconductors and pin-diodes were fabricated and tested with the view towards various optoelectronic device applications where group III–V compound semiconductors are functionally integrated onto non-single-crystal platforms.

Influence of electromagnetic radiation on the phase composition of clustered CN x films

The effect of white and UV radiation on the phase composition of amorphous CNx films are studied by X-ray diffraction analysis and visible-range spectroscopy. The films have variable-range atomic order and consist of amorphous graphite clusters (30 A) crystalline clusters (50–100 A) of graphite, diamond, and carbon nitride phases; and intercluster medium with long-range (1–2 A) atomic order. It is shown that irradiation of the films by white light facilitates the growth of fine graphite clusters. Irradiation by UV light suppresses the growth of the graphite and carbon nitride phases, favoring the growth of the diamond phase (1.5%). It is demonstrated that a change in the mesoscopic phase composition of the CNx films causes a change in the energy gap width in the visible range from Eg = 0.75 eV for the films irradiated by white light to Eg = 1.75 eV for those exposed to UV radiation.

Origin of grey spots on the surface of metallic glasses under Ar + ion irradiation: The surface and volume properties

The nature of grey spots on the surface of amorphous Fe 67Cr 18B 15 metglass ribbons irradiated by 30 keV Ar + ions is investigated. Changes in the surface and volume properties of samples are analyzed bearing in mind the presence of competitive processes of ordering and disordering in the structure under irradiation at initial stage of crystallization ( T = 500 °C). Changes in the volume properties of the samples become apparent in their structure, electrical and magnetic properties and are caused by radiation-enhanced diffusion at high gradients in the concentration of defects generated by Ar + irradiation. It is established that grey spots are shown by decrease in the reflection coefficient in a visible range. They emerge as a result of reorganization in the electronic structure of irradiated ribbons due to damage in the short-range order and formation of non-uniformly scaled atomic structure during transition from the medium-range to the long-range atomic order. Experimental data are in accord with the calculations in the framework of the free electrons model.

Microstructure and low field magnetic properties of bulk Fe61Co10Hf2.5Zr2.5Ti2W2B20 amorphous and partially crystallized alloy

The microstructure, initial magnetic susceptibility and its disaccommodation of the amorphous and partially crystallized Fe61Co10Hf2.5Zr2.5Ti2W2B20 alloy are investigated. From Mössbauer spectroscopy, X-ray diffraction and electron microscopy studies we have found that the samples in the as-quenched state are fully amorphous. Moreover, we have stated that in the annealed samples the crystalline α-FeCo phase shows the long range order of atoms. The amorphous rod exhibits the lower intensity of disaccommodation than the sample in the form of ribbon. It is due to annealing out of some free volumes during preparation.

Possible mechanisms for the nucleation of primary fracture zones during deformation-induced phase transformations in solids: I. deformation-induced instability of crystal lattices

The problems related to the search for the nucleation mechanisms of primary continuity defects, which serve as the sources of submicrocracks in an initially continuous solid body, are discussed. A crystal lattice in a solid body is shown to become unstable to the retention of its long-range atomic order, because of a high level of the hydrostatic tensile load generated by stress concentrators. As a result, localized phase trans-formations of a solid phase into other states of aggregation can occur near the stress concentrators. An analysis of these phenomena can serve as the basis for searching for fracture-zone nucleation mechanisms.

Growth, Interface Structure, and Magnetic Properties of Fe/GaAs and Fe3Si/GaAs Hybrid Systems

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Temperature effect on microstructure and mechanical properties of the nano-structured Ni 3Al coating

Phase composition, thin structure and morphology of the surface of the intermetallic coating are investigated by transmission electron microscopy, X-ray analysis and atomic force microscopy. It is shown that Ni 3Al is the main phase of the intermetallic coating for all investigated samples. There is a small amount of the NiAl and AlFe 3C phases. The application of temperature and ion beam treatment changes the atomic long-range order parameter and the elastic microstresses of the crystalline lattice.

Correlation between atomic order and the characteristics of the structural and magnetic transformations in Ni–Mn–Ga shape memory alloys

The evolution of the degree of L2 1 long-range atomic order and its influence on the martensitic transformation and Curie temperatures of a near-stoichiometric Ni 2MnGa shape memory alloy were studied during several thermal treatments. Both temperatures depend on the quenching temperature and increase on subsequent heating treatments. This increase is concurrent with the presence of an exothermic peak in differential scanning calorimetry measurements. Neutron diffraction experiments were carried out in order to correlate the evolution of the transformation temperatures with the evolution of the L2 1 order degree, which was estimated through the intensity of the (1 1 1) and (2 0 0) reflections. The results confirm that the quenched-in degree of long-range atomic order depends on the quenching temperature and prove that the evolution of the transformation temperatures is due to the evolution of the atomic order.

Disorder–order structural transition in amorphous LaSrMnO films

Using the x-ray diffraction method, the atomic rearrangement in amorphousLa0.6Sr0.2Mn1.2O3 films is investigated atgrowth temperatures Ts = 20–300 °C. Two stages in the disorder–order phase transition are discovered.In the first stage, amorphous structure undergoes ordering atTs = 100 °C up to a maximalconcentration of small (D≈30 Å) clusters which occupy 90% of the total volume. In theTs = 100–150 °C temperature interval, disordering in the structure and a change in short-rangeorder prepare the onset of a second stage. In the second stage, starting withTs = 150 °C,the progressive crystallization and transition from mesoscale order to long-range atomic order atTs = 300 °C take place. It is shown that all transitions produce changes in the conductivity mechanismsand magnetic properties of the films.

High-resolution x-ray diffraction study of MnO nanostructured within a MCM-48 silica matrix with a gyroidal system of channels

Antiferromagnetic MnO was synthesized within a mesoporous matrix MCM-48 with a gyroidal system of channels. Synchrotron radiation studies reveal that the embedded nanoparticles have a ribbonlike shape and a length of about 53(3) A . The peculiar diffraction line shape shows the loss of long-range atomic order. In spite of positional disorder, a transition from a cubic structure to a rhombohedral one, similar to the transition known for the bulk, is observed.

Novel TEM methods for large-area analysis of misfit dislocation networks in semiconductor heterostructures

Transmission electron microscopy (TEM) is a powerful tool for detailed investigations of individual misfit dislocations and misfit dislocation networks in semiconductor heterostructures. However, the characterization of large numbers of misfit dislocations distributed over large network areas has traditionally been a challenge. Such analyses are important for detailed understanding of phenomena, such as layer mosaicity, that originate from the misfit dislocations, but develop features on a length scale of several tens of microns. Moreover, quantitative interpretations of X-ray diffraction data, based on models that describe the arrangement of misfit dislocations in statistical terms, call for validation by a large-area analysis with microscopic details. In this paper, novel TEM sample preparation and analysis methods are presented, which allow for the efficient study of large numbers of misfit dislocations and provide new insight into the formation of misfit–dislocation networks. The methods discussed are: (i) a double-bevel-polish (DBP) sample preparation for systematic depth-sensitive large-area plan-view TEM imaging of three-dimensional misfit-dislocation networks, (ii) a bevel-polish and cleavage (BPC) sample preparation for efficient large-area Burgers vector analysis of misfit–dislocation networks, (iii) the use of bend contours for large-area plan-view analysis of the Burgers vectors of misfit dislocations in single networks and (iv) the post-growth retrieval of the sequence of dislocation formation by studying antiphase boundaries formed during dislocation glide in layers showing long-range atomic ordering. The feasibility of each method is demonstrated and their potential applications discussed.

Growth, interface structure, and magnetic properties of Fe/GaAs and Fe3Si/GaAs hybrid systems

Abstract Ferromagnetic Fe and Fe3Si films are grown on GaAs(001) substrates by molecular beam epitaxy. This review article summarizes the important aspects of the heteroepitaxial growth and the associated structural properties of the Fe/GaAs and Fe3Si/GaAs heterostructures. The magnetic anisotropy of the thin Fe epilayers is analyzed and correlated to the interface structure, which is determined by high-resolution transmission electron microscopy. The structural and electrical properties of Fe3Si alloys depend on the exact stoichiometry and the long-range atomic ordering that is controlled by the growth conditions during the molecular beam epitaxy. The interface reactivity and the thermal stability are critically discussed for both hetero-systems.

Order-Disorder Transformation in L1<SUB>0</SUB>-FePd Nanoparticles Studied by Electron Diffraction

Disappearance of long-range atomic order in 10-nm-sized L10-FePd (Fe–58 at%Pd) nanoparticles has been studied by electron diffraction using a specimen heating stage attached to a transmission electron microscope. Ordered structure is kept at lowest up to 949 K, while the intensity of superlattice reflection abruptly drops at 982 K. The transformation temperature decreases by about 80 K compared to that of the bulk alloy of Fe–58 at%Pd. Nanobeam electron diffraction from a particle shows a particle size dependence of the transformation temperature and the intensity of 110 superlattice reflection largely decreases below about 15 nm in diameter at 983 K. However, a weak intensity remains in the 110 superlattice reflection even at 983 K, which is attributed to short-range order in the nanoparticles.

Short- to Medium-Range Atomic Order and Crystallite Size of the Initial FeS Precipitate from Pair Distribution Function Analysis

Pair distribution function analysis (PDF) of X-ray diffraction data, collected at 11-IDC and 1-ID at the Advanced Photon Source, provides the atomic structure and primary crystallite size of FeS both freshly precipitated (FeSfresh) and aged (FeSaged). The short- to medium-range structure of both FeSfresh and FeSaged are nearly identical with that of highly crystalline (bulk) mackinawite. Attenuation in the observed range of structural coherence of the PDF for FeSfresh indicates an average crystallite size on the order of ∼2 nm. This range of structural coherence increased with aging of the sample under hydrothermal conditions due to growth of the individual crystallites, although the mechanism by which this growth occurs is not clear at present. Electron microscopic imaging confirms the presence of individual nanoscale crystallites and provides some insight into their aggregation behavior as larger clusters. The initial, fresh precipitate does not exhibit long-range atomic structure because it is nanocrystalline. The so-called X-ray amorphous nature of FeSfresh is the result of the limited range of structural coherence imposed by the size of the individual crystallites rather than the result of a lack of medium- and long-range atomic order. We propose that the discrepancies in the literature over crystallite size and the atomic structure of FeSfresh are due primarily to the varying degrees of aggregation of uniformly distributed and nanocrystalline FeS particles.