Non-cubic Phases Research Articles

Network Phases with Multiple-Junction Geometries at the Gyroid-Diamond Transition.

A novel phase sequence for the transition from the double diamond to the double gyroid cubic phases via two non-cubic intermediate phases, an orthorhombic Fmmm (O69) phase and a hexagonal P63/m (H176) phase, is reported for specifically designed bolapolyphiles composed of a linear rod-like bistolane core with sticky glycerol ends and two branched central and two linear peripheral side chains. These liquid crystalline (LC) phases represent members of a new class of unicontinuous network phases, formed by longitudinal rod bundles with polar spheres acting as junctions and the alkyl chains forming the continuum around them. In contrast to previously known bicontinuous cubic networks, they combine different junctions with different angles in a common structure, and one of them even represents a triple network instead of the usually found double networks. This provides new perspectives for the design of soft network phases with enhanced structural complexity, inspiring the search for new supramolecular networks, nano-particle arrays, and photonic band-gap materials.

Observation of grey cesium tin bromide with unusual phase transition

A new grey colour polymorph of CsSnBr3 with non-cubic phase at 295(±1) K is observed. Contrary to black and yellow polymorphs, it seems that not only the β-α transition at 292 K shifted to 304 K but also the new β 3-phase with second-order phase transition (β 2-β 3) can be seen in this new grey polymorph. The structural refinement shows the monoclinic phase with holohedry for β 3-CsSnBr3 against the group-subgroup theory. However, the parameters and are suggesting pseudo-tetragonality for β 3-CsSnBr3.

Recrystallization of Si Nanoparticles in Presence of Chalcogens: Improved Electrical and Optical Properties.

Nanocrystals of Si doped with S, Se and Te were synthesized by annealing them in chalcogen vapors in a vacuum at a high temperature range from 800 to 850 °C. The influence of the dopant on the structure and morphology of the particles and their optical and electrical properties was studied. In the case of all three chalcogens, the recrystallization of Si was observed, and XRD peaks characteristic of noncubic Si phases were found by means of electronic diffraction for Si doped with S and Se. Moreover, in presence of S and Te, crystalline rods with six-sided and four-sided cross-sections, respectively, were formed, their length reaching hundreds of μm. Samples with sulfur and selenium showed high conductivity compared to the undoped material.

Switching Chirophilic Self-assembly: From meso-structures to Conglomerates in Liquid and Liquid Crystalline Network Phases of Achiral Polycatenar Compounds.

Spontaneous generation of chirality from achiral molecules is a contemporary research topic with numerous implications for technological applications and for the understanding of the development of homogeneous chirality in biosystems. Herein, a series of azobenzene based rod-like molecules with an 3,4,5-trialkylated end and a single n-alkyl chain involving 5 to 20 aliphatic carbons at the opposite end is reported. Depending on the chain length and temperature these achiral molecules self-assemble into a series of liquid and liquid crystalline (LC) helical network phases. A chiral isotropic liquid (Iso1 [ *] ) and a cubic triple network phase with chiral I23 lattice were found for the short chain compounds, whereas non-cubic and achiral cubic phases dominate for the long chain compounds. Among them a mesoscale conglomerate with I23 lattice, a tetragonal phase (Tetbi ) containing one chirality synchronized and one non-synchronized achiral network, an achiral double network meso-structure with Ia d space group and an achiral percolated isotropic liquid mesophase (Iso1 ) were found. This sequence is attributed to an increasing strength of chirality synchronization between the networks, combined with a change of the preferred mode of chirophilic self-assembly between the networks, switching from enantiophilic to enantiophobic with decreasing chain length and lowering temperature. These nanostructured and mirror symmetry broken LC phases exist over wide temperature ranges which is of interest for potential applications in chiral and photosensitive functional materials derived from achiral compounds.

The Influence of the Elemental Composition, Crystal Structure, and Grain Structure of the Ferro-Piezoceramics of Various Degrees of the Ferro-Hardness on the Stability of the Polarized State

Ferro-piezoceramic materials (FPCM) with different degrees of ferrohardness were fabricated by double solid-phase synthesis followed by the sintering technique using hot pressing method. The X-ray studies carried out in a wide temperature range showed that with increasing temperature, each of the studied FPCM undergoes a series of phase transformations, accompanied by a change in the symmetry of the unit cell. In this case, near the phase transition to the nonpolar cubic phase, in each of the FPCM, the formation of a fuzzy symmetry region is observed, which is characterized by weak distortions and temperature–time instability of the crystal structure. The study of the piezoelectric modulus d33 in the quasi-static regime as a function of temperature made it possible to reveal the different nature of its behavior in materials of various degrees of ferrohardness. It was shown that the conservation of the state in ferrosoft materials above the Curie temperature is associated with the relaxation nature of the change in their properties, the existence of a region of fuzzy symmetry (noncubic phase) in them above the Curie temperature, and increased inertia of the system. The expediency of taking into account the presented results in the development of electromechanical converters based on FPCM of various degrees of ferrohardness, operated under temperature effects, including cyclic ones, was shown.

Self-organization of rectangular bipyramidal helical columns by supramolecular orientational memory epitaxially nucleated from a Frank-Kasper σ phase

Programming living and soft complex matter via primary structure and self-organization represents the key methodology employed to design functions in biological and synthetic nanoscience. Memory effects have been used to create commercial technologies including liquid crystal displays and biomedical applications based on shape memory polymers. Supramolecular orientational memory (SOM), induced by an epitaxial nucleation mediated by the close contact spheres of cubic phases, emerged as a pathway to engineer complex nanoscale soft matter of helical columnar hexagonal arrays. SOM preserves the crystallographic directions of close contact supramolecular spheres from the 3D phase upon cooling to the columnar hexagonal periodic array. Despite the diversity of 3D periodic and quasiperiodic nanoarrays of supramolecular dendrimers, including Frank-Kasper and quasicrystal, all examples of SOM to date were mediated by Im3¯m (body-centered cubic, BCC) and Pm3¯n (Frank-Kasper A15) cubic phases. Expanding the scope of SOM to non-cubic arrays is expected to generate additional morphologies that were not yet available by any other methods. Here we demonstrate the SOM of a dendronized triphenylene that self-organizes into helical columnar hexagonal and tetragonal P42/mnm (Frank-Kasper σ) phases. Structural analysis of oriented fibers by X-ray diffraction reveals that helical columnar hexagonal domains self-organize an unusual rectangular bipyramidal morphology upon cooling from the σ phase. The discovery of SOM in a non-cubic Frank-Kasper phase indicates that this methodology may be expanded to other periodic and quasiperiodic nanoarrays organized from self-assembling dendrimers and, most probably, to other soft and living complex matter.

Towards synthetic L10-FeNi: Detecting the absence of cubic symmetry in Laser-Ablated Fe-Ni nanoparticles

The L10 crystal structure underlines an important class of chemically ordered alloys that exhibits uniaxial magnetocrystalline anisotropy. The near-equiatomic L10-FeNi extracted from meteorites has demonstrated intriguing magnetic properties for permanent magnet applications. However, the synthesis of this chemically ordered non-cubic structure has been a longstanding challenge. Here, we demonstrate the absence of cubic symmetry in near-equiatomic Fe-Ni nanoparticles synthesized by picosecond-pulsed laser ablation in liquids. The non-cubic phase detected in these particles can only be L10-FeNi or hexagonal close-packed (HCP) FeNi, and the absence of cubic symmetry was unequivocal. The orientation relationship between the non-cubic phase and the adjacent cubic phase was characterized by a series of transmission electron microscopy (TEM) techniques, which consistently suggests that the formation of the non-cubic phase involves a martensitic transformation process.

Synthesis of Non-Cubic Nitride Phases of Va-Group Metals (V, Nb, and Ta) from Metal Powders in Stream of NH3 Gas under Concentrated Solar Radiation

Using a high-flux solar furnace, loosely compacted powders of Va-group transition metal (V, Nb, and Ta) were reacted with stream of NH3 gas (uncracked NH3 gas) being heated by concentrated solar beam to a temperature (T) range between 600 and 1000 °C. From V, sub-nitride V2N (γ phase) and hypo-stoichiometric mono-nitride VN possessing fcc (face-centered cubic) crystal lattice structure (δ phase) were synthesized. On the other hand, in the reaction product from Nb and Ta, hexagonal mono-nitride phase with N/M atom ratio close to 1 (ε phase) was detected. The reaction duration was normalized to be 60 min. In a conventional industrial or laboratory electric furnace, the synthesis of mono-nitride phase with high degree of crystallinity that yield sharp XRD peaks for Va-group metal might take a quite long duration even at T exceeding 1000 °C. In contrast, mono–nitride phase MN of Va-group metal was synthesized for a relatively short duration of 60 min at T lower than 1000 °C being co-existed with lower nitride phases.

Controlling liquid and liquid crystalline network formation by core-fluorination of hydrogen bonded supramolecular polycatenars

Photo-switchable materials with liquid crystalline (LC) properties and being capable of showing mirror-symmetry breaking are of special interest from a fundamental scientific point of view as well as for the development of π-conjugated materials with tailored superstructures for optical, emissive and photonic materials. Herein we report two new series of supramolecular photosensitive polycatenar (multi-chained) mesogens formed by hydrogen bonding interaction between a taper shaped triple-chain benzoic acid as the hydrogen bond-donor and fluorinated azopyridines with only a single chain as hydrogen bond-acceptors. The self-assembly of these materials was characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD). Depending on the position of the fluorine atom different types of LC phases were observed, including spontaneously chiral isotropic liquid (Iso1[*]) and bicontinous cubic phases (Cubbi[⁎]/I23), as well as achiral cubic (Cubbi/Ia3̄d) and non-cubic 3D phases with tetragonal symmetry. This report provides guidelines for the application of core fluorination at different positions as a powerful tool for controlling spontaneous helical self-assembly in supramolecular liquid and liquid crystalline materials.

Unraveling the Spatial Distribution of Catalytic Non-Cubic Au Phases in a Bipyramidal Microcrystallite by X-ray Diffraction Microscopy.

Tuning of crystal structures and shapes of submicrometer-sized noble metals have revealed fascinating catalytic, optical, electrical, and magnetic properties that enable developments of environmentally friendly and durable nanotechnological applications. Several attempts have been made to stabilize Au, knowing its extraordinary stability in its conventional face-centered cubic (fcc) lattice, into different lattices, particularly to develop Au-based catalysis for industry. Here, we report the results from scanning X-ray diffraction microscopy (SXDM) measurements on an ambient-stable penta-twinned bipyramidal Au microcrystallite (about 1.36 μm in length and 230 nm in diameter) stabilized in noncubic lattice, exhibiting catalytic properties. With more than 82% of the crystal volume, the majority crystallite structure is identified as body-centered orthorhombic (bco), while the remainder is the standard fcc. A careful analysis of the diffraction maps reveals that the tips are made up of fcc, while the body contains mainly bco with very high strain. The reported structural imaging technique of representative single crystallite will be useful to investigate the growth mechanism of similar multiphase nano- and micrometer-sized crystals.

Probing local structure of the morphotropic phase boundary composition of Na0.5Bi0.5TiO3–BaTiO3 using rare-earth photoluminescence as a technique

(1-y)Na0.5Bi0.5TiO3-(y)BaTiO3 (NBT-BT) is one of the most investigated lead-free piezoelectric system in the recent past. Unlike the conventional piezoelectric alloys wherein the morphotropic phase boundary (MPB) compositions exhibiting maximum piezoelectric response, exhibit coexistence of ferroelectric phases with different symmetries on the global scale, the MPB composition (y = 0.06) of NBT-BT shows a cubic-like phase in the unpoled state. On poling it transforms to a non-cubic ferroelectric phase. Here we exploit the sensitivity of the photoluminescence (PL) property of doped rare-earth ions with regard to the local symmetry of the host crystal to investigate the local structure of the cubic-like phase of the MPB composition of NBT-BT. We performed a comparative study of the PL response by doping separately Er and Eu in very dilute concentration in the (i) MPB composition (y = 0.06) exhibiting a cubic-like phase, (ii) a sub-MPB composition (y = 0.03) exhibiting rhombohedral phase, and (iii) above the MPB composition (y = 0.10) exhibiting tetragonal phase. We found that both the Er and the Eu PL spectra of the MPB composition (y = 0.06) exhibits more number of Stark lines in its unpoled cubic-like phase as compared to that in the field-stabilized rhombohedral phase. The additional Stark lines in the cubic-like global phase are identified to be that of the tetragonal structure. Our study therefore confirms that what appears as a cubic-like phase on the global scale has intimately connected tetragonal and rhombohedral local structures. The success of our experiments suggests that rare-earth PL can be used as a simple yet powerful tool to investigate local structures in scenarios wherein the global structure need not comply with the local structure.

Exploring the Crystal Structure Space of CoFe2P by Using Adaptive Genetic Algorithm Methods

Advances in theoretical and computational condensed matter physics have opened the possibility to predict and design magnetic materials for specific technological applications. In this paper, we use the adaptive–genetic algorithm technique for exploring the low-energy crystal structure configurations of Co0.25Fe0.5P0.25, aiming to find new low-energy non-cubic phases with high saturation magnetization that might be interesting for high-performance permanent magnet development.

Structural and magnetic characterization of the Ni55Fe19Ga26 shape memory alloy thin film

The structural evolution of the Pulsed Laser Deposited Ni55Fe19Ga26 ferromagnetic Heusler alloy thin films has been studied as a function of substrate temperature. Room temperature Grazing Incidence X-Ray Diffraction (GIXRD) measurement reveals the polycrystalline nature of the films consisting of cubic austenite and non-cubic martensite phases. The cubic phase consists of the mixture of an ordered (L21) and disordered (γ) phases, the disordered phase gets suppressed with an increase in the substrate temperature while low temperature X-Ray Diffraction study shows structural modification of the samples. Field Emission Scanning Electron Microscopy (FESEM) study shows the droplet-like morphology of the films. Extended X-ray Absorption Fine Structure (EXAFS) study agrees with the X-ray diffraction results about the phase evaluation and further reveals the local structure information of the constituent atoms which leads to the hybridization effect and finally govern the macroscopic properties of the material. In addition, magnetization studies show the ferromagnetic nature of the film with a mixture of anti-ferromagnetic ordering arising due to antisite disorder of the atoms revealed by the EXAFS measurements.

Identification and properties of the non-cubic phases of Mg2Pb

Mg2Pb occurs in the cubic fluorite structure and is a semimetal with a band structure strongly affected by spin-orbit interaction on the Pb p states. Its properties are therefore of interest in the context of topological materials. In addition a different phase of Mg2Pb was experimentally reported, but its crystal structure and properties remain unknown. Here we determine the structure of this phase using ab initio evolutionary methods and report its properties. The energy of one tetragonal phase, space group P4/nmm, is 2 meV per atom higher than that of the ground state structure supporting the experimental observation. We find this tetragonal phase to be a compenstated anisotropic metal with strong spin orbit effects. Many other metastable structures have also been identified, especially one orthorhombic structure, space group Pnma, of which energy is 17 meV per atom higher than that of ground state structure and which perhaps could be the phase that was reported based on similarity of lattice parameters.

Combinatorial Development of Fe-Co-Nb Thin Film Magnetic Nanocomposites.

A Fe-Co-Nb thin film materials library was deposited by combinatorial magnetron sputtering and investigated by high-throughput methods to identify new noncubic ferromagnetic phases, indicating that combinatorial experimentation is an efficient method to discover new ferromagnetic phases adequate for permanent magnet applications. Structural analysis indicated the formation of a new magnetic ternary compound (Fe,Co)3Nb with a hexagonal crystal structure (C36) embedded in an FeCo-based matrix. This nanocomposite exhibits characteristics of a two-phase ferromagnetic system, the so-called hard-soft nanocomposites, indicating that the new phase (Fe,Co)3Nb is ferromagnetic. Magnetic hysteresis loops at various angles revealed that the magnetization reversal process is governed by a domain wall pinning mechanism.

Rejuvenation of metallic glasses by non-affine thermal strain.

When a spatially uniform temperature change is imposed on a solid with more than one phase, or on a polycrystal of a single, non-cubic phase (showing anisotropic expansion-contraction), the resulting thermal strain is inhomogeneous (non-affine). Thermal cycling induces internal stresses, leading to structural and property changes that are usually deleterious. Glasses are the solids that form on cooling a liquid if crystallization is avoided--they might be considered the ultimate, uniform solids, without the microstructural features and defects associated with polycrystals. Here we explore the effects of cryogenic thermal cycling on glasses, specifically metallic glasses. We show that, contrary to the null effect expected from uniformity, thermal cycling induces rejuvenation, reaching less relaxed states of higher energy. We interpret these findings in the context that the dynamics in liquids become heterogeneous on cooling towards the glass transition, and that there may be consequent heterogeneities in the resulting glasses. For example, the vibrational dynamics of glassy silica at long wavelengths are those of an elastic continuum, but at wavelengths less than approximately three nanometres the vibrational dynamics are similar to those of a polycrystal with anisotropic grains. Thermal cycling of metallic glasses is easily applied, and gives improvements in compressive plasticity. The fact that such effects can be achieved is attributed to intrinsic non-uniformity of the glass structure, giving a non-uniform coefficient of thermal expansion. While metallic glasses may be particularly suitable for thermal cycling, the non-affine nature of strains in glasses in general deserves further study, whether they are induced by applied stresses or by temperature change.

An unconstrained DFT approach to microphase formation and application to binary Gaussian mixtures.

The formation of microphases in systems of particles interacting by repulsive, bounded potentials is studied by means of density-functional theory (DFT) using a simple, mean-field-like form for the free energy which has already been proven accurate for this class of soft interactions. In an effort not to constrain the configurations available to the system, we do not make any assumption on the functional form of the density profile ρ(r), save for its being periodic. We sample ρ(r) at a large number of points in the unit cell and minimize the free energy with respect to both the values assumed by ρ(r) at these points and the lattice vectors which identify the Bravais lattice. After checking the accuracy of the method by applying it to a one-component generalized exponential model (GEM) fluid with pair potential ϵexp[ - (r/R)(4)], for which extensive DFT and simulation results are already available, we turn to a binary mixture of Gaussian particles which some time ago was shown to support microphase formation [A. J. Archer, C. N. Likos, and R. Evans, J. Phys.: Condens. Matter 16, L297 (2004)], but has not yet been investigated in detail. The phase diagram which we obtain, that supersedes the tentative one proposed by us in a former study [M. Carta, D. Pini, A. Parola, and L. Reatto, J. Phys.: Condens. Matter 24, 284106 (2012)], displays cluster, tubular, and bicontinuous phases similar to those observed in block copolymers or oil/water/surfactant mixtures. Remarkably, bicontinuous phases occupy a rather large portion of the phase diagram. We also find two non-cubic phases, in both of which one species is preferentially located inside the channels left available by the other, forming helices of alternating chirality. The features of cluster formation in this mixture and in GEM potentials are also compared.

Structure and properties of arc evaporated nanoscale TiN/MoN multilayered systems

Using vacuum-arc evaporation method we fabricated periodic multilayered TiN/MoN structures with different bilayer periods λ ranging from 25 to 100nm. Rutherford backscattering (RBS), X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) and microhardness measurements were used for investigations of composition, structure and mechanical properties of the multilayered coatings. We found that molybdenum nitride and titanium nitride layers grown on steel show local partial epitaxy and columnar growth across interfaces. A molybdenum–titanium carbide interlayer was evidenced between the substrate and the multilayer. Molybdenum nitride and titanium nitride layers contain small (5–30nm) grains and are well crystallized with (100) preferred orientation. They were identified as stoichiometric fcc TiN and cubic γ-Mo2N. Non-cubic molybdenum nitride phases were also detected. The hardness of the obtained structures achieved great values and maximal hardness was 29–31GPa for multilayered structure with 50nm period. Hardness of the obtained coatings is 25% higher in comparison with initial single-layer nitride coatings, wherein plasticity index (H/E) of multilayered structure is 0.075.

Raman spectroscopic study of the size-dependent order parameter of barium titanate

Spherical barium titanate nanoparticles, having a diameter of 50–500 nm, were characterized using micro-Raman spectroscopy at 83 K. Proposed is a calibrated intensity method allowing a spectroscopic estimate of the relative fraction of two solid-state structural phases, where the phases have uniquely identifiable Raman bands. The Raman spectroscopy local probe is broadened to quantify macroscopic characteristics, such as the order parameter. Based on the intensity of the 311 cm−1 mode, it is revealed that the order parameter, η, of BaTiO3 decreases exponentially with particle size reduction. Similarly, an exponential decrease of the long-range (dipole-dipole) interaction is suggested, based on the Raman shift of the 716 cm−1 mode. An approximate long-range interaction decrease of 3% is estimated when the particle size is reduced from 500 nm to 50 nm. This study provides evidence that BaTiO3 nanoparticles can be modeled as being composed of coexisting cubic and non-cubic phases with relative fraction controlled by the degree of long-range Coulombic interactions.

Effect of Terminating Species on the Initial Growth of BN on Diamond Substrates

The details in the layer-by-layer formation of H (or F)-terminated boron nitride onto diamond (100) have been theoretically studied using ab initio density functional theory under periodic conditions. Cubic boron nitride, c-BN, is a very interesting and promising material due to its extreme properties. However, there are severe problems during the vapor phase synthesis of c-BN because of the formation of noncubic phases in the initial grow steps, why a gentle large area chemical vapor deposition (CVD) deposition is needed. The substrate material has been experimentally shown to be very important for an ideal growth of c-BN in the initial grow process. Diamond is a material that has been found to be a good substrate material for this purpose. By alternating H (or F)-terminated B and N layers, and calculating the resulting interfacial binding strengths and geometrical structures, the initial growth has been studied and compared to earlier theoretical results that have been made without terminating species. Two different structural alignments, with respect to the underlying diamond substrate, were initially constructed. One model was heterostructurally positioned on top of the diamond (100) substrate, while the other model had the x-axis of the c-BN lattice aligned with the y-axis of the diamond lattice. For the situation with a terminated monatomic B layer on top of diamond, the heteroepitaxially built structure was the most energetically favored. This result is just the opposite from the nonterminated situation, where the nonheteroepitaxial structures were favored. The binding energy for the heteroepitaxial terminated monolayer of N was calculated even stronger than the terminated B monolayer. When applying a second atomic layer of c-BN on top of the monatomic B layer, the heteroepitaxially build structure is energetically preferred. For the nonheteroepitaxial growth the adlayer actually bonds to the diamond substrate when H-terminated. Without terminating species the nonheteroepitaxial adlayer did not bind at all. Also for the situation with two atomic adlayer with N closest to the diamond substrate, the heteroepitaxial structure is energetically preferred over the nonheteroepitaxial structure. The opposite is true without any terminating species. When four, six, and eight atom layers are applied, with N atoms closest to the diamond substrate, the heteroepitaxially built structures are favored. The results showed that the terminating species helped to uphold the cubic sp3-formation. When using terminating species, the heteroepitaxially built interfaces had generally higher binding strengths than the nonheteroepitaxially built interfaces. While, when not using terminating species, the nonheteroepitaxially built interfaces were preferred instead. The c-BN structure seems to be more easily formed when several BN layers already are formed, and the interfacial binding strength is stabilized.