B2-type Structure Research Articles

Reactive magnetron cosputtering of hard and conductive ternary nitride thin films: Ti–Zr–N and Ti–Ta–N

Ternary transition metal nitride thin films, with thickness up to 300 nm, were deposited by dc reactive magnetron cosputtering in Ar–N2 plasma discharges at 300 °C on Si substrates. Two systems were comparatively studied, Ti–Zr–N and Ti–Ta–N, as representative of isostructural and nonisostructural prototypes, with the aim of characterizing their structural, mechanical, and electrical properties. While phase-separated TiN–ZrN and TiN–TaN are the bulk equilibrium states, Ti1−xZrxN and Ti1−yTayN solid solutions with the Na–Cl (B1-type) structure could be stabilized in a large compositional range (up to x=1 and y=0.75, respectively). Substituting Ti atoms by either Zr or Ta atoms led to significant changes in film texture, microstructure, grain size, and surface morphology, as evidenced by x-ray diffraction, x-ray reflectivity, and scanning electron and atomic force microscopies. The ternary Ti1−yTayN films exhibited superior mechanical properties to Ti1−xZrxN films as well as binary compounds, with hardness as high as 42 GPa for y=0.69. All films were metallic, the lowest electrical resistivity ρ∼65 μΩ cm being obtained for pure ZrN, while for Ti1−yTayN films a minimum was observed at y∼0.3. The evolution of the different film properties is discussed based on microstructrural investigations.

Microstructure and properties of age-hardenable Al xCrFe 1.5MnNi 0.5 alloys

Two promising high-entropy alloys (HEAs) Al x CrFe 1.5MnNi 0.5 ( x = 0.3 and 0.5) were designed from Al–Co–Cr–Cu–Fe–Ni alloys by substituting Mn for expensive Co and excluding Cu to avoid Cu segregation. Microstructures and properties were investigated and compared at different states: as-cast, as-homogenized, as-rolled and as-aged states. Al 0.3CrFe 1.5MnNi 0.5 alloy in the as-cast, as-homogenized and as-rolled states has a dual-phase structure of BCC phase and FCC phase, in which Al, Ni-rich precipitates of B2-type BCC structure disperse in the BCC phase. Al 0.5CrFe 1.5MnNi 0.5 alloy in the corresponding states has a matrix of BCC phase in which Cr-rich particles of BCC structure and Al, Ni-rich precipitates of B2-type BCC structure disperse. These three BCC phases have the same lattice constant. Both alloys are workable and show a hardness range of Hv 300–500 in the as-cast, as-forged, as-homogenized and as-rolled states. Al 0.5CrFe 1.5MnNi 0.5 alloy has a higher hardness level than Al 0.3CrFe 1.5MnNi 0.5 one because of its full BCC phase. Both alloys thus can be used as structural parts requiring stronger strength. Both alloys display a significant high-temperature age-hardening phenomenon. As-cast Al 0.3CrFe 1.5MnNi 0.5 alloy can attain the highest hardness, Hv 850, at 600 °C for 100 h, and Al 0.5CrFe 1.5MnNi 0.5 can get even higher hardness, Hv 890. The aging hardening is resulted from the formation of ρ phase (Cr 5Fe 6Mn 8-like phase). Prior rolling on the alloys before aging could significantly enhance the age-hardening rate and hardness level due to introduced defects. Al 0.5CrFe 1.5MnNi 0.5 alloy exhibits excellent oxidation resistance up to 800 °C, which is better than Al 0.5CrFe 1.5MnNi 0.5 alloy. Combining this merit with its high softening resistance and wear resistance as compared to commercial alloys Al 0.5CrFe 1.5MnNi 0.5 alloy has the potential for high-temperature structural applications.

Pressure-induced orthorhombic structure of PbS

The pressure-induced crystal structure of lead sulfide (PbS) above 2.2 GPa hasbeen studied with single-crystal x-ray diffraction in a diamond anvil cell atroom temperature. It has been found to be twinned and of the TlI type (Cmcm,Z = 4), in which the Pb atoms are surrounded by seven S atoms in a capped trigonal prismcoordination. The twin laws in relation to the parent B1 (NaCl) type structure (, Z = 4) at atmospheric pressure have been discussed.

Observation of Reverse Transformation in Metamagnetic Shape Memory Alloy Ni<SUB>40</SUB>Co<SUB>10</SUB>Mn<SUB>34</SUB>Al<SUB>16</SUB> by High-Field X-Ray Diffraction Measurements

The structural properties of metamagnetic shape memory alloy Ni40Co10Mn34Al16 were investigated by X-ray powder diffraction measurements in the temperature range from 283 to 473 K and magnetic fields up to 5 T. With increasing temperature from 283 K, the alloy undergoes the reverse martensitic transformation from the tetragonal L10-type structure with the lattice parameter at ¼ 0:3853 nm and ct ¼ 0:3299 nm into the B2-type cubic structure with the lattice parameter ac ¼ 0:2924 nm at As ¼ 383 K. At 408 K (>As), we directly observed the field-induced reverse martensitic transformation. [doi:10.2320/matertrans.M2010150]

Crystal structure of nanostructured PbS films at temperatures of 293–423 K

The crystal structure of lead sulfide films prepared by the hydrochemical deposition has been studied by X-ray diffraction. The thickness of the films synthesized is ∼100 nm, the size of coherent scattering regions is ∼70 nm, and the value of microstrains is ∼0.20%. It is established, for the first time, that the as-synthesized PbS films and the same films annealed in the temperature range 293–423 K have a cubic crystal structure (space group Fm3m) different from the B1-type structure. In the crystal lattice of the structure revealed, sulfur atoms are located not only in the 4(b) positions but also in the 8(c) positions. The occupancies of the 4(b) and 8(c) positions by the S atoms are ∼0.84 and 0.08, respectively.

Atomic displacements in the V52O64 superstructure and the short-range order in superstoichiometric cubic VO y vanadium monoxide with metal vacancies

Atomic displacements in the lattice of the tetragonal V52O64 superstructure have been experimentally determined. It has been found that atomic displacement waves, which are attributed to the formation of the short-range displacement order, appear in the vanadium and oxygen sublattices of this superstructure. It has been shown that the V52O64 superstructure is formed on the basis of disordered superstoichiometric cubic vanadium monoxide with the short-range order in the metallic sublattice. 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 superstoichiometric VO>1.0 vanadium monoxide has a cubic structure differing from the B1-type structure characteristic of most of the strongly nonstoichiometric cubic compounds MXy (X = C, N, O) of transition metals.

First-Principles Calculation and Experimental Investigations on Full-Heusler Alloy Co$_{2}$FeGe

First-principles calculation has been carried out for the full Heusler alloy Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeGe within the scheme of density functional theory using plane-wave self-consistent field method. The spin polarized band structure does not show any energy gap at the Fermi level for both up and down spin electrons. Atom resolved magnetic moment on each site was observed to be 1.3 mu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> (Co), 2.9 mu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> (Fe), and 0.0 mu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> (Ge). X-ray diffraction studies reveal a B2-type structure for the bulk sample and cubic L2 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> structure for the melt-spun ribbons. The lattice parameter value for the ribbon is 5.736 Aring, and its Curie temperature is around 981 K. The magnetic moment per formula unit at 5 K was observed to be 5.74 mu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> . The low temperature data ( < 100 K) follows the relationrho = rho <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> + AT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> + BT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4.5</sup> , indicating the presence of one magnon spin-flip scattering.

Structure and plasticity in hot deformed FeAl intermetallic phase base alloy

This paper constitutes part of research conducted on the possibility of forming alloys based on intermetallic phases from the Fe–Al system via thermoplastic processing. Insufficient plasticity, which is an inhibitor of further development of these intermetallics as construction materials, makes the range of their applications limited. In the paper, an analysis is conducted of the influence of deformation parameters on the structure of an alloy of Fe–Al with the B2 type structure. Axi-symmetric compression tests were carried out at temperatures ranging from 600 °C to 1200 °C and at a deformation rate from 0.001 s 1 to 10 s − 1 . Structural examination was carried out using light microscopy and transmission electron microscopy. A quantitative evaluation of the structure was made with the use of “Met-Ilo”. The results obtained will be used for the development of mathematical models determining the influence of deformation parameters on the alloy structure.

New crystalline phase in thin lead sulfide films

The crystal structure of thin lead sulfide (PbS) films fabricated by hydrochemical deposition is studied by X-ray diffraction analysis. It is established that both the synthesized PbS films and the same films annealed in the temperature interval of 293–423 K have a cubic (space group Fm\( \bar 3 \)m) crystal structure different from the B1-type structure. In this structure, sulfur (S) atoms are located not only in positions 4(b) (octahedral interstices of the face-centered sublattice of lead (Pb) atoms) but also in positions 8(c), i.e., in tetrahedral interstices. The occupations of positions 4(b) and 8(c) by S atoms are ∼0.84 and ∼0.08, respectively. Long-range order in the location of S atoms in positions of each type is absent, but correlations may be present. The new revealed structure of PbS films remains stable under a prolonged annealing in the temperature interval from 293 to 423 K.

Electronic structure, magnetic properties and order–disorder phenomena in Co2Mn1−xFexAl

The substitutional series of Heusler compounds Co2Mn1−xFexAl was investigated by experimental and theoretical methods. In the experiments a B2 type structure was found for the complete series by means of powder x-ray diffraction. The magnetic moment increases with the Fe content from about 4μB in Co2MnAl to above 5μB in Co2FeAl. All samples exhibit a soft magnetic character. The Curie temperature is above 700 K and governed by structural changes. The magneto-structural properties, as determined by Mössbauer spectroscopy, show that a statistical distribution of the hyperfine fields appears rather than a single value. The observed isomer shifts point to a transfer of charge between Mn and Fe atoms depending on the Fe content. The experimental findings are well supported by ab initio calculations of the electronic and magnetic structure for disordered systems.

The disorder-order transition in cubic vanadium monoxide with vacancies in the metal sublattice

The disorder-order transition in cubic vanadium monoxide VOy (y = 1.29, 1.30) possessing a B1 type structure and containing vacancies only in the metal sublattice has been studied by X-ray diffraction and symmetry analyses. It is established that the formation of a tetragonal (space group I41/amd) ordered V52O64 phase in cubic vanadium monoxide VOy proceeds in the form of the first-order phase transition via a channel involving 22 nonequivalent superstructural vectors of four stars ({k10}, {k4}, {k3}, and {k2}). The distribution function of V atoms in the tetragonal V52O64 superstructure is calculated, and it is found that the real ordered V51.6O64 phase exhibits significant atomic displacements. The boundaries of the domain of existence of the V52O64 phase at 54–60% O are determined in the phase diagram of the V-O system.

Exchange interactions and critical temperature of bulk and thin films of MnSi: A density functional theory study

Recent theoretical work [H. Wu et al., Phys. Rev. Lett. 92, 237202 (2004); M. Hortamani et al., Phys. Rev. B 74, 205305 (2006); M. Hortamani, Ph.D. thesis, Freie Universit\at, Berlin, 2006] predicted ferromagnetism at zero temperature in thin MnSi films of B2-type crystal structure on Si(100). The relevance of this finding for finite-temperature experiments needs to be clarified by further investigations, since bulk MnSi is a weak ferromagnet with an experimentally measured Curie temperature of only ${T}_{c}=30\text{ }\text{K}$, and ${T}_{\text{c}}$ is generally expected to be lower in thin films than in bulk materials. Here, we estimate ${T}_{c}$ of such MnSi films using a multiple-sublattice Heisenberg model with first- and second-nearest-neighbor interactions determined from density functional theory calculations for various collinear spin configurations. The Curie temperature is calculated either in the mean-field approximation (MFA) or in the random-phase approximation (RPA). In the latter case we find a weak logarithmic dependence of ${T}_{\text{c}}$ on the magnetic anisotropy parameter, which was calculated to be 0.4 meV for this system. In stark contrast to the above mentioned rule, large Curie temperatures of above 200 K for a monolayer (ML) MnSi film and above 300 K for a two ML MnSi film with B2-type structure on Si(100) are obtained within the RPA, and even higher values in MFA. Complementary calculations of MnSi bulk structures and thin unsupported MnSi films are performed in order to analyze these findings. We find that bulk MnSi in the cubic B2 structure is paramagnetic, in contrast to MnSi in the B20 ground-state structure in agreement with the Stoner criterion. In a tetragonally distorted B2 structure, the Mn atoms gradually develop a spin magnetic moment, passing through a low-spin and a high-spin state. However, the ferromagnetism of the MnSi/Si(100) films cannot be explained by tetragonal distortions alone, since the distorted B2 bulk structure is found to order antiferromagnetically. Comparison of the calculations of supported and unsupported films suggests that the reduced coordination of Mn atoms near surfaces and interfaces is crucial for the ferromagnetic ground state of the films. The coordination number of the Mn atoms in B2-type MnSi films on Si(100) constitutes a borderline case, where the spin magnetic moments of Mn are still large despite their sixfold coordination to Si, but the $sp\text{\ensuremath{-}}d$ hybridization with Si states gives rise to a sizable ferromagnetic coupling of the Mn spins. We conclude that the Curie temperatures predicted from the Heisenberg Hamiltonian make thin MnSi films an interesting subject for further experimental investigation of spintronics materials.

DISORDER ENHANCED MAGNETIC MOMENT IN Fe2CrAl RIBBONS

The electronic structure and magnetic properties of Fe 2 CrAl have been investigated theoretically and experimentally. The theoretical calculations show that there is an energy gap in the minority-spin band in the electronic structure of ordered Fe 2 CrAl with the L21 structure and that the spin-polarization ratio is as high as 93%. The calculated total spin moment is 0.97 μB/f.u. However, in melt-spun Fe 2 CrAl ribbons, the B2 type structure, an atomic-site disordered L21 structure, is found and this atomic site disorder gives rise to a magnetic moment of 2.02 μB in the ribbons which is much higher than the theoretical value. The calculations prove that the enhanced magnetic moment is mainly due to the Fe – Cr and Fe – Al types of disorder. In contrast, the Cr – Al type disorder has little influence on the total spin moment and the spin-polarization ratio.

Melting and solidification of TiNi alloys by cold crucible levitation method and evaluation of their characteristics

The addition of Re, Fe and Cr into Ti–50 mol.-%Ni has been carried out to improve the oxidation and mechanical properties. The mono phase consisting of TiNi with the B2 type structure was identified in micro-alloyed materials proposed on the basis of the d-electrons concept. Experimentally, TiNi alloys were melted and solidified by the cold crucible levitation melting (CCLM) method. The TiNi–(Cr, Fe, Re) alloys with high purity and without contamination from a crucible were prepared, and the homogeneous microstructure was achieved by the diffusion mixing effect of CCLM even in the as-cast alloys which contained Re and Cr with higher melting temperatures and different specific gravities. The transformation from austenite to martensite phases occurred in all alloys below or above room temperature. Some alloys had the ability of shape memory even at room temperature. Ternary alloys showed a higher flow stress level compared with the binary TiNi alloy. On the other hand, the oxidation at 1273 K was promoted by the formation of titanium oxides (TiO2) on the alloy surfaces. The oxidation resistance was improved by the formation of the continuous Cr2O3 film in TiNi–Cr alloys. The alloying effects by ternary elements (Re, Fe, Cr) in the intermetallic TiNi as well as metallic materials were explained well using two parameters used in the d-electrons concept.

Hyperfine field distribution in the Heusler compound Co2FeAl probed by 59Co nuclear magnetic resonance

The Heusler compound Co2FeAl is reported to occur in various structures ranging from the completely ordered L21 to the completely disordered A2 structure type. In this work, we use spin echo nuclear magnetic resonance (NMR) to probe the local structure of Co2FeAl bulk samples. The study of Co2FeAl bulk samples provides the unique possibility to verify the intrinsic generic structural properties. The 59Co NMR measurements reveal a distribution of Fe and Al not only in the first neighbouring shells of the 59Co nuclei but also in more distant shells. The analysis of 59Co NMR main resonance lines with an underlying sub-structure confirms that the local structure of the as-cast Co2FeAl bulk samples consists of a B2 type structure with contributions of the L21 type structure of about 10%. The observed sub-lines, which are attributed to a distribution of Fe and Al atoms in more distant shells, were previously not resolved in NMR spectra of Co2FeAl thin films, pointing to better long range order in bulk material than in thin films. We also show that the individual contributions of the structure types can be influenced by annealing.

Origin of diffuse scattering appearing in iron-doped Ti-Ni shape memory alloys

We have investigated diffuse scattering appearing in Ti-Ni based shape memory alloys to understand the origin of instability of the B2-type structure in the system. Transmission electron microscope observation and a first-principle electronic structure calculation have been made for Ti-(50-x)Ni-x Fe (x = 6, 7, 8, 10). They exhibit diffuse scattering in the electron diffraction pattern along 〈ξξ0 〉* direction on cooling with the intensity peak located at incommensurate positions of qdiffuse = 〈ζζ0 〉*. The value ζ increases with decreasing temperature for all alloys. The temperature at which the incommensurate scattering start to appear is close to the one at which the electrical resistivity exhibits a local minimum, Tmin, for all the alloys. The value of qdiffuse at Tmin is nearly the same as the nesting vector of Fermi surface at Tmin, which is obtained by calculating the generalized susceptibility of the B2-type structure along the 〈ξξ0 〉* direction. These results strongly suggest that the nesting effect of Fermi surface is responsible for the instability of the B2-type structure in Ti-Ni based shape memory alloys.

Iron content and temperature dependences of diffuse scattering in Ti–(50–x)Ni–xFe (6 ≤ x ≤ 10) alloys

Diffuse scatterings appearing in electron diffraction patterns of Ti–(50 − x)Ni–xFe (x = 6, 7, 8, 10, in at.%) alloys were investigated. In the alloys, martensitic (R-phase) transformation is suppressed down to 4.2 K, but the electrical resistivity exhibits a local minimum at T min (210 K, 195 K, 180 K and 140 K for x = 6, 7, 8, 10, respectively). The following results were obtained for all the alloys. Diffuse scattering appears below T min and its intensity maximum is located at an incommensurate position of g + ⟨ζζ0⟩*, where g is a reciprocal lattice vector of the B2-phase. The value ζ at T min is significantly smaller than 1/3 and increases with decreasing temperature; it decreases with increasing Fe content. The value of ζ at T min agrees with the length of nesting vector obtained by a band calculation, suggesting that the diffuse scattering is caused by the nesting effect of the Fermi surface in the B2-type structure.

The first principles study on LaS compound

The structural, electronic and dynamical properties of LaS compound in NaCl ( B1) and CsCl ( B2) structures are studied by performing ab initio calculations within the generalized gradient approximation (GGA). The calculated lattice constants, static bulk modulus, first-order pressure derivative of the bulk modulus and band structures are reported in both NaCl and CsCl structures and compared with theoretical and available experimental results. The transition pressure for the B1 type to B2 type structure is calculated to be about 20.8 GPa. A linear-response approach to density functional theory is used to derive phonon dispersion curves and density of states. The calculated phonon spectra are in good agreement with available experimental results. A detailed discussion of atomic displacement pattern is also presented.

Structural Studies of Metallic Powders Fe-Al-X (X= Fe, Ni, Cu, Cr) Type Obtained by the Self-Decomposition Method and Electro-Magneto-Mechanical Grinding

The Fe-Al-X and Fe-Al-Ni-X (X = Fe,Ni,Cu,Cr) metallic powders obtained by the selfdecomposition method and next intensive grinding in an electro-magneto-mechanical mill were examined by X-ray powder diffraction (XRD). Morphology of powders was determined by scanning electron microscopy (SEM) whereas energy dispersive spectroscopy (EDS) was applied to identify the composition of phases. The FeAlX and FeAlNiX phases both of B2 type structure were found besides small amounts of Al(OH)3 and rest of metallic additions.

Relation between Diffuse Scatterings and Fermi Surface Nesting in Iron Doped Ti-Ni Alloys

We have studied diffuse scatterings appearing in Ti-(50-x)Ni-xFe (x = 6, 7, 8, 10) alloy, which shows negative temperature coefficient in electrical resistivity below a temperature Tmin. Electron diffraction pattern of each alloy exhibits diffuse scatterings below Tmin on cooling. The intensity maximum of the diffuse scatterings is located at an incommensurate position of g+&lt;ζζ0&gt;, where g represents reciprocal lattice points of the B2-phase. The value ζ is slightly smaller than 1/3 and decreases with increasing Fe content. The diffuse scatterings of the present alloys are well explained by the nesting effect of the Fermi surface in the B2-type structure, i.e., ζ of each alloy agrees with the length of nesting vector in the [110] direction, which is evaluated by a first-principle calculation.