B2-type Structure Research Articles

Nanoscale structures of polarisation-modulated bent-core materials in thin films

We present small angle X-ray and cryogenic transmission electron microscopy (cryo-TEM) studies of three nitro-substituted (n-OPIMB-NO2, n = 7, 9 and16) bent-core liquid crystals (LCs) that form peculiar optical patterns known as B7 textures. Small angle X-ray scattering studies show two periodicities between a ~ 3.4–4.7 nm and bX ~ 9–16 nm. Cryo-TEM images of about 100 nm thick films do not show the a-periodicity, but only periodicities bT that are about 1 nm smaller than bX measured in bulk by X-ray. Additionally, an intensity modulation with a period c ~ 100–200 nm is observed. These observations can be explained by a B1-type structure, where there are half-layer step discontinuities at the boundaries of the splay domains, and where the molecules are everywhere normal to the substrate.

Structural stability and electronic structure of YCu ductile intermetallic compound by first-principal calculation.

We investigate the structural, elastic and electronic properties of cubic YCu intermetallic compound. Which crystallize in the CsCl- B2 type structure, the investigated using the first principle full potential linearized augmented plane wave method (FP-LAPW) within density functional Theory (DFT). We used generalized gradient approximation (GGA) and local density theory (LDA), which is based on exchange correlation energy optimization, are used to determinate the total energies and the electronic band structures. The latter provide us with the ground states properties such as lattice parameter, bulk modulus and its pressure derivative, elastic constants and the structural phase stability with respect to the B 1 , B 3 , and L 10 structures ¶of this compound. Our results are in good agreement with numerous experimental and theoretical data where available.

High spin polarization and spin splitting in equiatomic quaternary CoFeCrAl Heusler alloy

In this paper, we investigate CoFeCrAl alloy by means of ab-initio electronic structure calculations and various experimental techniques. The alloy is found to exist in the B2-type cubic Heusler structure, which is very similar to Y-type (or LiMgPdSn prototype) structure with space group F-43m (#216). Saturation magnetization (MS) of about 2µB/f.u. is observed at 8K under ambient pressure, which is in good agreement with the Slater–Pauling rule. MS values are found to be independent of pressure, which is a prerequisite for half-metals. The ab-initio electronic structure calculations predict half-metallicity for the alloy with a spin slitting energy of 0.31eV. Importantly, this system shows a high current spin polarization value of 0.67±0.02, as deduced from the point contact Andreev reflection measurements. Linear dependence of electrical resistivity with temperature indicates the possibility of reasonably high spin polarization at elevated temperatures (~150K) as well. All these suggest that CoFeCrAl is a promising material for the spintronic devices.

Magnetocaloric Properties of Melt-Spun Fe–Ni–Mn–Ga Ribbons

The effects of Ni substitution for Fe on phase constitutions, Curie temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , and magnetocaloric properties of melt-spun Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50-x</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">25</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">25</sub> (x = 0, 1, 3, 5 and 7) ribbons have been investigated. X-ray diffraction results show that the main phase in the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50-x</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">25</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">25</sub> (x = 0-7) alloy changed with the increase of Ni content from FCC structure for x = 0 into B2 structure for x = 1-7. Besides, the magnetic phase exhibits phase transition of ferromagnetic into paramagnetic state with increasing temperature for the samples with B2-type structure. The Curie temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of these ribbons varies in the temperature range of 232-257 K. The peak values of the maximal magnetic entropy change, -ΔS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sup> , are about 1.4-1.6 Jkg/K for Ni-substituted ribbons at a maximum applied field of 30 kOe. On the other hand, the relatively broader temperature range at the half maximum of ΔS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</sub> peak (~ 90 K), low-cost and nontoxic elements make Fe-Ni-Mn-Ga-based ribbons the promising candidates for magnetic refrigeration applications close to room temperature.

Un recorrido por las propiedades microestructurales de la aleación Al-Fe-Cr

Al-Fe-Cr alloy was rapidly solidified by means of melt spinning technique. Alloy structure has been studied by SEM, TEM and X-Ray diffraction. The ribbons were annealing at 500 ��C during 48 hours, in order to study the influence of thermal treatment onto the Al-Fe-Cr microstructure. The melt-spun Al- Fe-Cr microstructure underwent a considerable refinement as compared with same composition alloy produced by conventional methods of melting and casting. In melt�]spun ribbons the grain size decreases as the cooling increases. An ordered B2 type structure was obtained in all ternary rapidly solidified and annealed alloys at all cooling rates schedules.

Pressure-induced phase transitions in LnTe (Ln=La, Gd, Ho, Yb) and AmTe

The structural behaviour under compression of different lanthanide (La, Gd, Ho, Yb) and actinide (Am) monochalcogenides is studied by means of in situ high-pressure x-ray diffraction. All the investigated compounds crystallize at ambient conditions within a cubic (B1) NaCl-type structure but show different behaviours at high pressures. LaTe and AmTe undergo B1 to B2 (CsCl-type structure) phase transitions, starting at 9 GPa and 12 GPa, respectively. The high-pressure phase of AmTe exhibits an electronic transition, identified by an anomaly in the compression curve which is accompanied by a sample colour change. The other three monochalcogenides studied here show clear evidence of decomposition and amorphization under pressure and are, to the best of our knowledge, the first in the LnTe series to show a pressure-induced amorphization. The bulk moduli of all B1-type structure compounds are calculated using the third-order Birch–Murnaghan equation of state.

Local crystal structure of TiNiCu shape memory alloys

The features of long-range crystal structure and local environment of Ni and Cu atoms in shape memory alloy Ti50Ni25Cu25 are investigated using X-ray diffraction and extended X-ray absorption fine structure spectroscopy (EXAFS) in the temperature range of direct and reverse martensitic transformations. According to XRD study, alloy has the B2 type structure in austenitic phase and B19 type structure (space group Pmmb) in martensitic phase. EXAFS demonstrates the difference between Cu-Ti and Ni-Ti bond lengths ~ 0.09 Å not typical for common TiNi lattice. The static disordering is more significant in the local environment of Ni atoms than in local environment of Cu atoms.

Structure and properties of PbS films

The crystal structure of thin lead sulfide (PbS) films prepared by chemical deposition from aqueous solutions on a glass substrate is studied by X-ray diffraction analysis. It is established that the PbS films studied have a cubic (space group Fm3¯m) D03-type crystal structure different from the B1-type structure. Electron microscopic investigation of a film microstructure has shown that about half of all particles have a size of 60nm and less. In the wavelength range from 300 to 3200nm the optical transmission of PbS nanostructured cubic films with D03 structure and thickness from ∼120 to ∼400nm is measured. Decreasing of average size of nanoparticles in PbS films leads to increasing of energy band gap Eg from 0.85 to 1.5eV. These values of band gap are larger than that of coarse-grained PbS equal to 0.41eV. It testifies on blue shift of the absorption band in studied nanostructured lead sulfide films.

Synthesis, microstructure and magnetic properties of Heusler Co 2 FeSn nanoparticles

Heusler Co2FeSn nanoparticles in a B2-type cubic structure were synthesised for the first time using a simple solution reduction method. X-ray diffraction, a scanning electron microscope, an energy dispersive spectrometer and a vibrating sample magnetometer were used to characterise the synthesised products. The crystal structure and composition of the samples vary with the concentration of potassium hydroxide, synthesis temperature and reaction time. Magnetic measurement reveals that the obtained Co2FeSn nanoparticles are soft magnetic at room temperature. The Co2FeSn nanoparticles synthesised in this work are magnetically superior and economically feasible for spintronic applications.

The phase evolution, magnetic and exchange bias properties in Fe50Mn24+xGa26−x (x = 0–3) melt-spun ribbons

Effect of Ga content on the phase constitution, magnetic phase transition temperatures (TC), and exchange bias (EB) behaviors of Fe50Mn24+xGa26−x (x=0, 0.5, 1, 3) as-spun ribbons have been investigated. Fe50Mn24Ga26 ribbon exhibits a magnetic phase transition of B2-type structure from FM to PM state at TC1=190K. Another magnetic phase transition of face-center-cubic structure from AFM to FM takes place at TC2=240K for Fe50Mn25Ga25 sample. The values of exchange bias field (HE) at 10K after field-cooling (FC) at 30kOe from 300K are 14, 41, 1886, 1740Oe, respectively, as x is increased from 0 to 3. The phenomena are attributed to the change of exchange interactions at FM/AFM interfaces due to the change of proportion of FM and AFM phases in low temperature magnetic states. The maximum exchange bias field HE=2.4kOe at 10K after field-cooling at 40kOe is observed for Fe50Mn25Ga25.

Disorder and weak localization effects in Co2MnxTi1−xAl Heusler alloy thin films

The effects of disorder on the structural, magnetic and transport properties of stoichiometric Co2MnxTi1−xAl (0 ⩽ x ⩽ 1) thin films are reported. All the compositions exhibited B2-type structure with the actual composition of each film, as determined by Rutherford backscattering spectroscopy (RBS), being close to the nominal value. The values of saturation magnetization increase with increasing Mn content in the films and are in general agreement with the values obtained using the Slater–Pauling rule and the actual (RBS determined) compositions. Electrical resistivity as a function of temperature shows a change from metallic-type behaviour for x ⩽ 0.25 to a semiconducting type for x > 0.25. At lower temperatures (T < 25 K) metallic compositions show the presence of a resistivity minimum. The resistivity behaviour above low T upturn in metallic compositions follows a T2 dependence that is explained in terms of electron–electron scattering and one-magnon scattering. The presence of a resistivity minimum and the crossover from metallic to semiconducting type resistivity are explained in terms of localization effects originating mainly in the Co antisite disorder in these alloy films.

Surface morphology and texture of TiAlN/CrN multilayer coatings

Multilayer thin films of TiAlN/CrN were deposited at varying bilayer periods (λ=2, 4, 8 and 16nm), along with a co-deposited (TiAlCr)N film and constituent TiAlN and CrN films, in order to examine the effects of periodicity on grain structure, texture and surface morphology. Characterization was performed using electron microscopy, atomic force microscopy and X-ray diffraction analysis. All films exhibited a rock-salt B1-type structure. Transmission and scanning electron microscopy cross-sections showed a columnar morphology with lateral coarsening of the columns during film growth that is more pronounced in the multilayer films with smaller bilayer periods. The surface morphology was found to be dependent on period with sharp pyramidal surface features at λ=2nm, an increase in secondary faceting with increasing periodicity, and nearly rounded column tops at λ=16nm. Consistent with these morphological observations, surface roughness measurements showed an increasing roughness with decreasing bilayer period. X-ray diffraction pole figures revealed that the multilayer films had a crystallographic texture that could be described as preferential [200] poles tilted approximately 15° away from the substrate normal and clustered near the deposition direction. The distribution of the [111] poles was nearly random for the films with larger periodicities (8 and 16nm), but a small [111] texture component was noted for the 2 and 4nm period films. A mechanism for the effect of the multilayer period on the surface morphology is proposed wherein the higher interfacial energy density in the smaller period multilayers promotes a higher degree of (200) faceting at the film surface, reducing grain orientation effects on adatom mobility.

First principles study of structural, electronic, elastic and thermal properties of YX (X = Cd, In, Au, Hg and Tl) intermetallics

The structural, electronic, elastic and thermal properties of YX (X = Cd, In, Au, Hg and Tl) intermetallic compounds crystallizing in B2-type structure have been studied using first principles density functional theory within generalized gradient approximation (GGA) for the exchange correlation potential. Amongst all the YX compounds, YIn is stable in distorted tetragonal (P4/mmm) CuAu-type structure at ambient pressure with very small energy difference of 0.00681 Ry. but it undergoes to CsCl-type (B2 phase) structure at 23.3 GPa. Rest of the compounds are stable in B2 structure at ambient condition. The values of elastic moduli as a function of pressure are also reported. The ductility of these compounds has been analyzed using the Pugh rule. Our calculated results indicate that YTl is the most ductile amongst all the B2-YX compounds. YAu is the hardest and less compressible compound due to the largest bulk modulus. The elastic properties such as Young's modulus (E), Poisson's ratio (σ) and anisotropic ratio (A) are also predicted. The anisotropic factor is found to be unity for YHg which shows that this compound is isotropic.

Effect of high temperature annealing on ion-irradiation induced magnetization in FeRh thin films

Low temperature ferromagnetic FeRh with B2-type crystal structure was successfully synthesized by annealing of the excessively irradiated FeRh thin film samples having nonmagnetic A1-type crystal structure. The B2 phase transformed from the A1 phase by this process contained some amount of lattice defects, such as anti-site defects and vacancies, which made the magnetic spins of the sample aligned. These results imply that the combination of the process of the ion-beam irradiation and annealing of the film samples makes the magnetic state systematically controlled.

Phase and structural transformations in the Ti49.5Ni50.5 alloy with a shape-memory effect during torsion under high pressure

Results of investigations of structural and phase transformations that occur in the titanium-nickelide-based alloy Ti49.5Ni50.5 with a shape memory effect during severe plastic deformation by torsion under high pressure (HPT) are reported. The studies were performed using transmission and scanning electron microscopy, neutron and X-ray diffraction, and measurements of temperature dependences of electrical resistivity. The martensitic B2 → B19′ transformation was found to be induced in the alloy when applying a high pressure. After unloading, the martensitic B19′ phase is retained in the alloy. The fine structure of the B19′ martensite and its evolution into nanocrystalline and, subsequently, amorphous state during HPT with 1/4, 1/2, 1, 5, and 10 rev have been studied. It was shown that, after HPT, all nanosized crystallites whose sizes are less than 30–50 nm have a B2-type structure and, therefore, the reverse martensitic B19′ → B2 transformation is realized in the alloy at room temperature after unloading.

59Co nuclear magnetic resonance study of the local distribution of atoms in the Heusler compound Co2FeAl0.5Si0.5

In this work, the spin-echo nuclear magnetic resonance (NMR) technique is used to probe the local structure of Co2FeAl0.5Si0.5 bulk samples. The 59Co NMR spectrum of the Heusler compound Co2FeAl0.5Si0.5 consists of four main resonance lines with an underlying sub-structure. The splitting into the main resonance lines is explained by contributions of the B2 type structure. The sub-lines are attributed to a random distribution of Al and Si. By comparing the experimental results with an appropriate multinomial distribution, the fraction of the Al/Si intermixing and the ratio between the contributing structure types is assigned. The main structural contribution of as-cast bulk samples is of B2 type with 38% of L21 contributions. The L21 contribution can be enhanced to 59% by an appropriate annealing process. However, B2 contributions are still present after annealing. Additional foreign phases such as fcc-Co and Co-Al, with relative contributions of less than one percent, are also found in both as-cast and annealed samples. Resonance lines related to slight amounts of the ternary, parental Heusler compounds Co2FeAl and Co2FeSi are also observed.

First-principles calculations of phonon and thermodynamic properties of AlRE (RE = Y, Gd, Pr, Yb) intermetallic compounds

The phonon and thermodynamic properties of aluminum–rare-earth (AlRE, RE = Y, Gd, Pr, Yb) intermetallics with B2-type structure are investigated by performing density functional theory and density functional perturbation theory calculations within the quasiharmonic approximation. The phonon spectra and phonon density of states, including the phonon partial density of states and total density of states, are discussed. Our results demonstrate that the density of states is mostly composed of Al states at high frequency. The temperature dependences of various quantities such as thermal expansion, heat capacities at constant volume and pressure, the isothermal bulk modulus and the entropy are obtained. The electronic contribution to specific heat is discussed, and the present results show that the thermal electronic excitation affecting the thermal properties is inessential.

The temperature-dependent elastic properties of B2-MgRE intermetallic compounds from first principles

The temperature-dependent elastic modulus of MgRE (RE=Y, Dy, Pr, Sc, Tb) intermetallics with B2-type structure are presented from first-principles quasistatic approach, in which the static volume-dependent elastic constants are obtained by the first-principles total-energy method within density functional theory and the thermal expansion is obtained from the quasiharmonic approach based on density-functional perturbation theory. The comparison between the predicted results and the available experimental data for a benchmark material NiAl provides good agreements. At T=0 K, our calculated values of lattice parameter and elastic moduli for MgRE intermetallics show excellent agreement with previous theoretical results and experimental data. With temperature increasing, we find that the elastic constants satisfy the stability conditions for B2 structures and follow a normal behavior with temperature, i.e., decrease and approach linearity at higher temperature and zero slope around zero temperature. In addition, the sound velocities as a function of temperature for the NiAl and MgRE intermetallics are calculated and the relations to phonon spectrums have also been discussed.

Vacancy distribution in nonstoichiometric vanadium monoxide

Structural vacancy distribution in the crystal lattice of the tetragonal V 52O 64 superstructure which is formed on the basis of disordered superstoichiometric cubic vanadium monoxide VO y ≡V x O z is experimentally determined and the presence of significant local atomic displacements and large local microstrains in a crystal lattice of real ordered phase is established. It is shown that the relaxation of local microstrains takes place owing to the basic disordered cubic phase grain refinement and a formation of ordered phase domains. The ordered phase domains grow in the direction from the boundaries to the centre of grains of the disordered basic cubic phase. Isothermal evolution at 970 K of the average domain size 〈 D〉 in ordered VO 1.29 vanadium monoxide is established. It is shown that the short-range order presents in a metal sublattice of disordered cubic VO y vanadium monoxide. The character of the short-range order is such that vanadium atoms occupying tetrahedral positions are in the environment of four vacant sites of the vanadium sublattice. This means that the disordered VO y vanadium monoxide, especially superstoichiometric VO y>1.0 monoxide, has a cubic (space group F m 3 ¯ m ) D0 3-type structure differing from the B1-type structure typical for most of the strongly nonstoichiometric cubic compounds carbides, nitrides and oxides.

Influence of grain refinement and induced crystal defects on the magnetic properties of Ni50Mn36Sn14 Heusler alloy

The effect of high energy ball milling on the structural, magnetic and magnetocaloric properties of Ni50Mn36Sn14 Heusler-type alloy has been studied. X-ray diffraction results have revealed a reduction in the crystalline grain size concomitantly with defect inclusions in the crystalline lattice, favouring a chemical disorder effect that transforms the L21–B2-type disordered structure to a simple cubic B2-type structure for increasing milling time. From magnetometry and Mössbauer spectroscopy results, a decrease in the ferromagnetic exchange interaction contribution, an enhancement of the effective exchange bias field and a significant reduction in the magnetic entropy change of the milled alloy are observed.