Tetrahedrally Close-packed Research Articles

Architecture Effects in Complex Spherical Assemblies of (AB)n-Type Block Copolymers.

Molecular architecture plays a key role in the self-assembly of block copolymers, but few studies have systematically examined the influence of chain connectivity on tetrahedrally close-packed (TCP) sphere phases. Here, we report a versatile material platform comprising two blocks with substantial conformational asymmetry, A = poly(trifluoroethyl acrylate) and B = poly(dodecyl acrylate), and use it to compare the phase behavior of AB diblocks, ABA triblocks, and (AB)n radial star copolymers with n = 3 or 4. Each architecture forms TCP sphere phases at minority A block compositions (fA < 0.5), namely, σ and A15, but with differences in the location of order-order phase boundaries that are not anticipated by mean-field self-consistent field theory simulations. These results expand the palette of polymer architectures that readily self-assemble into complex TCP structures and suggest important design considerations when targeting specific phases of interest.

Mn39Si9Nx: Epitaxial Stabilization as a Pathway to the Formation of Intermetallic Nitrides.

The realization of the full potential of nitrogen-containing solid-state materials is limited by the inert and gaseous nature of N2. In this Communication, we describe the simple synthesis yet complex structure of the new phase Mn39Si9Nx (x = 0.84). The formation of this intermetallic subnitride appears to be facilitated by the high solubility of nitrogen in manganese metal, while its structural features are guided by the complementary internal packing strains of Mn-Si and Mn-N domains, an effect known as epitaxial stabilization. These domains intergrow into a composite structure based on the interpenetration of tetrahedrally close-packed (TCP) and Mackay cluster-like modules. We anticipate that other systems combining nitrogen with the TCP packing of metals will be similarly driven toward intergrowth, opening a path to a broader family of intermetallic nitrides.

Synthesis and Self-Assembly of ABn Miktoarm Star Polymers

The stability of tetrahedrally close-packed (TCP) phases in block copolymer melts is predicted by theory to depend on molecular architecture, yet no experimental studies to date have probed its effect. Motivated by this open question, here we report an efficient synthesis of asymmetric ABn miktoarm star polymers using functionalized sugars as cores for orthogonal grafting-from block copolymerizations. A combination of ring-opening and atom transfer radical polymerization produced model low dispersity materials comprising a single A = poly(lactide) (L) and multiple B = poly(dodecyl acrylate) (D) arms that amplify "conformational asymmetry" through two concerted effects: the mikto architecture and disparate block statistical segment lengths. Analyzing the self-assembly of LD2 and LD3 samples resulted in the discovery of two TCP phases, σ and A15, that remained stable to significantly higher A-block volume fractions as the number of B arms increased. These results experimentally establish the importance of conformational asymmetry and molecular architecture as powerful design tools for the self-assembly of block copolymers into nonclassical phases.

On the Functionality of Complex Intermetallics: Frustration, Chemical Pressure Relief, and Potential Rattling Atoms in Y11Ni60C6.

Intermetallic carbides provide excellent model systems for exploring how frustration can shape the structures and properties of inorganic materials. Combinations of several metals with carbon can be designed in which the formation of tetrahedrally close-packed (TCP) intermetallics conflicts with the C atoms' requirement of trigonal prismatic or octahedral coordination environments, as offered by the simple close-packings (SCP) of equally sized spheres. In this Article, we explore the driving forces that lead to the coexistence of these incompatible arrangements in the Yb11Ni60C6-type compound Y11Ni60C6 (cI154), as well as potential consequences of this intergrowth for the phase's physical properties. Our focus begins on the structure's SCP regions, which appear as C-stuffed versions of a AuCu3-type YNi3 phase that is not observed on its own in the Y-Ni system. DFT-Chemical Pressure (DFT-CP) calculations on this hypothetical YNi3 phase reveal large negative pressures within the Ni sublattice, as it is stretched to accommodate the size requirements of the Y atoms. In the Y11Ni60C6 structure, two structural mechanisms for addressing these CP issues appear: the incorporation of interstitial C atoms, and the presence of interfaces with CaCu5-type domains. The relative roles of these two mechanisms are investigated with the CP analysis on a hypothetical YNi3Cx series of C-stuffed AuCu3-type phases, the Y-Ni sublattice of Y11Ni60C6, and finally the full Y11Ni60C6 structure. Through these calculations, the C atoms appear to play the roles of relieving positive Y CPs and supporting relaxation at the AuCu3-type/CaCu5-type interfaces, where the cancellation occurs between opposite CPs experienced by the Y atoms in the two parent structures (following the epitaxial stabilization mechanism). The CP analysis of Y11Ni60C6 also highlights a sublattice of Y and Ni atoms with large negative CPs (and thus the potential for soft vibrational modes), illustrating how frustrated structures could lead to the full realization of the phonon glass-electron crystal concept.

Epitaxial Stabilization between Intermetallic and Carbide Domains in the Structures of Mn16SiC4 and Mn17Si2C4.

The concept of frustration between competing geometrical or bonding motifs is frequently evoked in explaining complex phenomena in the structures and properties of materials. This idea is of particular importance for metallic systems, where frustration forms the basis for the design of metallic glasses, a source of diverse magnetic phenomena, and a rationale for the existence of intermetallics with giant unit cells containing thousands of atoms. Unlike soft materials, however, where conflicts can be synthetically encoded in the molecular structure, staging frustration in the metallic state is challenging due to the ease of macroscopic segregation of incompatible components. In this Article, we illustrate one approach for inducing the intergrowth of incompatible bonding motifs with the synthesis and characterization of two new intermetallic carbides: Mn16SiC4 (mC42) and Mn17Si2C4 (mP46). Similar to the phases Mn5SiC and Mn8Si2C in the Mn-Si-C system, these compounds appear as intergrowths of Mn3C and tetrahedrally close-packed (TCP) regions reminiscent of Mn-rich Mn-Si phases. The nearly complete spatial segregation of Mn-Si (intermetallic) and Mn-C (carbide) interactions in these structures can be understood from the differing geometrical requirements of C and Si. Rather than macroscopically separating into distinct phases, though, the two bonding types are tightly interwoven, with most Mn atoms being on the interfaces. DFT chemical pressure analysis reveals a driving force stabilizing these interfaces: the major local pressures acting between the Mn atoms in the Mn-Si and Mn-C systems are of opposite signs. Joining the intermetallic and carbide domains together then provides substantial relief to these local pressures, an effect we term epitaxial stabilization.

Aperiodic Crystals at the Mesoscale

Materials with mesoscale structural characteristics have attracted great attention across the fields of chemistry, physics, and materials science. A typical example is mesoporous silica, which are synthesized in water/surfactant/silica systems, and has well-defined mesopores resulting in high surface area. Mesoporous silicas have two defining structural characteristics: (i) disorder at the atomic scale, i.e. only short-range order; and (ii) distinct order at the mesoscale, i.e. long-range order. Atomic-scale structural characterization by common diffraction techniques, such as X-ray single crystal diffraction, is challenging for these partially ordered materials. This is because of the difficulty in obtaining large (&gt; 10 µm) single crystals, and because large-distance periodic features cause diffraction intensities to fall off rapidly with scattering angle, so that only limited small-angle data are available. On the other hand, transmission electron microscopy (TEM) is a powerful tool for structural characterization at the mesoscale level due to the stronger interaction of electrons with matter compared to X-rays, enabling the use of very small crystals. In particular, high-resolution TEM (HRTEM) images give the phase and the amplitude of the crystal structure factors experimentally, leading to a 3D structural model by electron crystallography. Cage-type anionic-surfactant-templated mesoporous silicas display rich structural diversity. Among them, cage-type mesoporous silica with tetrahedrally close-packed (TCP) structures can be described by four types of polyhedra, 5^12, 5^12 6^2, 5^12 6^3, and 5^12 6^4.[1] A variety of structural polymorph have been observed and characterized by TEM. Their structures show a close resemblance to the Frank-Kasper phases, which are well known in intermetallic compounds. We found mesoporous silica with dodecagonal quasicrystalline ordering as one of the TCP structures (Figure).[2] In this presentation, I will discuss structural characterization of aperiodic crystals at the mesoscale, such as mesoporous silicas and binary colloidal crystals, by electron microscopy.

Metastable primary precipitation phases in multicomponent glass forming Fe-base alloys with metalloids

Structure transformations at the initial stages of devitrification have been investigated for alloys with good glass forming ability on the base of Fe–B–Si, Fe–P–C, Fe–P–B–C systems with various additions. Systematization of the metastable phases formed in these alloys showed that: (a) metastable phases at the initial stages of crystallization are tetrahedrally close packed (TCP) or aperiodic structures and possess compositions close to the mean composition of the alloy; (b) the observed TCP phases belong to one family of structures related to α-Mn for essentially different types of alloying systems, having in common a high glass forming ability; (с) for alloys based on the same system with different small alloying additions, structure of the metastable phase can change within this family of related phases. The obtained results show that Fe-based metallic glasses with metalloids are not essentially different from metal–metal type glasses.

The effect of solution annealing on the microstructural behavior of alloy 22 welds

Multipass gas tungsten arc welds of alloy 22 were subjected to solution annealing durations of 20 minutes, 24 hours, 72 hours, and 1 week at temperatures of 1075 °C, 1121 °C, 1200 °C, and 1300 °C. The specimens were studied in cross section by secondary electron microscopy to determine the effect of solution annealing on tetrahedrally close-packed (TCP) precipitate stability. Electron backscatter diffraction mapping was also performed on all of the specimens to determine the recrystallization behavior of the welds. It was found that complete TCP precipitate dissolution occurs after solution annealing at 1075 °C and 1121 °C for 24 hours, and at 1200 °C and 1300 °C for durations of 20 minutes. Regions of most rapid recrystallization were correlated to the regions of lowest solute content, highest plastic strain, and highest residual stresses. Texture analysis indicated that while the columnar dendrites originally present in the weld grew with a 〈001〉 orientation in the transverse direction (approximately opposite the heat flow direction), the recrystallized grains adopt a 〈101〉 orientation in the transverse direction when recrystallization and TCP phase dissolution occur simultaneously.

The effect of solution annealing on the microstructural behavior of alloy 22 welds

Multipass gas tungsten arc welds of alloy 22 were subjected to solution annealing durations of 20 minutes, 24 hours, 72 hours, and 1 week at temperatures of 1075 °C, 1121 °C, 1200 °C, and 1300 °C. The specimens were studied in cross section by secondary electron microscopy to determine the effect of solution annealing on tetrahedrally close-packed (TCP) precipitate stability. Electron backscatter diffraction mapping was also performed on all of the specimens to determine the recrystallization behavior of the welds. It was found that complete TCP precipitate dissolution occurs after solution annealing at 1075 °C and 1121 °C for 24 hours, and at 1200 °C and 1300 °C for durations of 20 minutes. Regions of most rapid recrystallization were correlated to the regions of lowest solute content, highest plastic strain, and highest residual stresses. Texture analysis indicated that while the columnar dendrites originally present in the weld grew with a 〈001〉 orientation in the transverse direction (approximately opposite the heat flow direction), the recrystallized grains adopt a 〈101〉 orientation in the transverse direction when recrystallization and TCP phase dissolution occur simultaneously.

The effect of solution annealing on the microstructural behavior of alloy 22 welds

Multipass gas tungsten arc welds of alloy 22 were subjected to solution annealing durations of 20 minutes, 24 hours, 72 hours, and 1 week at temperatures of 1075 °C, 1121 °C, 1200 °C, and 1300 °C. The specimens were studied in cross section by secondary electron microscopy to determine the effect of solution annealing on tetrahedrally close-packed (TCP) precipitate stability. Electron backscatter diffraction mapping was also performed on all of the specimens to determine the recrystallization behavior of the welds. It was found that complete TCP precipitate dissolution occurs after solution annealing at 1075 °C and 1121 °C for 24 hours, and at 1200 °C and 1300 °C for durations of 20 minutes. Regions of most rapid recrystallization were correlated to the regions of lowest solute content, highest plastic strain, and highest residual stresses. Texture analysis indicated that while the columnar dendrites originally present in the weld grew with a 〈001〉 orientation in the transverse direction (approximately opposite the heat flow direction), the recrystallized grains adopt a 〈101〉 orientation in the transverse direction when recrystallization and TCP phase dissolution occur simultaneously.

Second Phase Precipitation in As-Welded and Solution Annealed Alloy 22 Welds

The precipitation characteristics of tetrahedrally close-packed (TCP) phases during the welding and the subsequent solution annealing process of Alloy 22 1 1∕2in. thick plate double-U prototypical welds are investigated. Electron backscatter diffraction was used to provide large scale microstructural observation of the weld cross section, and scanning electron microscopy was used to map the location of the TCP phases. Analysis shows that TCP precipitation occurs congruent to the weld passes, with the solution annealing reducing the sizes of coarser precipitates.

Nb28Ni33.5Sb12.5, a New Representative of the X-phase

The first ternary compound in the Nb–Ni–Sb system, Nb28Ni33.5Sb12.5, has been synthesized and its structure has been determined by single-crystal X-ray diffraction methods. Nb28Ni33.5(2)Sb12.5(2) adopts the X-phase structure type (orthorhombic, space group Pnnm, Z=1, a=13.2334(5) Å, b=16.5065(7) Å, c=5.0337(2) Å), which belongs to the set of tetrahedrally close-packed (TCP) structures adopted by many intermetallic compounds. Typical of such TCP structures, the atoms reside in sites of high coordination number, with Ni and Sb in CN12 and Nb in CN14, -15, and -16 sites. The relative importance of various metal–metal bonding interactions is discussed on the basis of extended Hückel band structure calculations. Nb28Ni33.5Sb12.5 displays metallic behavior with a room-temperature resistivity of 2.3×10−4 Ω cm.

Simple shearing flow of dry soap foams with tetrahedrally close-packed structure

The microrheology of dry soap foams subjected to quasistatic, simple shearing flow is analyzed. Two different monodisperse foams with tetrahedrally close-packed (TCP) structure are examined: Weaire-Phelan (A15) and Friauf-Laves (C15). The elastic–plastic response is evaluated by using the Surface Evolver to calculate foam structures that minimize total surface area at each value of strain. The foam geometry and macroscopic stress are piecewise continuous functions of strain. The stress scales as T/V1/3, where T is surface tension and V is cell volume. Each discontinuity corresponds to large changes in foam geometry and topology that restore equilibrium to unstable configurations that violate Plateau’s laws. The instabilities occur when the length of an edge on a polyhedral foam cell vanishes. The length can tend to zero smoothly or abruptly with strain. The abrupt case occurs when a small increase in strain changes the energy profile in the neighborhood of a foam structure from a local minimum to a saddle...

Approximant and Frank-Kasper phases

Abstract It is shown by electron microscopy that planar defects in Frank-Kasper (or tetrahedrally close-packed (TCP)) phases have a very particular organization. In the three cases analysed in this paper, the icosahedral axes appear to be a useful reference in order to depict the microstructure. Our analysis reveals a hierarchical isocahedral organization in TCP phases. Since this approach is based on celi identification compared with the icosahedron, Frank-Kasper phases are consi-dered as auasicrystal approximants. The microstructural information, orientation relationships and defect organization emphasize the significance of such a description.

Structural and electronic properties of crystalline and glassy calcium-zinc compounds. I. Trigonal prismatic ordering or tetrahedral close packing

The authors present a detailed molecular dynamics study of the atomic structure of liquid and amorphous Ca-Zn alloys, based on inter-atomic forces derived from optimised first-principles pseudopotentials. In the Zn-rich limit, their results fit into the pattern characteristic for other sample-metal alloys; the liquid and the quench-condensed amorphous phase can be described as disordered tetrahedrally close packed. For the Ca-rich phases they find some distinctly different features; the most prominent is a pre-peak in the partial structure factor SZn-Zn(q) and in the number-density structure factor SNN(q), indicating topological short-range order. The analysis of the bond distances, coordination numbers and bond angles shows that the local order might be described as trigonal prismatic (distorted trigonal prisms of Ca, centred by Zn) in analogy to the trigonal prismatic structures of the crystalline intermetallic compounds Ca3Zn (Re3B-type), Ca5Zn3 (Cr5B3 type) and CaZn (CrB type).