Considerable Degree Of Disorder Research Articles

Site occupancies in a chemically complex σ-phase from the high-entropy Cr–Mn–Fe–Co–Ni system

Site occupancies in a single-phase σ-intermetallic compound of composition Cr46Mn15.2Fe16.3Co17Ni5.5 (in at.%) were studied. To the best of the authors’ knowledge, such a complex analysis with 5 elements occupying 5 different sites has never been tackled before and is addressed here using cutting edge X-ray diffraction techniques, absorption spectroscopy and simulations. Resonant powder diffraction data were collected at the K-edges of each of the five elements and the site occupancies were refined by a joint Rietveld analysis. Dispersion coefficients were retrieved independently from tabulated values, Kramers-Kronig relations and reference samples. Additional Extended X-ray Absorption Fine Structure (EXAFS) measurements were used to confirm the site occupancies and bond lengths. The site occupancies were independently obtained by a thermodynamic approach based on the enthalpies of formation of the 55 = 3125 configurations generated by the ordered distribution of the five components on the five sublattices, derived from a machine learning approach based on a DFT database. Overall, the calculations and experiments are in excellent agreement and reveal that the σ phase can accommodate a considerable degree of disorder, which consequences for thermodynamic modeling are discussed. The configurational entropy (S) of the σ phase represents 84% of that of a fully disordered alloy with the same composition, which is much larger than that expected for a fully ordered intermetallic phase (S = 0). The corresponding entropic stabilization is one of the reasons for the fact that the σ phase is one of the most important intermetallic that precipitates in high-entropy alloys and many engineering materials.

Optical and transport properties correlation driven by amorphous/crystalline disorder in InP nanowires

Indium phosphide nanowires with a single crystalline zinc-blend core and polycrystalline/amorphous shell were grown from a reliable route without the use of hazardous precursors. The nanowires are composed by a crystalline core covered by a polycrystalline shell, presenting typical lengths larger than 10 μm and diameters of 80–90 nm. Raman spectra taken from as-grown nanowires exhibited asymmetric line shapes with broadening towards higher wave numbers which can be attributed to phonon localization effects. It was found that optical phonons in the nanowires are localized in regions with average size of 3 nm, which seems to have the same order of magnitude of grain sizes in the polycrystalline shell. Regardless of the fact that the nanowires exhibit a crystalline core, any considerable degree of disorder can lead to a localized behaviour of carriers. In consequence, the variable range hopping was observed as the main transport instead of the usual thermal excitation mechanisms. Furthermore the hopping length was ten times smaller than nanowire cross-sections, confirming that the nanostructures do behave as a 3D system. Accordingly, the V-shape observed in PL spectra clearly demonstrates a very strong influence of the potential fluctuations on the exciton optical recombination. Such fluctuations can still be observed at low temperature regime, confirming that the amorphous/polycrystalline shell of the nanowires affects the exciton recombination in every laser power regime tested.

Relations Between Redundancy Patterns of the Shannon Code and Wave Diffraction Patterns of Partially Disordered Media

The average redundancy of the Shannon code, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rn</i> , as a function of the block length <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> , is known to exhibit two very different types of behavior, depending on the rationality or irrationality of certain parameters of the source: It either converges to 1/2 as <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> grows without bound, or it may have a nonvanishing, oscillatory, (quasi-) periodic pattern around the value 1/2 for all large <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> . In this paper, we make an attempt to shed some insight into this erratic behavior of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rn</i> , by drawing an analogy with the realm of physics of wave propagation, in particular, the elementary theory of scattering and diffraction. It turns out that there are two types of behavior of wave diffraction patterns formed by crystals, which are correspondingly analogous to the two types of patterns of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rn</i> . When the crystal is perfect, the diffraction intensity spectrum exhibits very sharp peaks, a.k.a. Bragg peaks, at wavelengths of full constructive interference. These wavelengths correspond to the frequencies of the harmonic waves of the oscillatory mode of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rn</i> . On the other hand, when the crystal is imperfect and there is a considerable degree of disorder in its structure, the Bragg peaks disappear, and the behavior of this mode is analogous to the one where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rn</i> is convergent.

The structure of long period stacking/order Mg–Zn–RE phases with extended non-stoichiometry ranges

We propose structural models of the unique long period stacking/order (LPSO) phases formed in Mg–Zn–RE alloys, based on Z-contrast scanning transmission electron microscopy observations and first principles calculations. The LPSO structures are long period stacking derivatives of the hcp Mg structure, and the Zn/RE distributions are restricted at the four close-packed atomic layers forming local fcc stacking (i.e. a local ABCA stacking). Chemical order is well developed for the LPSO phases formed in Mg 97Zn 1Er 2 (14H type) and Mg 85Zn 6Y 9 (18R type) alloys with pronounced superlattice reflections, and the relevant Zn/RE distributions clearly emerge in the Z-contrast atomic images. Initial ternary ordered models were constructed by placing all the atoms at the ideal honeycomb sites, leading to plausible space groups of P6 3/ mcm for the 14H type and C2/ m, P3 112 or P3 212 for the 18R type. The characteristic ordered features are well represented by local Zn 6RE 8 clusters, which are embedded in the fcc stacking layers in accordance with the L1 2 type short-range order. Energy favored structural relaxations of the initial model cause significant displacement of the Zn/RE positions, implying that strong Zn–RE interactions may play a critical role in phase stability. The LPSO phases seem to tolerate a considerable degree of disorder at the Zn and RE sites with statistical co-occupations by Mg, extending the non-stoichiometric phase region bounded along the Zn/RE equiatomic line from ∼Mg 94.0Zn 2.0Y 4.0 to ∼Mg 83.3Zn 8.3Y 8.3.

Synchrotron micro-diffraction analysis of the microstructure of cryogenically treated high performance tool steels prior to and after tempering

The phase transformation and strain changes within cryogenically (−196 °C) treated high performance tool steels (AISI H13) before and after tempering have been examined using both laboratory XRD and synchrotron micro-diffraction. The martensitic unit cell was found to have very low tetragonality as expected for low carbon steel. Tempering resulted in the diffusion of excess carbon out of the martensite phase and consequent unit cell shrinkage. In addition on tempering the martensite became more homogeneous as compared to the same samples prior to tempering. For cryogenically treated samples, the effect was most pronounced for the rapidly cooled sample which was the least homogenous sample prior to tempering but was the most homogenous sample after tempering. This suggests that the considerable degree of disorder resulting from rapid cryogenic cooling results in the beneficial release of micro-stresses on tempering thus possibly resulting in the improved wear resistance and durability observed for cryogenically treated tool steels.

Crystal Structure of Thrombin Bound to the Uncleaved Extracellular Fragment of PAR1

Abundant structural information exists on how thrombin recognizes ligands at the active site or at exosites separate from the active site region, but remarkably little is known about how thrombin recognizes substrates that bridge both the active site and exosite I. The case of the protease-activated receptor PAR1 is particularly relevant in view of the plethora of biological effects associated with its activation by thrombin. Here, we present the 1.8 A resolution structure of thrombin S195A in complex with a 30-residue long uncleaved extracellular fragment of PAR1 that documents for the first time a productive binding mode bridging the active site and exosite I. The structure reveals two unexpected features of the thrombin-PAR1 interaction. The acidic P3 residue of PAR1, Asp(39), does not hinder binding to the active site and actually makes favorable interactions with Gly(219) of thrombin. The tethered ligand domain shows a considerable degree of disorder even when bound to thrombin. The results fill a significant gap in our understanding of the molecular mechanisms of recognition by thrombin in ways that are relevant to other physiological substrates.

The Stokes−Einstein Relationship and the Levitation Effect: Size-Dependent Diffusion Maximum in Dense Fluids and Close-Packed Disordered Solids

We report a molecular dynamics study of a binary mixture consisting of a large (host) particle and a smaller (guest) particle whose radius is varied over a range. These simulations investigate the possible existence of a diffusion anomaly or levitation effect in dense fluids, previously seen for guest molecules diffusing within porous solids. The voids in the larger component have been characterized in terms of void and neck distributions by means of Voronoi polyhedral analysis. Four different mixtures with differing ratios of guest to host diffusivities (D) have been studied. The results suggest that the diffusion anomaly is seen in both close-packed solids with disorder and dense fluids. In the latter, the void network is constantly and dynamically changing and possesses a considerable degree of disorder. The two regimes, viz., the linear regime (LR) and the anomalous regime (AR), found for porous solids are shown to exist for a dense medium as well. The linear regime is characterized by D(g) proportional to 1/sigma(gg)(2), where sigma(gg) is the diameter of the guest. The anomalous regime exhibits a maximum in D up to rather high temperatures (T = 1.663), even though in porous solids the maximum disappears at higher temperatures. In agreement with previous studies on porous solids, a particle in the AR is associated with lower activation energy, lower friction, and less backscattering in the velocity autocorrelation function when compared to a particle in the LR. Wavevector dependent self-diffusivity, Delta, and decay of the intermediate scattering function, F(s)(k, t), exhibit contrasting behaviors for the LR and AR. For LR, Delta exhibits a minimum at values of k at which there are spatial correlations in S(k) while a smooth decrease with k is seen for AR. For LR, F(s)(k, t) shows a biexponential decay corresponding to two different time scales of motion. Probably, the fast decay is associated with motion within the first shell of solvent neighbors and the slow decay with motion past these shells. For AR, a single-exponential decay is seen. The results indicate a breakdown of the Stokes-Einstein (SE) relationship. The relevant quantity that determines the validity of the SE relationship is the levitation parameter which is indirectly related to the solute/solvent radius ratio and not either the size of the solute or the solvent alone.

The tetraethylammonium cation in a double three-ring silicate heteronetwork clathrate and a polyhedral clathrate hydrate: low-temperature single-crystal X-ray diffraction studies on (NEt 4) 6[Si 6O 15] · 40.8H 2O and NEt 4OH · 9H 2O

A reinvestigation of the crystal structure of the tetraethylammonium silicate hydrate with the approximate chemical composition (C 8H 20N) 6[Si 6O 15] · 40.8H 2O ( 1) is reported. The crystal structure of tetraethylammonium hydroxide nonahydrate, (C 8H 20N)OH · 9H 2O ( 2), has been determined for the first time. The structure of 1 has been refined to much higher precision in the centrosymmetric space group P 1(Z=2) with a considerable degree of disorder, compared to a previous structure analysis in the non-centrosymmetric space group P1 without disorder (Sov. Phys. Crystallogr. 29 (1984) 421). 1 may be considered as a host–guest compound, in which trigonal-prismatic double three-ring silicate anions and water molecules form a complicated hydrogen-bonded host structure. The organic guest cations reside in straight channels and very irregular, not completely closed cages. The [Si 6O 15] 6− oligoanions are stabilized in a specific organic–water environment by O–H ⋯ O hydrogen bonds and electrostatic and van der Waals O ⋯ CH 3 and O ⋯ CH 2( N) anion–cation interactions. The NEt + 4 cations occur as conformers of approximate 4 ̄ 2m(D 2d) and 4 ̄ (S 4) molecular symmetry, depending on the location around the oligoanion. 2 is the first representative of a polyhedral clathrate hydrate of the NEt + 4 cation. The novel host structure is built from hydrogen-bonded water molecules and hydroxide ions.

The structure of satellite panicum mosaic virus at 1.9 Å resolution

The crystal structure of satellite panicum mosaic virus (SPMV) has been solved by multiple isomorphous replacement and molecular replacement and refined at 1.9 A resolution. SPMV, a T = 1 icosahedral virus, is the smallest virus structure determined. The coat protein is an eight-stranded 'jelly roll' beta-barrel with an amino-terminal strand that extends into the interior of the virus, presumably interacting with the RNA. Regions of electron density on the interior of the protein capsid may be RNA, although it is not possible to construct any detailed model of the nucleic acid. Basic amino acid residues in contact with the nucleic acid show a considerable degree of disorder. The carboxy-terminal strand of the virus coat protein interacts with adjacent subunits, forming an additional beta-strand.

The As-terminated reconstructions formed by GaAs(001): a scanning tunnelling microscopy study of the (2 × 4) and c(4 × 4) surfaces

Atomic resolution scanning tunnelling microscopy (STM) has been used to study the As-terminated reconstructions formed by GaAs(001) surfaces grown in situ by molecular beam epitaxy (MBE). Specific emphasis has been placed on the transition from a (2 × 4) to c(4 × 4) surface with increasing amounts of As. STM images of the initial (2 × 4) surface, corresponding to the β phase, showed an ordered structure with unit cells containing two As dimers. With increasing amounts of As, the intensity of the 2 4 streak in the RHEED pattern weakened considerably. Although STM images of this (2 × 4) phase again only showed two As dimers per unit cell, the surface was characterized by a considerable degree of disorder and a large number of kinks. The results are consistent with the (2 × 4) β phase having a structure with unit cells based on two As dimers and Gn absent from the missing dimer trenches. The kinks formed on the more As-rich (2 × 4) structure are then caused by the additional As occupying these vacant Ga sites producing an electron rich site. Quenching to lower temperatures in the presence of As leadsto the c(4 × 4) structure. STM images of this surface indicate that the top layer of the structure is based on rectangular units, which when complete, consist of a total of six As atoms. The wide coverage range for which this reconstruction can be maintained is explained by a varying number of missing As atoms from the basic six atom structural unit. A new structural model is proposed for the c(4 × 4) structure based on its formation from the starting (2 × 4) surface and involves a mixed third layer containing both Ga and As.

Systematic study of the growth-temperature dependence of structural disorder and superconductivity in YBa2Cu3O7- delta thin films.

The growth-temperature dependence of the crystal structure and superconductivity of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin films has been investigated quantitatively for a series of sputter-deposited films prepared at substrate temperatures ranging from 593 to 726 \ifmmode^\circ\else\textdegree\fi{}C. X-ray diffraction revealed that the reflection intensities of the films, especially I(004), I(005), and I(006), change systematically with substrate temperature. Structure analyses suggested that there is a considerable degree of disorder between Y and Ba atoms in the structure of the thin films deposited at lower substrate temperatures. This degree of disorder decreases rapidly from 14 to 3 % (percentage of the Y site substituted by Ba atoms) with increase in the substrate temperatures up to 630 \ifmmode^\circ\else\textdegree\fi{}C, and then decreases slowly to zero percent and remains zero even at higher substrate temperatures. Electrical-resistivity measurements showed that the superconducting transition temperatures of the thin films increase with increasing substrate temperature, in good agreement with the recovery of the disordered structure. The results indicate that there is an intrinsic relationship between the cation disorder and ${\mathit{T}}_{\mathit{c}}$, which are both strongly dependent on the growth temperature.

Structure of the microporous titanosilicate ETS-10

INORGANIC microporous framework solids such as zeolites are of considerable technological importance as shape-selective catalysts, ion-exchange materials and molecular sieves1. Most microporous materials known until recently were silicates, aluminosilicates1 or aluminophosphates2–4, all of which contain tetrahedrally coordinated metal atoms. In 1989, a family of microporous titanosilicates (generically denoted ETS) was discovered in which the metal atoms (Ti4+) are octahedrally coordinated5–8. A full understanding of the potential of any microporous solid to act as a molecular sieve and selective catalyst, and of the nature of the catalytic centres, requires that its structure be known. But that of the ETS materials has proved elusive because of the considerable degree of disorder that they contain. Using a combination of high-resolution electron microscopy, electron and powder X-ray diffraction, solid-state NMR, molecular modelling and chemical analysis, we have now been able to solve the structure of a prominent member of this family, ETS-10. This structure comprises corner-sharing SiO4 tetrahedra and TiO6 octahedra linked through bridging oxygen atoms. The pore system contains 12-membered rings and displays a considerable degree of disorder. Many ordered variants of ETS-10 exist, some of which are chiral.

Non-tandem arrangement and divergent transcription of chicken histone genes

We have located chicken histone genes within 14-kilobase pair inserts of two genomal clones, λCH-01 and λCH-02. The genes were mapped with chicken histone cDNA and gene-specific probes, and the identity and polarity of each gene confirmed by DNA sequencing. As previously indicated, histone mRNAs are transcribed from both DNA strands. The gene organization differs in the two clones. Although there is some clustering of genes, no repeating unit is observed. λCH-01 contains all core histone genes, but two copies of H2A occur before the full complement of histone genes is represented. λCH-02 contains no H4 or H2A genes, but notably has two adjacent H3 genes which are divergently transcribed. H2A, H2B and H3 genes all appear more than once in the combined inserts of the two clones, but in no case do they occur in equivalent environments. This indicates a considerable degree of disorder of the histone genes in the chicken genome.