Basic Stacking Research Articles

The Zintl Concept Applied to Intergrowth Structures: Electron‐Hole Matching, Stacking Preferences, and Chemical Pressures in Pd5InAs

AbstractEnumerating the potential stacking sequences of layers is a fundamental way to account for the structure diversity of solid state compounds. In many cases, these stacking variations represent polymorphs with only small energetic differences. Here, we examine a compound for which the preferred stacking pattern instead reveals key aspects about its chemical bonding: Pd5InAs. Its structure is based on the intergrowth of slabs of the AuCu3 and PtHg2 (or alternatively, fluorite) structure types. Two basic stacking arrangements are available to this compound, represented by the Pd5TlAs and HoCoGa5 structure types. DFT total energy calculations reveal that the former outcompetes the latter by a staggering 0.65 eV/formula unit. Through a combination of DFT‐reversed approximation Molecular Orbital (DFT‐raMO) and DFT‐Chemical Pressure (DFT‐CP) analysis we trace this preference to two factors. First, with DFT‐raMO analysis, we derive a Zintl‐like bonding scheme of Pd5InAs. This scheme, along with the inspection of selected crystal orbitals, is then connected to preferred stacking through the coordination environments of the Pd atoms at the interface between the Pd−In and Pd−As layers. In the hypothetical HoCoGa5‐type and observed Pd5InAs‐type structures, different Pd coordination environments arise at the interfaces. The hypothetical structure features square planar PdIn2As2 units, in each of which the same 4d‐orbital serves in the Pd sublattice's role as both Lewis acid (for interactions with the As) and Lewis base (for interactions with the In). In the observed structure, tetrahedral PdIn2As2 units occur instead, so that these contradictory roles are distributed to separate 4d‐orbitals, leading to more effective bonding. DFT‐CP analysis illustrates that this driving force for the Pd5TlAs‐type arrangement is supplemented by a favorable alignment of the packing tensions in the parent structures. Altogether, the resulting picture demonstrates how the reaction of simple intermetallic structures to form intergrowths can be guided by recognizable chemical interactions.

Maximum Stacking Base Pairs: Hardness and Approximation by Nonlinear Linear Programming-Rounding.

Maximum stacking base pairs is a fundamental combinatorial problem from ribonucleic acid (RNA) secondary structure prediction under the energy model. The basic maximum stacking base pairs problem can be described as: given an RNA sequence, find a maximum number of base pairs such that each chosen base pair has at least one parallel and adjacent partner (i.e., they form a stacking). This problem is NP-hard, no matter whether the candidate base pairs follow the biology principle or are given explicitly as input. This article investigates a restricted version of this problem where the base pairs are given as input and each base is associated with at most k (a constant) base pairs. We show that this restricted version is still APX-hard, even if the base pairs are weighted. Moreover, by a nonlinear LP-rounding method, we present an approximation algorithm with a factor [Formula: see text]. Applying our algorithms on the simulated data, the actual approximation factor is in fact much better than this theoretical bound.

Three-Dimensional Blade Stacking Strategies and Understanding of Flow Physics in Low-Pressure Steam Turbines—Part II: Stacking Equivalence and Differentiators

Optimization of blade stacking in low-pressure (LP) steam turbine development constitutes one of the most delicate and time-consuming parts of the design process. This is the second part of two papers focusing on stacking strategies applied to the last stage guide vane and represents an attempt to discern the aerodynamic targets that can be achieved by each of the well-known and most often used basic stacking schemes. The effects of lean and twist have been investigated through an iterative process, involving comprehensive 3D computational fluid dynamics (CFD) modeling of the last two stages of a standard LP, where the basic lean and twist stacking schemes were applied on the last stage guide vanes while keeping the throat area (TA) unchanged. It has been found that it is possible to achieve the same target value and pattern of stage reaction by applying either tangential lean or an equivalent value of twist. Moreover, the significance of axial sweep on hub reaction has been found to become pronounced when the blade sweep is carried out at constant TA. The importance of hub-profiling has also been demonstrated and assessed. Detailed analysis of the flow fields has provided an overall picture, revealing the differences in the main flow parameters as produced by each of the alternative basic stacking schemes.

On the mechanism of hardening wurtzitic boron nitride under an indenter

The possibility of hardening wurtzitic boron nitride through plastic shears along the (0001) basal plane in the indentation normal to this plane has been considered based on the analysis of the crystal geometry of wurtzitic lattice. It has been concluded that the hardening may be caused by the formation of basic stacking faults (as interlayers of BN cubic phase) as a result of plastic shears directly in double close packed layers of the wurtzite lattice and appearance of stresses of the dimensional mismatch in parameters between the lattices of the wurtzitic and cubic phase, which forms from the wurtzitic one due to the crystaloriented layer-by-layer transformation of the cubic phase.

Dimeric aggregates of five-coordinated methyl(phthalocyaninato)Rh(III): 1H NMR evidence for staggered and slipped cofacial dimers

The 1 H NMR spectra of the five-coordinated rhodium phthalocyanine complex Me ( RPc ) Rh (III) (1, RPc = dianion of 1,4,8,11,15,18,22,25-octa-n-pentylphthalocyanine) in toluene indicate that it exists as a discrete monomer at low concentrations. The complex forms a dimer aggregate reversibly in the concentration range of 2.6–10.2 mM at -90 < T < 22 ° C . The dimerization constants at -60, -80, and -90 °C are 2.0 × 102, 5.7 × 102, and 1.0 × 103 M -1, respectively. In chloroform, aggregation of the complex occurs at ~0.3 mM, reflecting the lower solvating power of chloroform for this complex. The 1 H NMR spectra of the chloroform solutions indicate that at concentrations greater than 1.0 mM, two discrete dimers in the ratio of 87:13 are reversibly formed. The 1 H NMR spectra of the major isomer, 2a, suggest that it is the 45 °-staggered cofacial dimer, a dimer with a D4 d symmetry. The 1 H NMR spectra of the minor isomer, 2b, suggest that it is a slipped cofacial dimer with a C2 h symmetry, which is formed by slipping a ( RPc ) Rh plane of the eclipsed dimer (a D4 h dimer) along a line connecting two opposing meso nitrogens of ( RPc ) Rh . Interestingly, the solution conformation of 2b is similar to that of the basic stacking unit found in the crystal structure of 1.

Investigation of boron-enriched Cottrell atmospheres in FeAl on an atomic scale by three-dimensional atom-probe field-ion microscopy

In 1949, on the basis of theoretical considerations, Cottrell proposed the concept of 'atmospheres' (called later by his name) to explain some specific behaviour of materials during plastic deformation, such as sharp yield-point formation or the Portevin-LeChatelier effect. In this letter, atomic-scale observations and three-dimensional analyses of a Cottrell atmosphere are reported. They have been performed by three-dimensional atom-probe field-ion microscopy techniques. The ability of this new experimental method to provide atomic-resolution images, both structural and chemical, was confirmed; the basic stacking structure of (001) planes in FeAl could be visualized with success. Moreover the presence of a <001> edge dislocation was also detected in the analysed zone. Further, B enrichment was measured in the vicinity of this defect; the B-rich region appeared as a pipe 5 nm in diameter, parallel to the dislocation line. The concentration of B in the core reached 3 at.%; this local enrichment in boron was accompanied by an Al depletion of more than 10 at.%. Boron in FeAl has a well known tendency to segregate to internal interfaces. In this letter, we show experimental evidence of the solute segregation to dislocation lines. The observed effects of this segregation on mechanical properties of FeAl, both at room temperature and high temperatures, are also discussed.

Changes in structure and electronic state from C60 fullerene to amorphous diamond.

Changes in electronic state and structure from ${\mathrm{C}}_{60}$ fullerene to amorphous diamond under shock compression were examined by electron energy-loss spectroscopy and electron diffractometry for the recovered samples from the 50 to 55 GPa compressions. Three transient states were distinguished. In the first transient state, the fcc structure was almost unchanged, and the \ensuremath{\pi} plasmon was prohibited owing to decreasing intercluster distance. In the second state, the basic stacking structure of clusters was barely retained, and intercluster bondings were generated. In the third state, the fullerene structure collapsed to be amorphous, while ${\mathit{sp}}^{3}$ bonding was partly produced. Finally, complete ${\mathit{sp}}^{3}$ bonds were entirely reproduced, whereas the diamond structure in the long-range order was not reconstructed.

Current Status of Planar SOFC Development at Tokyo Gas

Planar single cells of 5 × 5, 10 × 10, and 23 × 23 cm2 have been successfully manufactured and tested. The newly devised process for carefully controlling the electrode microstructure drastically reduced the interfacial resistance, which led to a high power density of 0.67 W/cm2 in 5 × 5 cm2 cells and a high total power of 97 W in 23 × 23 cm2 cells. Basic stacking technology has been established through fabricating and testing ten-cell stacks of 5 × 5 cm2 cells and 25-cell stacks of 10 × 10 cm2 cells. A total power of as high as 420 W was generated in the 25-cell stack. A brief description of fundamental research topics is also given.