Directions In Crystal Lattice Research Articles

Reconstruction of martensite variant microstructures in grains of deformed NiTi shape memory alloy by TEM

Two transmission electron microscopy (TEM) based methods were applied to reconstruct martensite variant microstructures in entire grains of plastically deformed nanocrystalline NiTi wires. First method consists in manual TEM analysis of composite electron diffraction patterns and dark field images of microstructures taken using carefully selected Bragg spots. Second method involves automated orientation/phase mapping of martensite variant microstructures in TEM (ASTAR). Application of these two methods helped us to discover new mechanism of the plastic deformation of monoclinic B19’ martensite in NiTi shape memory alloys called kwinking as it combines dislocation slip based kinking and deformation twinning in martensite.In this work, we describe how the martensite variant microstructures in nanograins of plastically deformed NiTi shape memory wire are reconstructed using the TEM based methods and discuss their advantages and disadvantages. The application of the first manual method requires tilting the TEM lamella so that all martensite variants in the analysed grain are oriented in a common low index zone. Such common crystal lattice direction exists in each grain of plastically deformed martensitic NiTi wire because the microstructures were created by the kwinking deformation mechanism. The requirement to orient the selected grain into a common unique direction in monoclinic lattice, on the other hand, is not unique to kwinking and represents a severe limitation of the SAED-DF method due to the constraints on tilting the TEM lamella. Application of the automated crystal orientation mapping ASTAR method enables detailed orientation analysis of nanoscale martensite variant microstructures in generally oriented grains of deformed NiTi wires as well as pointwise ex-situ analysis of interfaces with excellent resolution in real and orientation space.

NbCX (X = F, Cl, Br, I) with Highly Anisotropic Optical, Mechanical and Electric Transport Properties

Two-dimensional materials with anisotropic properties (for example, mechanical, optical, and electrical transmission anisotropy) have been sought for a long time, especially two-dimensional materials with strong electrical transport anisotropy. Here, we report a class of two-dimensional NbCX(X=F,Cl,Br,I) monolayers with stable performance. Using density functional theory combined with the method of non-equilibrium Green's function, we found that the optical absorption characteristics of two-dimensional NbCX are highly dependent on the crystal lattice direction. The calculated electrical transport properties also show that the NbCX monolayer exhibits strong anisotropy in the bias voltage range of 0.1∼2.0 V. NbCX prefers to transmit current in the a direction, which can be orders of magnitude up to µA. The anisotropy, adjustable band gap and unique transport properties of two-dimensional NbCX can have potential applications in nano-optoelectronics.

Momentum-resolved superconducting energy gaps of Sr2RuO4 from quasiparticle interference imaging

Sr2RuO4 has long been the focus of intense research interest because of conjectures that it is a correlated topological superconductor. It is the momentum space (k-space) structure of the superconducting energy gap [Formula: see text] on each band i that encodes its unknown superconducting order parameter. However, because the energy scales are so low, it has never been possible to directly measure the [Formula: see text] of Sr2RuO4 Here, we implement Bogoliubov quasiparticle interference (BQPI) imaging, a technique capable of high-precision measurement of multiband [Formula: see text] At T = 90 mK, we visualize a set of Bogoliubov scattering interference wavevectors [Formula: see text] consistent with eight gap nodes/minima that are all closely aligned to the [Formula: see text] crystal lattice directions on both the α and β bands. Taking these observations in combination with other very recent advances in directional thermal conductivity [E. Hassinger et al., Phys. Rev. X 7, 011032 (2017)], temperature-dependent Knight shift [A. Pustogow et al., Nature 574, 72-75 (2019)], time-reversal symmetry conservation [S. Kashiwaya et al., Phys. Rev B, 100, 094530 (2019)], and theory [A. T. Rømer et al., Phys. Rev. Lett. 123, 247001 (2019); H. S. Roising, T. Scaffidi, F. Flicker, G. F. Lange, S. H. Simon, Phys. Rev. Res. 1, 033108 (2019); and O. Gingras, R. Nourafkan, A. S. Tremblay, M. Côté, Phys. Rev. Lett. 123, 217005 (2019)], the BQPI signature of Sr2RuO4 appears most consistent with [Formula: see text] having [Formula: see text] [Formula: see text] symmetry.

Microscale and Mesoscale Crystallographic Textures of Nanocrystalline Ni-Based Electrodeposits

The microscale and mesoscale crystallographic textures observed in nanocrystalline Ni, Ni-20 pct Fe, and Ni-50 pct Fe electrodeposits are described. The nanosized grains are arranged in coarse mesoscale colonies. In the as-deposited state, the bulk texture of the Ni-20 pct Fe alloy displays a dominant 〈001〉 fiber parallel to the macroscopic deposition direction (DD). The grains are elongated along the 〈001〉 crystal lattice direction, which is mostly parallel to the local DD, producing a well-defined 〈001〉//DD fiber microtexture on a local scale. The grain misorientation histograms show some content of low-angle boundaries, frequently associated with the presence of grain clusters, and are dominated by high-angle boundaries with a possible enhanced frequency of ∑5 and ∑7 coincident site lattice boundaries and a significant content of ∑3 twin boundaries. For all three alloys, the coarsened grains, obtained by annealing, within the mesoscale colonies show a fiber mesotexture characterized by a 〈111〉 axis approximately perpendicular to the colony hemispherical growth surface (parallel to the local DD). It is surmised that a similar “cobblestone” mesotexture with a 〈001〉 fiber axis already exists in the as-received state, as previously proposed by a number of the present authors and as supported by the results on the Ni-20 pct Fe alloy presented here.

Azimuthal dependence and accurate determination of the half-angle in RBS channeling

Rutherford Backscattering Channeling (RBSC) is often used in the studies of single crystalline materials with regard to issues hinging upon the location of specific atomic species within the crystal lattice. Results are deduced from the characteristics of an angular scan curve representing the yield of close collision events from the constant scattering of the probing projectiles from the crystalline sample in a sequence of directions lying in a plane perpendicular to one particular crystal lattice direction. Such angular scans exhibit dips near major channels, and their angular widths are of concern. On a fundamental level, accurate determination of these angular widths in the case of axial channeling and their interpretation come up against uncertainties arising out of the absence of a preferred choice for the plane of scan. Here, we shall illustrate these uncertainties in a representative case, channeling of 2 MeV He + ions in the 〈1 0 0〉 direction of Si.

Proton dynamics in superprotonicCsHSO4

We present a detailed study of proton dynamics in the hydrogen-bonded superprotonic conductor $\mathrm{Cs}\mathrm{H}\mathrm{S}{\mathrm{O}}_{4}$ from first-principles molecular dynamics simulations, isolating the subtle interplay between the dynamics of the O---H chemical bonds, the O\ensuremath{\cdots}H hydrogen bonds, and the $\mathrm{S}{\mathrm{O}}_{4}$ tetrahedra in promoting proton diffusion. We find that the Grotthuss mechanism of proton transport is primarily responsible for the dynamics of the chemical bonds, whereas the reorganization of the hydrogen-bond network is dominated by rapid angular hops in concert with small reorientations of the $\mathrm{S}{\mathrm{O}}_{4}$ tetrahedra. Frequent proton jumping across the O---H\ensuremath{\cdots}O complex is countered by a high rate of jump reversal, which we show is connected to the dynamics of the $\mathrm{S}{\mathrm{O}}_{4}$ tetrahedra, resulting in a diminished $\mathrm{Cs}\mathrm{H}\mathrm{S}{\mathrm{O}}_{4}∕\mathrm{Cs}\mathrm{D}\mathrm{S}{\mathrm{O}}_{4}$ isotope effect. We also find evidence of multiple time scales for $\mathrm{S}{\mathrm{O}}_{4}$ reorientation events, leading to distinct diffusion mechanisms along the different crystal lattice directions. Finally, we employ graph-theoretic techniques to characterize the topology of the hydrogen-bond network and demonstrate a clear relationship between the likelihood for diffusive jump events and the presence of connectivity configurations favoring linear network chains over closed network rings.

Point-defect dynamics in two-dimensional colloidal crystals

We study the topological configurations and dynamics of individual point defect vacancies and interstitials in a two-dimensional crystal of colloids interacting via a repulsive Yukawa potential. Our Brownian dynamics simulations show that the diffusion mechanism for vacancy defects occurs in two phases. The defect can glide along the crystal lattice directions, and it can rotate during an excited topological transition configuration to assume a different direction for the next period of gliding. The results for the vacancy defects are in good agreement with recent experiments. For interstitial point defects, which were not studied in the experiments, we find several of the same modes of motion as in the vacancy defect case along with two additional diffusion pathways. The interstitial defects are more mobile than the vacancy defects due to the more two-dimensional nature of the diffusion of the interstitial defects.

Hyperfine interactions in the heavy fermion CeMIn5 systems

The CeMIn5 heavy fermion compounds have attracted enormous interest since their discovery six years ago. These materials exhibit a rich spectrum of unusual correlated electron behaviour, and may be an ideal model for the high-temperature superconductors. As many of these systems are either antiferromagnets, or lie close to an antiferromagnetic phase boundary, it is crucial to understand the behaviour of the dynamic and static magnetism. Since neutron scattering is difficult in these materials, often the primary source of information about the magnetic fluctuations is nuclear magnetic resonance. Therefore, it is crucial to have a detailed understanding of how the nuclear moments interact with conduction electrons and the local moments present in these systems. Here, we present a detailed analysis of the hyperfine coupling based on anisotropic hyperfine coupling tensors between nuclear moments and local moments. Because the couplings are symmetric with respect to bond axes rather than crystal lattice directions, the nuclear sites can experience non-vanishing hyperfine fields even in high symmetry sites.

Number-theoretic method for practical indexing of crystal directions

Abstract The paper presents new number-theoretic approaches to the solution of orientation problems in single crystal studies. A fast method for indexing any rational crystal lattice direction was developed to be a new 3D generalization of the Euclid algorithm. Based on the latter, a method for indexing directions in any crystal lattice was developed, where the practical problem is reduced to the search of the shortest primitive lattice vector within a certain angular error for the direction in analysis. A multipurpose personal computer system for testing both real and reciprocal lattices of familiar crystals is devised. The compact-in-memory software represents a principally new “bottomless” atlas of any crystal stereographic projections, where each point of the projection field is indexed within a desired accuracy.

Effect of compression force on the crystal properties of erythromycin acistrate tablets

The crystal properties of compressed and powdered erythromycin acistrate tablets were studied by the X-ray powder diffraction (XRPD) method. Detailed analysis of X-ray powder diffraction line profiles was performed. Diffraction peak intensities and full width at half maximum (FWHM) values of the peaks corresponding to three different crystal lattice directions were determined. Crystallite size was calculated by Scherrer's equation using the data of integral breadth of the peaks. The preferred orientation of the crystallites is also discussed. According to the results, the crystallite size increased on the tablet surface after a small compression force (4 kN) in all crystal lattice directions studied. Even small compression forces caused recrystallization. With higher compression forces (8–18 kN) the crystallite size and the FWHM values remained rather constant. After the compression force of 18 kN the peaks in different crystal lattice directions behaved differently. In the lattice directions of diffraction maxima 2 and 3, the effect was the same with the small (4 kN) and the high compression force (22 kN). Further recrystallization occurred with 22 kN. However, in the crystal lattice direction of diffraction maximum 1 at the compression force of 8 kN the crystallite broke and crystallinity decreased. These were not seen in the powdered tablet samples. It could be concluded that the effect of compression force on the crystal properties of erythromycin acistrate tablets was seen on the tablet surface but not in the powdered tablets. Compression force also affected the preferred orientation of crystallites on the tablet surface and especially in the lattice direction of diffraction maximum 3. This was not seen in the powdered tablets.

Hopping rates of H(D) bound to heavy interstitials in Ta as investigated by mechanical spectroscopy at MHz frequencies

The stress-induced reorientation of O(N)–H(D) pairs has been investigated at MHz frequencies in Ta by propagating longitudinal ultrasonic pulses along a <111> crystal lattice direction. The resulting anelastic relaxation is found to be composed of two processes which are attributed to two distinct configurations of the pair. A comparison of the transition rates of free and trapped H(D) atoms in Nb and Ta suggests that both processes observed in Ta involve transitions of H(D) atoms between equivalent nn sites, which are probably slightly displaced from normal tetrahedral sites (dis-1T↔dis-1T transitions). In Nb bound H(D) atoms should be in a state delocalized over two equivalent sites, probably slightly displaced from normal tetrahedral sites (dis-2T state), and the reorientation in this metal would take place via (dis-2T↔dis-2T) transitions.

X-ray photoelectron spectroscopy and calculation of the electronic structure in iron based alloys under phase transformation

An experimental method which allows us to survey alterations in the X-ray photoelectron spectra under ▪- to ▪- transition is suggested. The densities of states of valence electrons calculated in the multi-band coherent potential approximation (CPA) reflect general features of XPS measurements. Our calculations have allowed us to determine contributions from electrons which occupy e g- and t 2g- orbitals, oriented along different directions in crystal lattice to the cohesive energy.

Grain Boundaries in Metals

ACCORDING to the transition lattice theory, since in pure metals and single-phase alloys the only difference between the two grains which meet at a boundary is one of direction, the atoms at the boundary take up positions representing a compromise between the two crystal lattice directions. There thus exists a region, a few atomic diameters in thickness, over which a state of disorder exists, the extent of which will be expected to depend upon the relative orientation of the two lattices; there will exist angles for which this disorder is a minimum, an obvious example being the twinning angle. In addition, the direction of the boundary itself relative to the grains would be expected to produce an effect. Lateral misalignment of crystal planes would also be expected to produce ‘lack of fit’ at the grain boundaries, but this effect is most probably smoothed out by the imperfections in the crystal structure. It would be expected, therefore, that any phenomenon depending upon the degree of disorder existing at the boundary would vary in magnitude according to the relative orientations of the grains meeting at the boundary, and with position in the boundary between two given crystal grains if the boundary changed in direction. It is reasonable to suppose that where precipitation of a second phase occurs from the supersaturated solid solution, it occurs more readily in those regions where disorder is greatest, and it would, therefore, be expected that grain boundary precipitation would vary in this manner.