Commensurate-incommensurate phase transition in muthmannite,AuAgTe2: first evidence of a modulated structure at low temperature

To study the temperature-dependent structural changes and to analyze the crystal chemical behavior of silver as a function of temperature, a crystal of muthmannite, AuAgTe₂, has been investigated by X-ray single-crystal diffraction methods at 300 K and 110 K. At room temperature, muthmannite was confirmed as belonging to the space group P2/m, while at low temperature (110 K) it undergoes a reversible commensurate–incommensurate phase transition with a modulation wave vector q = 0.215(1)a* + 0.379(2)c*. Muthmannite reconverts to the commensurate type upon returning to room temperature, thus indicating that the phase transition is completely reversible in character. The average structure of the low-temperature muthmannite remains monoclinic, space group P2/m, and shows only normal thermal compression over the entire temperature range investigated. Crystal-chemical characteristics are compared with published data on the other members of the system Au–Ag–Te. Speculations on the possible origin of the modulated structure at low temperature are also given.     

Dislocation configurations in twin-free melt-textured YBa2Cu3O7 processed at high pressure and temperature

Melt-textured YBa₂Cu₃O₇ samples were deformed under high pressure–high temperature (HP–HT) conditions (P?=?2?GPa, T?=?900 and 800°C, t?=?15?min). During the HP–HT treatment, samples were in contact with annealed zirconia, preventing oxygen loss, in such a way that the orthorhombic superconductor phase was preserved at the end of the experiment. The deformed microstructure, investigated by transmitted electron microscopy, exhibited original features characterized by a very low twin density, a memory effect on the dislocations shape and a non-classical faulted loops distribution in the matrix. A mechanism, based on the interaction between twin and gliding dislocations, is proposed to explain the nucleation of these aligned loops.     

In situ transformation from P phase to μ phase in rhenium-containing single-crystal superalloy during thermal exposure

An in situ phase transformation from P to μ in a rhenium-containing superalloy during high-temperature exposure has been investigated by transmission electron microscopy. The orientation relationship between the phases is found as (1–12)_μ // (1?0?0)_P and [2?4?1]_μ // [0?1?1]_P. An atomic model of the μ/P interface was built to reveal a perfectly coherent interface with lattice misfits of only 0.421 and 1.098%. Considering the small lattice distortions and the similar elemental composition of the phases, it is concluded that the phase transformation from P to μ is favourable.     

Structural stability of W2B5 under high pressure

High-pressure structural stability studies have been carried out on tungsten boride W₂B₅ up to maximum pressure of 36 GPa using a Mao-Bell diamond-anvil cell at beamline BR-12 of the ELETTRA synchrotron facility (λ = 0.68881 Å). The hexagonal phase (S.G:P6₃/mmc) of W₂B₅ is stable up to the maximum pressure studied. The bulk modulus is estimated to be ~347 GPa using the Birch–Murnaghan equation of state. The variation of lattice parameters and bond lengths B–B and W–B have been studied and the c-axis is seen to be marginally more compressible than the a-axis.     

Structural flexibility of cuprate superconductors

We identify the lowest local energy state in the CuO₂ plane of cuprate superconductors, the vibration that does not involve distortions of constituent units of the plane, the Rigid Unit Mode (RUM). On lowering the temperature, correlated atomic motion in the plane is related to the RUM vibration. We discuss the relevance of this vibration for superconductivity, including electron–phonon coupling, d-wave symmetry of the order parameter, structural state of the CuO₂ plane and spatial variations of the electronic structure.     

Controllable electron-momentum filter in a δ-doped magnetically modulated semiconductor nanostructure

ABSTRACT

We have theoretically investigated the control of wave-vector filtering (WVF) by introducing δ-doping into a magnetically modulated nanostructure fabricated by depositing ferromagnetic stripes on top and bottom of a GaAs/Al_xGa_{1 ?x}As heterostructure. With the help of an improved transfer matrix method, the Schrödinger equation for electrons in this semiconductor nanostructure is solved exactly and the transmission efficient calculated numerically. We demonstrate that the WVF efficiency is associated closely with the weight and position of the δ-doping, which may be helpful for designing a controllable electron-momentum filter based on such a magnetically modulated semiconductor nanostructure.     

Synthesis and characterization of nanocrystalline MoBi2Te5 thin films for photoelectrode applications

Molybdenum bismuth telluride thin films have been prepared on clean glass substrate using arrested precipitation technique which is based on self-organized growth process. As deposited MoBi₂Te₅ thin films were dried in constant temperature oven at 110°C and further characterized for their optical, structural, morphological, compositional, and electrical analysis. Optical absorption spectra recorded in the wavelength range 300–800?nm showed band gap (E _g) 1.44?eV. X-ray diffraction pattern and scanning electron microscopic images showed that MoBi₂Te₅ thin films are granular, nanocrystalline having rhombohedral structure. The compositional analysis showed close agreements in theoretical and experimental atomic percentages of Mo⁴⁺, Bi³⁺, and Te^2? suggest that chemical formula MoBi₂Te₅ assigned to as deposited molybdenum bismuth telluride new material is confirmed. The electrical conductivity and thermoelectric power measurement showed that the films are semiconducting with n-type conduction. The fill factor and conversion efficiency was characterized by photoelectrochemical (PEC) technique. In this article, we report the optostructural, morphological, compositional, and electrical characteristics of nanocrystalline MoBi₂Te₅ thin films to check its suitability as photoelectrode in PEC cell.     

Pressure-induced structural transition in UGa2

High-pressure X-ray diffraction has been performed on UGa₂ up to 20?GPa using a diamond anvil cell. UGa₂ exhibits the AlB₂-type structure with space group P6/mmm at room temperature and atmospheric pressure. At about 16?GPa a reversible structural transformation to a tetragonal phase was observed. The bulk modulus of the AlB₂-type phase has been determined to be ～100?GPa, which is comparable to rare earth digallides like TmGa₂ and HoGa₂.     

Strain-induced submicrocrystalline grains developed in austenitic stainless steel under severe warm deformation

Strain-induced grain evolution in a 304 type austenitic stainless steel has been studied in multiple compression with the loading direction being changed in each pass. The tests were carried out to total strains above 6 at 873 K (0.5 T _m) at a strain rate of about 10^-3 s^-1. Multiple deformation promotes the rapid formation of many mutually crossing subboundaries because various slip systems operate from pass to pass. The gradual rise in misorientations across dislocation subboundaries with increasing strain finally leads to the evolution of very fine grains with large-angle boundaries. It is concluded that a new grained structure can result from a kind of continuous reaction during deformation, namely continuous dynamic recrystallization. Such deformation-induced grains are characterized by relatively low densities of dislocations, and considerable lattice curvatures developed in their interiors. The latter observations suggest that high elastic distortions are developed in the grain interiors and so such strain-induced grain structures are in a non-equilibrium state.     

Reaction of a Ni-coated Al nanoparticle to form B2-NiAl: A molecular dynamics study

The kinetic reaction in a Ni-coated Al nanoparticle with equi-atomic fractions and diameter of approximately 4.5 nm is studied by means of molecular dynamics simulation, using a potential of the embedded atom type to model the interatomic interactions. First, the large driving force for the alloying of Ni and Al initiates solid state amorphization of the nanoparticle with the formation of Ni₅₀Al₅₀ amorphous alloy. Amorphization makes intermixing of the components much easier compared to the crystalline state. The average rate of penetration of Ni atoms can be estimated to be about two times higher than Al atoms, whilst the total rate of inter-penetration can be estimated to be of the order of 10^?2 m/s. The heat of the intermixing with the formation of Ni₅₀Al₅₀ amorphous alloy can be estimated at approximately ?0.34 eV/at. Next, the crystallization of the Ni₅₀Al₅₀ amorphous alloy into B2-NiAl ordered crystal structure is observed. The heat of the crystallization can be estimated as approximately ?0.08 eV/at. Then, the B2-NiAl ordered nanoparticle melts at a temperature of approximately 1500 K. It is shown that, for the alloying reaction in the initial Ni-coated Al nanoparticle, the ignition temperature can be as low as approximately 200 K, while the adiabatic temperature for the reaction is below the melting temperature of the nanoparticle with the B2-NiAl ordered structure.     

Light emission from ferroelectric barium titanate nanocrystals

Thin laminates (thickness δ = 20–45 nm) of barium titanate, BaTiO₃ (BT), have been synthesised by the sol–gel method followed by heating of the amorphous precursor powder in air. An orthorhombic BT polymorph forms along with a tetragonal phase (t-BT) after 2 h of heating the precursor at 600°C, as evidenced by a well-defined X-ray diffraction pattern. The average crystallite size D ～ 21 nm compares the δ-value obtained from the scanning electron micrograph. On heating at temperatures as high as 750°C, the structure remains mostly t-BT (D ? 44 nm), which emits violet–blue–green light at a wavelength of 380–580 nm with a maximum at around 422 nm. The emission extends to wavelengths shorter than ～300 nm owing to a quantum-size effect in smaller crystals processed at lower temperatures. Electron paramagnetic resonance (EPR), which occurs at a g-value of 1.995 (linewidth ΔH = 1.12 mT) in a sample heated at 400°C, shows an order of magnitude of lower intensity at g = 1.993 (ΔH = 3.69 mT) on annealing out the paramagnetic defects at 600°C in air. No EPR signal arises in t-BT free from such defects in larger crystals.     

Effect of network connectivity on thermal properties of As30Te70– x Tl x glasses

Alternating differential scanning calorimetry (ADSC) studies were undertaken to investigate the effect of Tl addition on the thermal properties of As₃₀Te_{70– x} Tl _x (6?≤?x?≤?22 at%) glasses. These include parameters such as glass-transition temperature (T _g), changes in specific heat capacity (ΔC _p) and relaxation enthalpy (ΔH ^NR) at the glass transition. It was found that T _g of the glasses decreased with the addition of Tl, which is in contrast to the dependence of T _g in As–Te glasses on the addition of Al and In. The change in heat capacity ΔC _p through the glass transition was also found to decrease with increasing Tl content. The addition of Tl to the As–Te matrix may lead to a breaking of As–Te chains and the formation of Tl⁺Te^?AsTe_2/2 dipoles. There was no significant dependence of the change of relaxation enthalpy, through the glass transition, with composition.     

The crystal structure of Al20.6Ta22.4 and its relation to Frank–Kasper phases

The monoclinic phase in the Al–Ta system has been identified by means of X-ray single-crystal structure analysis as Al_{19+ x} Ta_{24? x} (x?=?1.6), Pearson symbol mP86. Al_20.58Ta_22.42 crystallizes in the space group P2₁/n, with a?=?987.86?pm, b?=?990.12?pm, c?=?1489.45?pm and β?=?99.958°. Twinned crystals of the title compound can be obtained by iodine-promoted reactions of the elements. In the experiments described here, the reactions took place in sealed tantalum ampoules at 1400°C. Single-phase samples were obtained by a metallurgical powder method, which minimizes aluminium loss by incongruent vapourization. The composition was found to span the range 0.51?≤?x _Ta?≤?0.53. Al_{19+ x} Ta_{24? x} decomposes in the solid state at 1450°C into Al₆₉Ta₃₉ and σ-AlTa₂. The crystal structure is analyzed in relation to tetrahedral close-packed structures. The kinship with Cr₃Si-type and H-type structures is accentuated.     

Type II twinning in devitrite,Na2Ca3Si6O16

Nanometre-scale twins found in the triclinic crystalline phase devitrite, Na₂Ca₃Si₆O₁₆, are shown to exhibit the relatively rare form of twinning, Type II twinning, in which the crystal structure of one of the parts is related to the crystal structure of the other by a rotation of 180° about η ₁?=?[1?0?0]. Although formally, the twin plane K ₁ is irrational, it is in essence (0?1?0).     

Investigations of the optical spectra and local structure for nickel(II) ions in XF2 (X = Mg,Zn) single crystals

A complete diagonalization energy matrix method (CDM) is proposed for 3d ⁸ ions in orthorhombic site (D _{2 h} ) symmetry. As an application, the optical spectra of XF₂ : Ni²⁺ (X = Mg, Zn) is well explained on the basis of both the CDM and the semi-self-consistent field d-orbital (semi-SCF d-orbital) theory. In addition, by establishing the relationship between the optical spectra and local structure, it has been possible to calculate the crystal structure parameters for XF₂:Ni²⁺ from optical measurements on Ni²⁺ ions. The theoretical results are in good agreement with experimental findings.     

Frequency-temperature dependent dynamics of dielectrics in ferric oxoborate Fe3BO6 of cocktail structure of nanorods

Oxoborates have both dielectric and magnetic properties useful for magnetodielectric devices, sensors, or biological tools. Such compounds Fe₂BO₄, Fe₃BO₅, or Fe₃BO₆ are known to grow easily as single crystals in a liquid flux. A polycrystalline phase forms only on controlled conditions of a solid state reaction of the basic oxides. In this study, we report highly dielectric Fe₃BO₆ when grown in a specific shape of nanorods (～200?nm diameter and 50–100?µm length) from an iron borate glass, which offers devisable shapes of sheets, discs, and fibers. Frequency (f)-temperature reliant dynamics of dielectric constant ε_r is studied over 25–300°C at 0.1–10^3?kHz frequencies. At low frequency such as 100?Hz, a large ε_r -value 40,000, better than most of high ε_r -value ferroelectrics, incurs at room temperature. At f?≥?50?kHz, although only an order of diminished ε_r -value lasts, it increases steadily with temperature, possibly due to increasing electrical conductivity in a specific resistor–capacitor network. Suppressed dielectric relaxation and spin-flops share a merely weak spin-reorientation transition near 160°C. A stable power loss ≤0.5 lasts at f?>?10?kHz useful for possible applications of magnetodielectric materials.     

Domains and Na substitution in the superconductor Y0.6Na0.4Ba2Cu2.7Zn0.3O7– δ

The high-temperature superconductor Y_0.6Na_0.4Ba₂Cu_2.7Zn_0.3O_{7– δ} , with a T _c of about 60?K, has been studied by electron diffraction and high-resolution electron microscopy. High densities of twin boundaries and anti-phase boundaries are observed and analysed. Darkness-level profiles at the positions of Ba and Y atoms in the [100] experimental and simulated images are used to determine the position and content of the Na atoms. It is shown that Na substitutes for Y in the crystal structure and the atomic ratio between Na and Y can be estimated quasi-quantitatively.     

Morphological and magnetic response of copper-substituted nickel ferrite nanoparticles

Ferrite nanoparticles are interesting materials owing to their unique physical and chemical properties. The metal-doped ferrites have well-defined structures and magnetic response, such as high permeability for a specific frequency range. In this study, copper-substituted nickel ferrite (Ni_1?xCu_xFe₂O₄) nanoparticles with a compositional range of 0?≤?x?≤?0.3 were synthesised through a co-precipitation technique. Metal chlorides were used as precursors and NaOH as a precipitating agent for the growth of ferrite nanoparticles. To minimise the internal stresses and maximise the magnetic response, ferrite nanoparticles were annealed in a furnace at 700°C for 6 h. The structural and magnetic response of Ni_1?xCu_xFe₂O₄ ferrite with different values of x were investigated using Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FT-IR), Vibrating Sample Magnetometer (VSM) and X-ray Diffraction (XRD) techniques. XRD analysis confirmed the formation of cubic spinel structure of single phase for all the compositions. The lattice constant decreased with increase in the value of x. FT-IR study showed two main metal oxygen bonds in the range 500–700 cm^?1 confirming the formation of a single-phase cubic inverse structure of Cu-substituted Ni ferrite. VSM results revealed the formation of ferrimagnetic nanoparticles. The optical and magnetic response of the ferrite nanoparticles changed with Cu content.     

Growth of carbon nanotubes: effect of Fe diffusion and oxidation

The diffusion and surface oxidation rates of Fe deposited on Si and barrier layers of Al/SiO₂ and Al₂O₃/SiO₂ have been comparatively studied and correlated with the growth of carbon nanotubes (CNTs). Initially, Fe/Si, Fe/Al/SiO₂/Si and Fe/Al₂O₃/SiO₂/Si samples were subjected to thermal chemical vapour deposition (CVD) at ～650°C for ～30?min to grow the CNTs. Scanning electron microscopy analysis showed that the height of the CNTs on the Fe/Al₂O₃/SiO₂/Si samples was relatively high (～9.5–11?µm), as compared with the other samples. To investigate this, a few as-prepared samples were thermally annealed at ～650°C for ～30?min and characterized by dynamic secondary ion mass spectroscopy (D-SIMS) and X-ray photoelectron spectroscopy (XPS). The D-SIMS results showed that the diffusion depth, x _Fe, and magnitude of the diffusivity, D _Fe, of the Fe atoms are highest for the Fe/Si sample. This is attributed to vacancy-mediated migration, which leads to the formation of unstable, non-stoichiometric Fe–Si and Fe–O–Si phases. However, for the Fe/Al₂O₃/SiO₂/Si samples, the magnitudes of x _Fe and D _Fe are found to be the lowest, which indicates steric hindrance to Fe by the Al₂O₃ layers. The XPS analysis revealed that the surface metallic state, after annealing, is almost unaffected for the Fe/Al₂O₃/SiO₂/Si samples, whereas the majority of the Fe precipitate was observed to be oxidized in the case of the other samples.