B2 Crystal Structure Research Articles

Density-functional study of Mn monosilicide on the Si(111) surface: Film formation versus island nucleation

The stability of thin films and of small crystallites of Mn monosilicide MnSi on the Si111 surface is investigated by density-functional theory calculations. Extending previous studies of MnSi/Si001, our calculations indicate that MnSi films on Si111 have similar electronic and magnetic properties, i.e., large magnetic moments at the Mn atoms near the surfaces and interfaces and a high degree of spin polarization at the Fermi level. Hence, such MnSi films could be interesting as a spintronics material compatible with silicon. Moreover, from our calculated total energies we conclude that the Si111 substrate should be more suitable to grow MnSi layers than the Si001 substrate. This result is obtained by analyzing the conditions for the formation of three-dimensional 3D MnSi islands, either in the B20 crystal structure or as pseudomorphic islands in the B2 structure: On Si001, 3D islands, even if they are just a few lattice constants wide, are found to be already more stable than a homogeneous MnSi film. A bipyramidal “iceberg” island consisting of MnSi in the B20 structure on the Si001 substrate is found to be most stable among the structures investigated. For MnSi on Si111, however, our calculations show that the nucleus for forming a 3D island is larger. Therefore, Mn deposition initially leads to the formation of flat 2D islands. On Si111, the lowest-energy structure for such islands is found to be similar to the B20 structure of bulk MnSi, whereas on Si001 this structure is incompatible with the substrate lattice. Our results are in agreement with the experimental observations, formation of an almost closed film with 33 structure on Si111, and 3D island formation on Si001.

Dependence of structural properties of ZnO on high pressure

We have carried out the first-principles total energy calculations to investigate the structural properties of ZnO, we report total energy calculations for ZnO in the B4 (wurtzite), B3 (zinc blende), B2 (cesium chloride), and B1 (rocksalt) crystal structure over a range of pressure and we present a description of the transition phases of ZnO. The calculations are based on the density functional theory and we have used both the generalized gradient approximation (GGA) and local density approximation (LDA) for the exchange and correlation potential. Our results for transition pressures and equilibrium lattice parameters as well as their dependence on pressure were compared with experimental data and previous calculations.

Crystal structure of TiNi nanoparticles obtained by Ar ion beam deposition

Nanoparticles are a state of matter that have properties different from either molecules or bulk solids, turning them into a very interesting class of materials to study. In the present work, the crystal structure of TiNi nanoparticles obtained by ion beam deposition is characterized. TiNi nanoparticles were obtained from TiNi wire samples by sputtering with Ar ions using a Gatan precision ion polishing system. The TiNi nanoparticles were deposited on a Lacey carbon film that was used for characterization by transmission electron microscopy. The nanoparticles were characterized by high-resolution transmission electron microscopy, high-angle annular dark-field imaging, electron diffraction, scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. Results of nanodiffraction seem to indicate that the nanoparticles keep the same B2 crystal structure as the bulk material but with a decreased lattice parameter.

An Interatomic Potential for Studying CuZr Bulk Metallic Glasses

Glass forming ability has been found in only a small number of binary alloys, one being CuZr. In order to simulate this glass, we fitted an interatomic potential within Effective Medium Theory (EMT). For this purpose we use basic properties of the B2 crystal structure as calculated from Density Functional Theory (DFT) or obtained from experiments. We then performed Molecular Dynamics (MD) simulations of the cooling process and studied the thermodynamics and structure of CuZr glass. We find that the potential gives a good description of the CuZr glass, with a glass transition temperature and elastic constants close to the experimental values. The local atomic order, as witnessed by the radial distribution function, is also consistent with similar experimental data.

Magnetic Properties of Single-Crystalline CoSi Nanowires

We have observed unusual ferromagnetic properties in single-crystalline CoSi nanowire ensemble, in marked contrast to the diamagnetic CoSi in bulk. High-density freestanding CoSi nanowires with B20 crystal structure are synthesized by a vapor-transport-based method. The reaction of cobalt chloride precursor with a Si substrate produces high-aspect-ratio CoSi nanowires. The high-resolution transmission electron microscopy and electron diffraction studies reveal superlattice structure in CoSi nanowires with twice the lattice parameter of simple cubic CoSi lattice. The zero-field-cooled and field-cooled (ZFC-FC) measurements from the nanowire ensemble show freezing of the disordered surface spins at low temperatures. The magnetoresistance (MR) measurements of single nanowire devices show a negative MR whose magnitude gets larger at lower temperatures.

Microscopic model for spiral ordering along (110) on the MnSi lattice

We study an extended Heisenberg model on the MnSi lattice. In the cubic B20 crystal structure of MnSi, Mn atoms form lattices of of corner-shared equilateral triangles. We find an ubiquitous spiral ordering along (110) for J1 <0, and J2=J3 >0, where J1, J2, and J3 are 1st, 2nd and 3rd nearest neighbor Heisenberg interactions, respectively. While the ordering direction of (110) is reasonably robust to the presence of the Dzyaloshinskii-Moriya interaction, it can be shifted to the (111) direction with the introduction of a magnetic anisotropy term for small J2/|J1|. We discuss the possible relevance of these results to the partially ordered state recently reported in MnSi.

Ruthenium-containing bond coats for thermal barrier coating systems

Bond coats for zirconia-based thermal barrier coating systems applied to nickel-based superalloys are typically composed of the B2 NiAl phase. Since RuAl has the same B2 crystal structure but a melting point 400°C higher than NiAl, ruthenium-modified aluminide bond coats could provide improved system temperature capability. Creep experiments on ternary Al−Ni−Ru alloys demonstrate greatly improved creep properties with increasing ruthenium content. Processing paths for ruthenium-modified NiAl-based bond coatings have been established within the bounds of commercially available coating systems. The oxidation resistance of ruthenium-modified bond coats during thermal cycling has been examined, and potential thermal barrier coating system implications are discussed.

Growth and microstructural characterisation of CoTi and CoTi(Zr) crystals

Methods have been developed to produce CoTi and CoTi(Zr) large-grained polycrystals and single crystals by reducing the formation of titanium oxide particles during melting and crystal growth. Oxide particles were found to form when powdered starting materials were used, even though the metals were handled and melted under an inert gas atmosphere. Alloys produced from bulk starting materials contained fewer oxide particles than alloys from powders, but adding a small amount of Al to getter the oxygen was not sufficient to prevent TiO 2 formation. However, the use of a slightly reducing atmosphere during initial melting was effective in reducing the formation of oxide. Crystal growth carried out in Ar did not reduce the total amount of oxide but only resulted in redistribution of TiO 2 particles to grain boundaries, where they obstructed grain growth. Crystal growth in a vacuum was found to be essential in producing oxide-free crystals. A seed selection technique was developed and used to grow CoTi single crystals. The microstructures of these materials were examined by optical microscopy, scanning electron microscopy and transmission electron microscopy, the B2 crystal structure was confirmed and the morphologies, grain sizes and oxide distributions were determined.

Differential barothermal analysis in the course of reactive powder barothermal processing of RuAl alloys

Thermostable high-temperature structural alloys based on the refractory (melting temperature Tm=2060°C) RuAl intermetallic (IM) with an ordered B2 crystal structure (CsCl type) are developed. This IM surpass other aluminides (NiAl, TiAl and Ni3Al) used as the base for the development of high-temperature alloys and matrices of high-temperature composites (CM) intended for hot parts of supersonic engines, which serve at the temperatures exceeding operation temperatures of modern nickel-base superalloys. The differential barothermal analysis was used to develop the basic technological process of barothermal reaction sintering to produce near-net shape billets from RuAl-based structural materials.

Effects of growth temperature on the structural and magnetic properties of epitaxial Ni2MnIn thin films on InAs (001)

Heusler alloy Ni2MnIn thin films have been grown on InAs (001) by molecular beam epitaxy at growth temperatures ranging from 120 to 300 °C. For growth at 120 °C, transmission electron diffraction confirms the epitaxial growth of Ni2MnIn in the B2 crystal structure on InAs (001) with an epitaxial relationship of Ni2MnIn(001)⟨100⟩‖InAs(001)⟨100⟩. Magnetic measurements show that the Ni2MnIn film is ferromagnetic with a Curie temperature ∼170K. However, for growth at 120 °C followed by a postgrowth anneal at 200 °C, a Curie temperature as high as 330 K was obtained. The increase in Curie temperature is attributed to the formation of partial L21 ordering in the Ni2MnIn film, as determined by convergent beam electron diffraction.

Ｘ線回折と回折パターンシミュレーションによるＮｉ‐Ｔｉ形状記憶合金の結晶学的検討

The martensitic transformation of Ni-Ti shape memory alloy was investigated by X-ray diffraction analysis aided X-ray diffraction pattern simulation. Using the wire shape samples the X-ray diffraction patterns were measured at every 10°C from 0°C to 80°C. The software that simulated the X-ray diffraction patterns using by the data of already known lattice constants was developed on the basis of the Bragg's law and the intensity laws. First we confirmed that the X-ray pattern measured by arranging wire samples and attaching the sample holder coincided that of the powder method. Next the X-ray pattern at austenite phase (80°C) was compared with the simulated pattern of B2 crystal structure by the program and the state was identified as B2 structure. On the other hand at martensite phase (0°C) B2 and B19' structures were mixed in the crystal. Although the shape memory effect was expressed at this environmental temperature, it is considered that not only B19' structure but B2 structure is also related to the effect of shape memory effect.

Theoretical studies on the high pressure phases of Lanthanum monochalcogenides

We report total energy and electronic structure calculations for lanthanum monochalcogenides in B1 (NaCl) and B2 (CsCl) crystal structures over a range of unit cell volumes. We employed the tight binding linear muffin-tin orbital approach to density functional theory within the local density approximation to expand the crystal orbitals and periodic electron density. In agreement with the experiment we find that B1 phase is lower in energy than B2 phase, and that the compounds transforms to B2 structure under applied pressure. This is the first qualitative prediction of the transition in La monochalcogenides and should be testable with diamond-anvil technique.

Mechanical properties of soft magnetic FeCo alloys

The brittle fracture of stoichiometric FeCo has long been a puzzle given its high-symmetry B2 crystal structure, 1/2〈111〉{110} slip, low ordering temperature and relatively low strength. Macroalloying with vanadium (≈2%) improves ductility significantly in the disordered state, but only moderately in the ordered state. Brittle fracture is intergranular in the binary stoichiometric alloy, but is usually transgranular cleavage in the vanadium-containing alloy. Boron and carbon additions are shown not to suppress grain-boundary fracture nor improve the ductility of stoichiometric FeCo, but they produce significant ductility improvements in the ternary FeCo–2V alloy. The mechanism appears to be slip refinement by fine precipitates. It is also shown that for boron-doped (30 wppm) stoichiometric FeCo, the optimum vanadium content for good ductility is 2.1–2.5 wt.%.

Epitaxial growth of ferromagnetic Ni2MnIn on (001) InAs

The ferromagnetic Heusler alloy Ni2MnIn has been grown on InAs (001) by molecular-beam epitaxy. In situ reflection high-energy electron diffraction, ex situ x-ray diffraction, Rutherford backscattering spectrometry, and transmission electron microscopy indicate the high-quality epitaxial growth of Ni2MnIn with the B2 crystal structure on InAs (001). Superconducting quantum interference device magnetometry shows that the Ni2MnIn film is ferromagnetic with a Curie temperature ∼170 K.

Specific Heat Measurement of the Single-Crystalline FeSi and its Theoretical Analysis

The precise measurements of the specific heat have been done on high-quality samples of FeSi and CoSi with the B20 crystal structure. The magnetic and electronic contributions are extracted by explicitly subtracting phonon contributions. The results are discussed based on the spin fluctuation theory for itinerant electron magnets.

LDA and GGA calculations for high-pressure phase transitions in ZnO and MgO

We report total energy and electronic structure calculations for ZnO in the B4 (wurtzite), B3 (zinc blende), B1 (rocksalt), and B2 (CsCl) crystal structures over a range of unit cell volumes. We employed both the local-density approximation (LDA) and the PBE96 form of the generalized gradient approximation (GGA) together with optimized Gaussian basis sets to expand the crystal orbitals and periodic electron density. In agreement with earlier ab initio calculations and with experiment, we find that the B4 phase of ZnO is slightly lower in energy than the B3 phase, and that it transforms first to the B1 structure under applied pressure. The equilibrium transition pressure ${p}_{T1}$ is 6.6 GPa at the LDA level of theory and 9.3 GPa in the GGA, compared to experimental values around 9 GPa. This confirms a trend seen by other workers in which the LDA underestimates structural transition pressures which are more accurately predicted by the GGA. At much higher compression, we predict that the B1 phase of ZnO will transform to the B2 (cesium chloride) structure at ${p}_{T2}=260\mathrm{GPa}$ (LDA) or 256 GPa (GGA) indicating that gradient corrections are small for this material at megabar pressures. This is the first quantitative prediction of this transition in ZnO, and should be testable with diamond-anvil techniques. We predict that ZnO remains a semiconductor up to ${p}_{T2}.$ For comparison we find that the B1 to B2 transition in MgO occurs at 515 GPa with either LDA or GGA, in excellent agreement with other ab initio predictions.

Elevated temperature compressive slow strain rate properties of several directionally solidified NiAl–(Nb,Mo) alloys

Three NiAl-based alloys containing 3Nb–10Mo, 5Nb–10Mo or 13.6Nb–18Mo (at%) were directionally solidified to develop three dimensional Mo-based dendrite networks. Examination of the alloys indicated that the desired chemistry was achieved for the 3Nb–10Mo and 5Nb–10Mo versions but the composition of the highly alloyed ingot was NiAl–14.6Nb–13.2Mo. The as-grown structure for all three materials consisted of three major phases: essentially unalloyed B2 crystal structure NiAl, Laves NiAlNb phase alloyed with ∼8.5 Mo, and a bcc metallic Mo solid solution containing 27Nb–7Ni–7Al. Compressive properties were measured between 1200 to 1400 K in air under constant velocity and constant load creep conditions with strain rates ranging from ∼10 −4 to ∼10 −8 s −1. The flow strengths of the two alloys with 10Mo were nearly identical and much weaker than those for NiAl–14.6Nb–13.2Mo under all conditions. Comparison the properties of this latter alloy with other directionally solidified NiAl-based eutectics revealed that it was the strongest material under lower temperature/fast deformation conditions, but this advantage was lost at higher temperatures and/or slower strain rates.

PVD NiAl intermetallic coatings: microstructure and mechanical properties

NiAl intermetallic coatings were fabricated using an rf magnetron sputtering system and a specially-designed composite target made from pure Ni and Al metals. The coatings which were 10 μm thick were deposited on a variety of substrates including 1100Al, commercial purity nickel, 316 type stainless steel and borosilicate glass. X-Ray diffraction (XRD) showed that the coatings produced for all substrates were the NiAl intermetallic compound with the ordered cubic B2 crystal structure. The coating deposited on a borosilicate glass substrate had a (111) preferred orientation. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) confirmed that the grain size of the NiAl was 10 nm. The hardness and elastic modulus of the NiAl coatings were measured using an ultra-microhardness indentation system and found to be 11.52 GPa and 143.42 GPa, respectively. The NiAl intermetallic coating worn against a steel bearing pin exhibited a low wear rate (2.9 × 10 −4 mm 3/m). The NiAl coating had a lower value of coefficient of friction (0.23) than the aluminum substrate (0.28) as measured using a scratch tester.

INVESTIGATION OF CRYSTALLOGRAPHIC SLIP IN POLYCRYSTALLINE Fe3Al USING SLIP TRACE MEASUREMENT AND MICROTEXTURE DETERMINATION

An intermetallic Fe72Al28 alloy (doped with Cr, Zr, Mo, and C) with an imperfectly ordered B2 crystal structure was rolled at 830–860 K to ϵ = 20%. To investigate crystallographic slip an etching technique was developed which allowed slip traces to be determined in grain interiors rather than at the sample surface. To derive the prevalent glide systems both the slip traces and the corresponding orientations were determined in grain scale. Three types of slip systems were identified, namely {110}〈111〉, {112}〈111〉, and {123}〈111〉. However, the slip traces produced by {123}〈111〉 systems appeared wavy and were interpreted in terms of macroscopic or effective rather than crystallographic slip. The critical resolved shear stress ratio of the slip systems involved was fitted from experiment using a Relaxed Constraints Taylor model. The best correspondence between predicted and experimentally observed slip systems was attained for a critical resolved shear stress ratio of τ{110}/τ{112} = 1.05/1.0. © 1997 Acta Metallurgica Inc.

The ordering scheme in N b aluminides with the B2 crystal structure

This research may be summarized as follows. 1. 1. The OTLs for the three alloys Nb-15Al-xTi, where x = 10,125 or 40, have been determined. The trends in site occupancy in these compounds has been identified, and the apparent sublattice compositions have been determined. 2. 2. From these OTLs, it has been shown that while both compounds containing 10Ti and 40Ti exhibit the B2 crystal structure, their ordering schemes are rather different. 3. 3. Simulation of the ALCHEMI results using the dynamical theory of electron diffraction has provided estimates of the end-points of the OTLs. The estimated long range order parameters for the alloys with Ti contents of 10,25 and 40 have been estimated to be 0.88,0.25 and 0.5, respectivley. 4. 4. In all three compounds studied, the most ordered state would correspond to the Al atoms being confined to one sublattice, and this appears to be the most significant factor influencing the stability of the B2 crystal structure adopted by these alloys.