Structure Of Various Phases Research Articles

Electron–phonon interactions and superconductivity of β -Nb2N thin films

Niobium nitride (NbN) has attracted scientific interest due to its diverse physical properties and a variety of structural phases. The structure and superconductivity of the cubic δ-NbN phase are well established, but its hexagonal phases are not explored hitherto. In the present work, we report a simple synthesis route and a detailed study of hexagonal β-Nb2N thin films. Thermal annealing of sputtered grown α-NbN leads to a single phase β-Nb2N at 973 K as confirmed by x-ray diffraction and absorption spectroscopy. The electrical transport measurements revealed a dominance of electron–phonon interactions with a superconducting transition around 4.74 K and an upper critical field [HC2(0)] of 3.99 T. The estimated HC2(0) is well below the calculated Pauli limit, and the Maki parameter value (α < 1) indicates that HC2(0) is dominated by an orbital pair breaking effect. Finally, the obtained value of electron–phonon coupling constant (λ) is in excellent agreement with a weak coupling Bardeen–Cooper–Schrieffer value of conventional superconducting materials.

Absence of superconductivity in micrometer-sized ɛ -NbN single crystals

It is important to study the properties of high quality single crystal in order to resolve the issue of an interesting material in which certain debatable fundamental properties exist. However, it is unfortunate that a sizable single crystal for experimental measurements is not always available. NbN is one of the examples; it has attracted scientific and engineering interest due to its diverse physical properties and a variety of structural phases. Until now superconductivity is only observed in cubic $\ensuremath{\delta}$- and tetragonal $\ensuremath{\gamma}$-NbN but not in hexagonal $\ensuremath{\varepsilon}$-NbN. Recently, Zou et al. reported the observation of superconductivity with ${T}_{c}\ensuremath{\sim}11.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in a hexagonal $\ensuremath{\varepsilon}$-NbN based on the measurement on a multiphase powder specimen. In order to resolve the issue, the work used the electron backscattering diffraction technique to characterize phases of micron-size NbN crystals from commercial powders and measure their transport properties. Our results unambiguously confirm that the hexagonal $\ensuremath{\varepsilon}$-NbN phase is not superconducting.

Chiral Symmetry Breaking in Liquid Crystals: Appearance of Ferroelectricity and Antiferroelectricity

The study of chiral symmetry breaking in liquid crystals and the consequent emergence of ferroelectric and antiferroelectric phases is described. Furthermore, we show that the frustration between two phases induces a variety of structural phases called subphases and that resonant X-ray scattering is a powerful tool for the structural analysis of these complicated subphases. Finally, we discuss the future prospects for clarifying the origin of such successive phase transition.

Ab Initio Simulation of Ta2O5: A High Symmetry Ground State Phase with Application to Interface Calculation

AbstractIt is reported that the recently predicted triclinic γ‐phase ground state Ta2O5 by Yang and Kawazoe can be assigned a much more symmetric I41/amd space group, and is isomorphic to P‐Nb2O5. Interestingly, the well‐known high temperature α‐phase Ta2O5 also has the I41/amd symmetry, but is unstable at zero temperature according to our phonon dispersion calculation. A thorough energy comparison of the βAL, δ, λ, Β, LSR, βR, Pm, Cmmm, γ, and α phases of Ta2O5 is carried out using density functional theory under the generalized gradient approximation (GGA). The GGA‐1/2 method is applied in calculating the electronic structure of various phases, where the tetragonal γ‐phase demonstrates a 4.24 eV indirect band gap, close to experimental value. The high symmetry tetragonal phase together with computationally efficient GGA‐1/2 method greatly facilitates the ab initio simulation of Ta2O5‐based devices. As an example, the Ohmic contact nature between metal Ta and Ta2O5 by calculating an interface model of b.c.c. Ta and tetragonal γ‐Ta2O5, using GGA‐1/2 has been explicitly shown.

Heat capacity and thermodynamic functions of silica-doped γ-Al2O3

Abstract Alumina can exist in a variety of structural phases, including the more common alpha and gamma phases. The gamma phase is particularly useful as a support material for catalysts, but compared to the alpha phase there is little thermodynamic data reported for it. Even less data are available for alumina prepared with dopants such as silica. These materials have improved thermal stability over their un-doped counterparts. This makes them especially useful in high temperature catalytic applications, and while they have been, in many regards, well characterized, gaps still exist in the available thermodynamic data. Additionally, water from synthesis conditions or catalytic preparation tends to bind to gamma phase alumina, further complicating thermodynamic data collection and analysis. We collected heat capacity data for a set of silica-doped alumina samples calcined at 300 °C (chemical formula Al2O3 0.089 SiO2 1.190 H2O), 600 °C (Al2O3 0.089 SiO2 1.027 H2O), 900 °C (Al2O3 0.089 SiO2 0.321 H2O), and 1100 °C (Al2O3 0.089 SiO2 0.187 H2O) using a Quantum Design Physical Properties Measurement System (PPMS). The data were fit to mathematical functions, and from these fits entropy, enthalpy, and Gibbs energy functions were calculated. The standard molar entropies for silica-doped alumina calcined at 300 °C, 600 °C, 900 °C, and 1100 °C are 101.69 J K−1 mol−1, 90.206 J·K−1·mol−1, 73.758 J·K−1·mol−1, and 66.444 J·K−1·mol−1 respectively.

Magnetostructural, mechanical and electronic properties of manganese tetraboride

Magnetostructural stabilities, mechanical behaviors and electronic structures of various phases of manganese tetraboride (MnB4) have been investigated systematically by density functional theory (DFT) based first-principles methods. It is found that MnB4 undergoes temperature-induced phase transitions from the nonmagnetic (NM) monoclinic mP20 structure to the ferromagnetic (FM) orthorhombic oP10 structure at 438 K, then to the antiferromagnetic (AFM) orthorhombic oP10 structure at 824 K. We reveal that the NM insulating mP20 phase stabilizes by the Peierls distortion breaking the structural degeneracy, while the FM and AFM metallic oP10 phases stabilize by the Stoner magnetism lifting the spin degeneracy. Furthermore, the calculated mechanical properties show that the NM mP20, FM oP10, and AFM oP10 phases exhibit low compressibility and high hardness, which originate from their three-dimensional covalent boron networks. Therefore, this unique temperature-assisted insulator-metal transition, strong stiffness and high hardness suggest that MnB4 may find promising technological applications as thermoelectric switches and field effect transistors at the extreme conditions.

Effect of inhomogeneous deformation on the electronic structure of SnO2 and Sn x Sb1–x O2 phases

The electronic structures of various phases in the Sn–Sb–O2 system under pressure and under tetragonal, monoclinic, and orthorhombic deformation are studied. Calculations are performed using the first-principles pseudopotential method. It is established that SnO2 undergoes the following phase transitions under pressure: rutile–pyrite (17 GPa) and pyrite–fluorite (138 GPa). It is also found that doping SnO2 with Sb leads to a shift of the Fermi level to the conduction band and to additional resonant states below the valence band. Inhomogeneous deformation of Sn x Sb1–x O2, x = 1.00; 0.94; 0.88, at δ ≤ 0.2 causes a stress up to 6.2 GPa, depending on the strain. An analysis of the density of electron states in the bandgap of the deformed structures shows that the gap broadens under tetragonal deformation and narrows under orthorhombic and monoclinic deformation. The theoretical results obtained should be taken into account in interpreting the piezoresistive properties of Sn–Sb–O2-based thick films.

Phase Diagram and Structures in Mixtures of Poly(styrenesulfonate anion) and Alkyltrimethylammonium Cations in Water: Significance of Specific Hydrophobic Interaction

Mixtures of polyelectrolytes and oppositely charged surfactants show very rich phase behavior that is influenced by surfactant-ion and polyion properties and by water content. A general feature is associative phase separation as a result of strong electrostatic interactions, whereas the effect of eventual more specific interactions (e.g., hydrophobic) has not been thoroughly investigated. In this paper, we report a detailed study on phase behavior and structures in poly(styrenesulfonate anion) (PSS(-))-cetyltrimethylammonium cation (CTA(+))-water mixtures that are characterized by a hydrophobic interaction between the styrene groups of PSS(-) and the micelle interior. Structures of various phases were determined by small-angle X-ray scattering, and results indicated the presence of a disordered micellar and an ordered hexagonal phase; no cubic phase was found. The general conclusion is that the highlighted hydrophobic interaction promotes dissolution of CTAPSS when the polyion salt is added and provides further stabilization of the dense phase when the surfactant salt is added.

Orientational prewetting of planar solid substrates by a model liquid crystal

We present grand canonical ensemble Monte Carlo simulations of prewetting transitions in a model liquid crystal at structureless solid substrates. Molecules of the liquid crystal interact via anisometric Lennard-Jones potentials and can be anchored planar or homeotropically at the substrates. Fluid-substrate attraction is modeled by a Yukawa potential of variable range. By monitoring the grand-potential density and the nematic order parameter as functions of the chemical potential μ, several discontinuous prewetting, wetting, and isotropic-nematic phase transitions are observed. These transitions depend on both the range of the fluid-substrate attraction and the specific anchoring at the substrate. Our results show that at substrates characterized by degenerate anchoring prewetting occurs at lower μ compared with cases in which the anchoring is monostable. This indicates that prewetting transitions are driven by orientational entropy because degenerate anchoring allows for more orientationally distinct configurations of molecules compared with monostable anchoring. In addition, by analyzing local density and various local order parameters, a detailed picture of the structure of various phases emerges from our simulations.

Influence of interstitial impurities on the valence electron structures and phase transformation behavior in intermediate Ti–Al alloys

Development of intermetallic compounds in Ti–Al alloys for high-temperature structural applications has long been impeded due to their embrittlement. To overcome the embritttlement, it is necessary to understand thoroughly its chemical bonding nature and the resultant phase transformation behavior. In this study, based on the Empirical Electron Theory of Solids and Molecules (EET), effects of interstitial impurities on the valence electron structures and phase transformations in the intermediate Ti–Al alloys were investigated. It was demonstrated that for such alloy systems, because interstitial impurities can enhance the hybridization states of Ti and Al atoms, the valence electron structures of various phases became considerably anisotropic. As a result, some phase transformations at high temperatures was hindered, leading to the occurrence of complex metastable phases at the room temperature. Such theoretical calculations clarified some unclear results from experimental observations in the literature.

Dependence of the normal modes on the electronic structure of various phases of ice as calculated by ab initio methods

The charge-density distribution in various phases of ice is used to explore the information that can be obtained about the preferred directions of motion of atoms so as to investigate the possibility of the creation of more efficient and computationally cost-effective dynamical matrices. PACS Nos.: 63.20Dj, 31.90+s, 71.10-w

Uniplanar smectic phases in free-standing films

Structures of various phases that appear in antiferroelectric liquid crystals can be described within a simple discrete phenomenological model of antiferroelectric liquid crystals, which for the smectic-C-alpha (SmC(alpha)) phase suggests a short-pitch helicoidal structure. In a paper presented here the same model is used to analyze theoretically free-standing films, formed by a finite number of smectic layers. In the bulk sample a transition from the smectic-A (SmA) phase is either to the ferroelectric SmC* phase, the SmC(*)(A) phase, or the antiferroelectric SmC(alpha) phase, as observed from experiments as well as comprehended in the model. However in free-standing films uniplanar structures, which in a bulk sample were not observed experimentally nor predicted by the model, are shown to appear immediately below the transition from the high-temperature SmA phase. In free-standing film uniplanar structure remains stable in a narrow temperature region between SmA and SmC(alpha) phase.

Morphology and structure of various phases at the bonding interface of Al/steel formed by explosive welding

The bonding interface of explosively-welded aluminium and steel in three explosive conditions have been investigated by using scanning electron microscopy, transmission electron microscopy, electron diffraction and electron probe microanalysis methods. The results show that all the interfaces have the shape of waves with curled front formed by process of superplasticity and some discontinuous reacted zones. They consist of amorphous and nano sized crystals and quasi-crystals as well as the compounds such as AlFe, Al2Fe, Al3Fe and Al6Fe with various shapes. The basal steel crystal near the interface has structure of martensite and perlite crystals which are deformed by the process of superplasticity. The size of reacted zone becomes large with increasing amount of explosive charge powder and separation of the driver Al plate from the basal steel plate.

Structural relationships between compounds based on the stacking of mixed layers related to hexagonal perovskite-type structures

Ever since the discovery of high-temperature superconductivity in layered copper oxides, interest in materials exhibiting two-dimensional crystal structures related to perovskites (AMX3) has greatly increased. This review deals with crystal structures of various phases derived from perovskite with mixed stacking of [AX3] and [AnXm](n+m⩽4) layers. Based on their structural relationships with the perovskite-related hexagonal polytypes, the possibility of forming compounds adopting new structure types derived from mixed stacking of various layers will be discussed. In addition, this review will also highlight the similarities between the perovskite structure and the structures of phases as varied as vanadates, phosphates etc., in which part of the structure is made up of cations present in a tetrahedral environment.

Vibrational spectra of and phase transitions in caesium hydrogen sulphate

AbstractThe Raman and infrared spectra of the superionic protonic conductor CsHSO4(CsDSO4) were investigated in the 18–450 K range. Polarized single crystal Raman and infrared reflection spectra were also studied at room temperature. An assignment of bands due to internal and external vibrations is given in terms of symmetry species and approximate type of motion. The spectroscopic results, in particular the behaviour of the Raman bands as a function of temperature, are used to discuss the crystalline structures of various phases, hydrogen bonding, the type and degree of structural disorder, phase transitions and conductivity.