Peierls–Nabarro model for dislocations in MgSiO3 post-perovskite calculated at 120 GPa from first principles

We present here the first numerical modelling of dislocations in MgSiO₃ post-perovskite at 120?GPa. The dislocation core structures and properties are calculated through the Peierls–Nabarro model using the generalized stacking fault (GSF) results as a starting model. The GSFs are determined from first-principle calculations using the VASP code. Dislocation properties such as planar core spreading and Peierls stresses are determined for the following slip systems: [100](010), [100](001), [100](011), [001](010), [001](110), [001](100), [010](100), [010](001), ½[110](001) and ½[110](110). Our results confirm that the MgSiO₃ post-perovskite is a very anisotropic phase with a plasticity dominated by dislocation glide in the (010) plane.     

三羟甲基甲胺单晶缺陷的X射线衍射形貌研究

三羟甲基甲胺(Trihydroxymethylaminomethane,缩写为TAM)是一种新型的X射线分光晶体,综合分光性能优于季戍四醇(PET)。X射线形貌鉴定结果表明,除了宏观包裹物和包裹列外,主要是门类繁多的位错,还有面缺陷干涉条纹。文中扼要讨论了缺陷形成的结构影响因素。 关键词：     

The nucleation of faulted dipoles at intersection jogs in γ-TiAl

Single crystal samples of n-(Ti-54.7 at.% Al) deformed to a permanent strain of 2% at room temperature under multiple-slip conditions contain faulted dipoles (FDs) whose density exhibits some dependence on load orientation. Although FDs are hard to observe after compression along [210], they are profuse and congregated in places in the [1 1 8.6] load orientation. They exhibit most of the topological characteristics of FDs formed under single slip as reported by Grégori and Veyssière such as elongation in the screw direction of the primary d011] slip direction and a noticeable shape asymmetry. It is shown further that, in the [1 1 8.6] samples, bundles of FDs originate at jogs that result from intersection with forest dislocations of appropriate Burgers vectors. A mechanism for FD nucleation is proposed on the basis of asymmetrical dissociation of the parent d011] dislocation and specific impingements between the various partials on two adjacent octahedral planes. Implications of the FD nucleation at jogs on the load orientation dependence of the FD density are discussed.     

High-resolution identification of ½⟨110⟩ stacking faults in epitaxial Ba0.3Sr0.7TiO3 thin films

The near-interface region of an epitaxial Ba_0.3Sr_0.7TiO₃ thin film grown on LaAlO₃ (001) was found to consist of a high density of ½?110? stacking faults bounded by partial dislocations. The stacking faults can extend over large distances (greater than 50 nm). Various possible atomic configurations of the faults were considered. The atomic structures of the faults were identified using high-resolution electron microscopy and simulation as well as energy-filtered imaging. The ½[101] and faults (where [001] is normal to the film plane) were found to lie predominately on the {100} and {110} planes. The ½lsqb;101] faults on (010), (110) or (1&1tilde;0) have never been observed before in perovskites. The stacking faults on [100] have a structure consisting of a double layer of edge-sharing TiO₆ octahedra. The excess of Ti was detected by energy-filtered imaging. The formation of the extended stacking faults is probably related to a small amount of excess Ti during the film deposition, which may originate from the non-stoichiometry of the ceramic targets BaTiO₃ and SrTiO₃. It is also enhanced by the misfit-induced compressive strain in the early stages of the film growth.     

Analysis of contrasts and identifications of Burgers vectors for basal-plane dislocations and threading edge dislocations in 4H-SiC crystals observed by monochromatic synchrotron X-ray topography in grazing-incidence Bragg-case geometry

Contrasts of dislocations in the sub-surface region of the Si-face of a 4H-SiC wafer were observed by monochromatic synchrotron X-ray topography in grazing-incidence Bragg-case geometry. Basal-plane dislocations show very characteristic contrast depending on their Burgers vectors, running directions, and types of dislocations, whether they are screw dislocations, C-core edge dislocations, or Si-core edge dislocations. The rules for contrasts of basal-plane dislocations are summarized. It is shown that by observing those contrasts at fixed diffraction conditions, Burgers vectors of the basal-plane dislocation can be identified without performing a g?·?b analysis in some cases. Threading edge dislocations also have very characteristic contrasts depending on the angles between the projected g and their Burgers vectors. It is shown that Burgers vectors of threading edge dislocations can be determined uniquely by observing their characteristic contrasts without performing g?·?b analysis. Contrast mechanisms for these dislocations in grazing-incidence X-ray topography are discussed.     

Heavy-ion irradiations of Fe and Fe–Cr model alloys Part 2: Damage evolution in thin-foils at higher doses

A study of heavy-ion damage in Fe and Fe–Cr alloys started in Part 1 1 Yao, Z, Hernandez-Mayoral, M, Jenkins, ML and Kirk, MA. 2008. Phil. Mag., 88: 2851[Taylor & Francis Online] , [Google Scholar] was continued with an investigation of damage development in UHP Fe and Fe–8%Cr at higher doses up to 2 × 10¹⁹ ions m^?2 (～13 dpa). In thin-foil irradiations with 150 keV Fe⁺ ions at 300°C and room temperature (RT), more complex microstructures started to develop in thicker regions of the foils at doses greater than about 2 × 10¹⁸ ions m^?2, apparently involving cooperative interaction, alignment and coalescence of smaller loops. First strings of loops all with the same ½?111? Burgers vectors formed. In UHP Fe irradiated at 300°C the damage then developed into colonies of resolvable interstitial loops with ½?111? Burgers vectors. By a dose of 2 × 10¹⁹ ions m^?2, large (several hundred nanometre) finger-shaped loops with large shear components had developed by the growth and subsequent coalescence of smaller loops. Similar but finer-scale damage structures developed in UHP Fe irradiated at RT and in Fe–8%Cr irradiated at both RT and 300°C.     

Observations of glide and decomposition of a⟨101⟩ dislocations at high temperatures in Ni-Al single crystals deformed along the hard orientation

Ni-44 at.% Al and Ni-50 at.% Al single crystals were tested in compression in the hard d001 ¢orientation. The dislocation processes and deformation behaviour were studied as a function of temperature, strain and strain rate. A slip transition in NiAl occurs from a?111? slip to non-a?111? slip at intermediate temperatures. In Ni-50 at.% Al single crystals, only a?010? dislocations are observed above the slip transition temperature. In contrast, a a?101?{101} glide has been observed to control deformation beyond the slip transition temperature in Ni-44 at.% Al. a?101? dislocations are observed primarily along both ?111? directions in the glide plane. High-resolution transmission electron microscopy observations show that the core of the a?101? dislocations along these directions is decomposed into two a?010? dislocations, separated by a distance of approximately 2 nm. The temperature window of stability for these a?101? dislocations depends upon the strain rate. At a strain rate of 1.4 210^?4 s^?1, a?101? dislocations are observed between 800 and 1000 K. Complete decomposition of a?101? dislocations into a?010? dislocations occurs beyond 1000 K, leading to a?010? climb as the deformation mode at higher temperatures. At lower strain rates, decomposition of a?101? dislocations has been observed to occur along the edge orientation at temperatures below 1000 K. Embedded-atom method calculations and experimental results indicate that a?101? dislocations have a large Peierls stress at low temperatures. Based on the present microstructural observations and a survey of the literature with respect to vacancy content and diffusion in NiAl, a model is proposed for a?101?{101} glide in Ni-44 at.% Al, and for the observed yield strength versus temperature behaviour of Ni-Al alloys at intermediate and high temperatures.     

An approach to the elastic field of a hybrid dislocation

ABSTRACT

The transfer of plastic sliding through a crystalline interface involves at least a dislocation having a branch in each crystal. The elastic field associated with this elemental configuration has been processed in the past by Belov et al. (1983 A.Y.Belov, A.Chamrov, V.L.Indenbom and J.Lothe , Elastic fields of dislocations piercing the interface of an anisotropic bicrystal , Phys. Stat. Sol. (B) 119 (1983), pp. 565–578. Available at https://doi.org/10.1002/pssb.2221190216 .[Crossref] , [Google Scholar], 1992) but has never been verified or used, to the author’s knowledge. With typographical corrections and various verifications, the results obtained in this work confirm the validity of the theory for isotropic and/or anisotropic crystals. A general explicit solution to the elastic field is derived in the case of two different isotropic crystals. The theory fails when one branch is along the interface while the other lies in a crystal (hybrid dislocation). On the other hand, if a branch is very little inclined relative to the interface (quasi-hybrid dislocation), the theory applies fully. In this context, the combination of two quasi-hybrid dislocations solves in practice the problem of the triple node anchored to the interface.     

Ultrafast Soft X-ray Probing of Gas Phase Molecular Dynamics

The molecular ability to selectively and efficiently convert sunlight into other forms of energy like heat, bond change, or charge separation is truly remarkable. The decisive steps in these transformations often happen on a femtosecond timescale and require transitions among different electronic states that violate the Born-Oppenheimer approximation (BOA) [1 W. Domcke, D. Yarkony, and H. Köppel, Conical Intersections Electronic Structure, Dynamics &; Spectroscopy, World Scientific, Singapore (2004).[Crossref] , [Google Scholar]]. Non-BOA transitions pose challenges to both theory and experiment. From a theoretical point of view, excited state dynamics and nonadiabatic transitions both are difficult problems [2 F. Plasser, Theor. Chem. Acc. 131, 233902 (2012).[Crossref], [Web of Science ®] , [Google Scholar], 3 J. C. Tully, J. Chem. Phys. 137, 22A301 (2012).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]] (see Figure 1(a)). However, the theory on non-BOA dynamics has advanced significantly over the last two decades. Full dynamical simulations for molecules of the size of nucleobases have been possible for a couple of years [4 H. R. Hudock, J. Phys. Chem. A 111, 8500–8508 (2007).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar], 5 J. J. Szymczak, J. Phys. Chem. A 113, 12686–12693 (2009).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]] and allow predictions of experimental observables like photoelectron energy [6 S. Ullrich, Phys. Chem. Chem. Phys. 6, 2796 (2004).[Crossref], [Web of Science ®] , [Google Scholar]] or ion yield [7 H. Tao, J. Chem. Phys. 134, 244306 (2011).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]–9 M. Assmann, T. Weinacht, and S. Matsika, J. Chem. Phys. 144, 34301 (2016).[Crossref], [Web of Science ®] , [Google Scholar]]. The availability of these calculations for isolated molecules has spurred new experimental efforts to develop methods that are sufficiently different from all optical techniques. For determination of transient molecular structure, femtosecond X-ray diffraction [10 M. P. Minitti, Phys. Rev. Lett. 114, 255501 (2015).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar], 11 J. Küpper, Phys. Rev. Lett. 112, 083002 (2014). [Google Scholar]] and electron diffraction [12 J. Yang, Nat. Commun. 7, 11232 (2016).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]] have been implemented on optically excited molecules.     

Fourth User Workshop on High-Power Lasers at the Linac Coherent Light Source

The fourth international user workshop focusing on high-power lasers at the Linac Coherent Light Source (LCLS) was held in Menlo Park, CA, USA, on October 3–4, 2016 [1 R. Falcone, S. Glenzer, and S. Hau-Riege, Synchrotron Radiation News 27(2), 56–58 (2014).[Taylor &; Francis Online] , [Google Scholar]–3 C. Bolme, S. Glenzer, and A. Fry, Synchrotron Radiation News 29(2), 14–17 (2016).[Taylor &; Francis Online] , [Google Scholar]]. The workshop was co-organized by Los Alamos National Laboratory and SLAC National Accelerator Laboratory (SLAC), and garnered the attendance of more than 110 scientists. Participants discussed the warm dense matter and high-pressure science that is being conducted using high-power lasers at the LCLS Matter in Extreme Conditions (MEC) endstation. During the past year, there have been seven journal articles published from research at the MEC instrument [4 L. B. Fletcher et al., Review of Scientific Instruments 87(11), 11E524 (2016).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]–10 P. Sperling et al., Physical Review Letters 115(11), 115001 (2015).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]]. The specific topics discussed at this workshop were experimental highlights from the past year, current status and future commissioning of MEC capabilities, and future facility upgrades that will enable the expanded science reach of the facility.     

Simulation and Experimental Study of Wavefront Measurement Accuracy of the Pencil-Beam Method

Ultra-bright and high-coherence X-rays are now being used in synchrotron radiation facilities and X-ray free electron laser facilities. X-ray focusing techniques are essential to take full advantage of these excellent X-ray light sources. To meet the strong demand, high-quality X-ray focusing optics have been developed owing to the advancement of ultraprecision machining and measurement. State-of-the-art refractive lenses [1 C.G. Schroer, Applied Physics Letters 87, 124103 (2005).[Crossref], [Web of Science ®] , [Google Scholar]], zone plates [2 T. Chen, Optics Express 19, 19919 (2011).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]], and Laue lenses [3 H. Yan, Optics Express 19, 15069 (2011).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]] can be used to achieve X-ray focusing to a spot a few tens of nanometers.     

Femtosecond Structural Dynamics of Proteins

Proteins are the workhorses of living cells, providing essential functions such as structural support, signal transduction, enzymatic catalysis, transport and storage of small ligands. Atomic-resolution structures obtained with conventional X-ray crystallography show proteins essentially as static. In reality, however, proteins move and their motion is crucial for functioning. Although the structure and dynamics of proteins are intimately related, they are not equally well understood. A very large number of protein structures have been determined, but only a few studies have been able to monitor experimentally the dynamics of proteins in real time. In the last two decades, the availability of short (~100 ps) and intense (~10⁹–10¹⁰ photons) X-ray pulses produced by third-generation synchrotrons have allowed the implementation of structural methods like time-resolved X-ray crystallography and time-resolved X-ray solution scattering that have allowed us to monitor protein motion in the nanosecond-to-millisecond timescale [1 K. Moffat, Chem. Rev. 101, 1569–1582 (2001).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]–4 J. G. Kim, Acc. Chem. Res. 48, 2200–2208 (2015).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]]. Time-resolved X-ray crystallography has been used to monitor processes such as the migration of a ligand from the protein active site to the surrounding solvent [5 V. Srajer, Science 274, 1726–1729 (1996).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]–7 D. Bourgeois, Proc. Natl. Acad. Sci. U. S. A. 100, 8704–8709 (2003).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]] or tertiary structural changes associated with allosteric transitions [8 J. E. Knapp, Proc. Natl. Acad. Sci. 103, 7649–7654 (2006).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar], 9 J. G. Kim, Struct. Dyn. 3, 023610 (2016). [Google Scholar]]. On the other hand, time-resolved X-ray scattering in the so-called wide-angle X-ray scattering (WAXS) region [10 M. Cammarata, Nat. Methods 5, 881–886 (2008).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]] has been used to track conformational changes corresponding to large-amplitude protein motions such as the quaternary R-T transition of human hemoglobin [11 M. Cammarata, J. Mol. Biol. 400, 951–962 (2010).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]–13 A. Spilotros, Soft Matter 8, 6434–6437 (2012).[Crossref], [Web of Science ®] , [Google Scholar]], the relative motion of bacteriorhodopsin α-helices following retinal isomerization [14 M. Andersson, Structure 17, 1265–1275 (2009).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]], or the open-to-close transition in bacterial phytochromes [15 H. Takala, Nature 509, 245–8 (2014).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]].     

Serial Femtosecond Crystallography

X-ray free-electron lasers produce brief flashes of X-rays that are of about a billion times higher peak brightness than achievable from storage ring sources. Such a tremendous jump in X-ray source capabilities, which came in 2009 when the Linac Coherent Light Source began operations, was unprecedented in the history of X-ray science. Protein structure determination through the method of macromolecular crystallography has consistently benefited from the many increases in source performance from rotating anodes to all generations of synchrotron facilities. But when confronted with the prospects of such bright beams for structural biology, enthusiastic proposals were tempered by trepidation of the effects of such beams on samples and challenges to record data [1 M. Wilmanns, J. Synchr. Rad. 7, 41 (2000).[Crossref], [PubMed] , [Google Scholar]]. A decade after these discussions (and others in the USA) on the applications of X-ray FELs for biology, the first experiments took place at LCLS, giving results that fulfilled many of the dreams of the early visionaries. In particular, the concept that diffraction representing the pristine object could be recorded before the X-ray pulse completely vaporizes the object was validated [2 H.N. Chapman, Nature 470, 73 (2011).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]], confirming predictions [3 R. Neutze, Nature 406, 753 (2000).[Crossref], [Web of Science ®] , [Google Scholar]] that established dose limits could be vastly exceeded using femtosecond-duration pulses. The first experiments illuminated a path to achieve room-temperature structures free of radiation damage, from samples too small to provide useful data at synchrotron facilities, as well as providing the means to carry out time-resolved crystallography at femtoseconds to milliseconds. In the five years since, progress has been substantial and rapid, invigorating the field of macromolecular crystallography [4 J.C.H. Spence and H.N. Chapman, Phi. Trans. Roy. Soc. B 369, 20130309 (2014).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar], 5 I. Schlichting, IUCrJ 2, 246 (2015).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]]. This phase of development is far from over, but with both the LCLS and the SPring-8 Ångström Compact Free-electron Laser (SACLA) providing facilities for measurements, the benefits of X-ray FELs are already being translated into new biological insights.     

Ultrafast Electronic and Molecular Dynamics Induced by Ultrashort FEL Pulses

In recent years, short wavelength free electron lasers (FELs) have opened up access to ultrafast electronic and structural dynamics in matter. Currently, four FEL facilities are in operation in the world. FLASH [1 W. Ackermann, Nat. Photonics 1, 336 (2007).[Crossref], [Web of Science ®] , [Google Scholar]] in Germany and FERMI [2 P. Emma, Nat. Photonics 4, 641 (2010).[Crossref], [Web of Science ®] , [Google Scholar]] in Italy cover the range from extreme ultraviolet (EUV) to soft X-rays, while LCLS [3 T. Ishikawa, Nat. Photonics 6, 540 (2012).[Crossref], [Web of Science ®] , [Google Scholar]] in the U.S. and SACLA [4 E. Allaria, Nat. Photonics 6, 699 (2012).[Crossref], [Web of Science ®] , [Google Scholar]] in Japan provide pulses in the hard X-ray regime. In addition, an upgrade version of SCSS [5 T. Shintake, Nat. Photonics 2, 555 (2008).[Crossref], [Web of Science ®] , [Google Scholar]], nicknamed SCSS+, has also just started user operation as a beamline of SACLA [6 See http://xfel.riken.jp/eng/users/index.html [Google Scholar]]. These FELs deliver coherent pulses combining unprecedented power densities up to ~10²⁰ W/cm² and extremely short pulse durations down to a few femtoseconds. The intense coherent FEL pulse focused down to ~1 μm² makes single-shot diffractive imaging of nano-crystals or even non-crystallized bio-samples as well as other small objects a reality. Time-resolved spectroscopic and structural studies on the timescale of femtoseconds, having FEL pulses as a probe, allow us to probe electrons and atoms in action. Additionally, since FEL pulses are in a new regime of intensity, they are opening up new research fields that exploit the interaction between intense short wavelength pulses and matter, leading to matter at extremely high energy. Relevant theories dealing with such extreme conditions are also rapidly growing.     

Seeded Multicolor FEL Pulses: Status and Future Plans

The advent of FEL sources delivering two synchronized pulses of different wavelengths has made available a whole range of novel pump-probe experiments [1 E. Ferrari, Nat. Commun. 7, 10343 (2016).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]], allowing the exploration of the dynamics of matter driven to extreme non-equilibrium states by an intense ultrashort X-ray pulse and then probing the sample response at variable time delay with a second pulse [2 E. Allaria, Nat. Commun. 4, 2476 (2013).[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]].