B2-type Structure Research Articles

Magnetic properties of the off-stoichiometric Heusler alloys Rh2Mn1+xSn1-x

We report the results of experimental investigation of the crystal structure, magnetization and 119Sn Mössbauer spectroscopy of Rh2Mn1+xSn1-x (0 ≤ x ≤ 0.6). The samples crystallize in the cubic L21-type structure for 0 ≤ x ≤ 0.4 and the cubic B2-type structure for x = 0.5 and 0.6. X-ray diffraction patterns with the samples with 0 ≤ x ≤ 0.4 indicate that the excess Mn atoms occupy the Sn sites. The samples with 0 ≤ x ≤ 0.4 exhibit the typical ferromagnetic behavior. The magnetic moment at 5 K of 4.27 μB/f.u. for x = 0 (Rh2MnSn) initially increases with increasing x and saturates for x ≥ 0.2. On the other hand, the Curie temperature for 0 ≤ x ≤ 0.4 decreases with increasing x. It is presumed that both samples with x = 0.5 and 0.6 are in the heterogeneous magnetic state. The 119Sn Mössbauer spectroscopy measurements showed that the hyperfine field at 90 K for 0 ≤ x ≤ 0.6 increases with increasing x. The concentration dependence of the magnetic moment and the Curie temperature for Rh2Mn1+xSn1-x (0 ≤ x ≤ 0.6) are discussed qualitatively based on the Mn-Rh and Mn-Mn interactions in the crystal.

Theoretical Investigation of Pressure Dependent Structural and Elastic Properties of MnSi Compound

MnSi is a metallic compound that exhibits a structural phase transition as a function of pressure. At ambient conditions, MnSi has a cubic structure such as NaCl type crystal structure. However, at high pressures, it undergoes a structural phase transition to a CsCl type crystal structure. The pressure-dependent structural phase transition in MnSi has been studied using various experimental and theoretical techniques. In the present work, we have used an efficient inter-ionic potential approach to predict pressure dependent structural phase change and associated volume collapse in MnSi. Therein, the potential includes the long-range Coulomb, van der Waals (vdW) interaction, and the short-range repulsive interaction up to second neighbour ions. It has been demonstrated that the Hafemeister and Flygare approach successfully evaluates the equation of state for change in volume with respect to applied pressure. We have identified a structural phase transition from B1(NaCl) type structure to B2 (CsCl) type structure in this compound. The estimated value of phase transition pressure (Pt) is 42 GPa, which is consistent with the available reported data. The identified first order phase transformation showed a volume collapse of about 10% in the vicinity of transition. In addition, we have also investigated second order of elastic constants for MnSi compound.

Microstructure, mechanical properties, electrical resistivity, and corrosion behavior of (AlCr)x(HfMoNbZr)1-x films

Refractory high-entropy alloys (RHEAs) have emerged as a new class of materials due to their exceptional properties such as high strength, high-temperature stability, and potential for corrosion resistance. In this study, we investigate the effect of Al and Cr co-alloying on the crystal structure, mechanical properties, electrical resistivity, and corrosion resistance of HfMoNbZr film. (AlCr)x(HfMoNbZr)1-x films were grown using direct current magnetron sputtering, with Al50Cr50 and Hf25Mo25Nb25Zr25 target co-sputtering. Our findings reveal that all films possess a B2-type BCC fine-grained crystallite structure. Moreover, an increase in AlCr content results in an increase in hardness and electrical resistivity, which can be attributed to the stronger directionally angular bonds and higher local lattice distortion caused by Al atoms, as confirmed by density functional theory (DFT) calculations. Additionally, the (AlCr)x(HfMoNbZr)1-x films exhibit excellent corrosion resistance in 3.5 wt% NaCl solution, as indicated by a corrosion current below 10−8 A/cm2. Remarkably, at PAlCr = 90 W, the current density reaches a minimum value of 4.85 ± 0.43 × 10-9 A/cm2. These results demonstrate the tunable properties of RHEAs through element alloying, which offers promising opportunities for various applications.

Structure and density of silicon carbide to 1.5 TPa and implications for extrasolar planets.

There has been considerable recent interest in the high-pressure behavior of silicon carbide, a potential major constituent of carbon-rich exoplanets. In this work, the atomic-level structure of SiC was determined through in situ X-ray diffraction under laser-driven ramp compression up to 1.5 TPa; stresses more than seven times greater than previous static and shock data. Here we show that the B1-type structure persists over this stress range and we have constrained its equation of state (EOS). Using this data we have determined the first experimentally based mass-radius curves for a hypothetical pure SiC planet. Interior structure models are constructed for planets consisting of a SiC-rich mantle and iron-rich core. Carbide planets are found to be ~10% less dense than corresponding terrestrial planets.

Formation of quaternary all-d-metal Heusler alloy by Co doping fcc type Ni2MnV and mechanical grinding induced B2-fcc transformation

The structure of the all-d-metal alloy Ni50–x Co x Mn25V25 (x = 0–50) is investigated by using theoretical and experimental methods. The first-principles calculations indicate that the most stable structure of the Ni2MnV alloy is face-centered cubic (fcc) type structure with ferrimagnetic state and the equilibrium lattice constant is 3.60 Å, which is in agreement with the experimental result. It is remarkable that replacing partial Ni with Co can turn the alloy from the fcc structure to the B2-type Heusler structure as Co content x > 37 by using the melting spinning method, implying that the d–d hybridization between Co/Mn elements and low-valent elements V stabilizes the Heusler structure. The Curie temperature T C of all-d-metal Heuser alloy Ni50–x Co x Mn25V25 (x > 37) increases almost linearly with the increase of Co due to that the interaction of Co–Mn is stronger than that of Ni–Mn. A magnetic transition from ferromagnetic state to weak magnetic state accompanying with grinding stress induced transformation from B2 to the dual-phase of B2 and fcc has been observed in these all-d-metal Heusler alloys. This phase transformation and magnetic change provide a guide to overcome the brittleness and make the all-d-metal Heusler alloy interesting in stress and magnetic driving structural transition.

Theoretical studies of pressure induced structural phase transformation and elastic properties in nickel silicide

In this study, we have explored pressure dependent structural phase transformation and related volume collapse in the vicinity of transition in Nickel Silicide (NiSi) with the help of an effective inter-ionic potential approach. It was observed that by employing an effective interionic interaction approach, which is basically based on phenomenological models of lattice dynamics, includes the long-range Coulomb, van der Waals (vdW) interaction, and the short-range repulsive interaction in the framework of the Hafemeister and Flygare approach. Our results indicate that the material parameters evaluated by such an approach provide a successful evaluation of the equation of state for change in volume with respect to the applied pressure. The estimated value of phase transition pressure (Pt) is 25 GPa for NiSi, which is in line with the information that we have in the literature. This transition corresponds to the transformation of a B1 (NaCl) type structure to a B2 (CsCl) type structure and is a first-order phase transition. This first order phase transformation showed a volume collapse of about 11.44% at 25 GPa for NiSi. Later on, the relationship between pressure and the variation of second-order elastic constants is examined in greater detail.

Enhanced magnetocaloric performances and tunable martensitic transformation in Ni35Co15Mn35−xFexTi15 all-d-metal Heusler alloys by chemical and physical pressures

The solid-state magnetic cooling (MC) method based on the magnetocaloric effect (MCE) is recognized as an environmentally friendly and high-energy-efficiency technology. The search or design of suitable magnetic materials with large MCEs is one of the main targets at present. In this work, we apply the chemical and hydrostatic pressures in the Ni35Co15Mn35−xFexTi15 all-d-metal Heusler alloys and systematically investigate their crystal structures, phases, and magnetocaloric performances experimentally and theoretically. All the alloys are found to crystallize in an ordered B2-type structure at room temperature and the atoms of Fe are confirmed to all occupy at sites Mn(B). The total magnetic moments decrease gradually with increasing Fe content and decreasing of volume as well. The martensitic transformation temperature decreases with the increase of Fe content, whereas increases with increasing hydrostatic pressure. Moreover, obviously enhanced magnetocaloric performances can also be obtained by applied pressures. The maximum values of magnetic entropy change and refrigeration capacity are as high as 15.61(24.20) J (kg K)−1 and 109.91(347.26) J kg−1 with ΔH = 20(50) kOe, respectively. These magnetocaloric performances are superior to most of the recently reported famous materials, indicating the potential application for active MC.

Hydrogen in nonstoichiometric cubic titanium monoxides: X-ray and neutron diffraction, neutron vibrational spectroscopy and NMR studies

Hydrogen-induced changes in the properties of transition-metal oxides have attracted much recent attention due to numerous applications of these materials including catalysis, H2 production, low-temperature H2 sensing, solar cells, and air purification. However, basic properties of hydrogenated titanium monoxides have not been investigated so far. In the present work, we report the results of the first studies of the crystal structure, vibrational spectra, and mobility of H atoms in TiO0.72H0.30 and TiO0.96H0.14 using X-ray diffraction (XRD), neutron powder diffraction, neutron vibrational spectroscopy, and nuclear magnetic resonance (NMR). The hydrogenated compound TiO0.72H0.30 is found to retain the disordered cubic B1-type structure of the initial titanium monoxide, where H atoms exclusively occupy vacancies in the oxygen sublattice. It has been revealed that hydrogenation of the disordered cubic TiO0.96 leads to the formation of the two-phase compound TiO0.96H0.14, where the disordered B1-type phase coexists with the monoclinic phase of Ti5O5 type with an ordered arrangement of vacancies. In both phases, H atoms are found to occupy only vacancies in the oxygen sublattice. The low-temperature inelastic neutron scattering spectra of TiO0.72H0.30 and TiO0.96H0.14 in the energy transfer range of 40–180 meV exhibit a single peak due to optical oxygen vibrations (centered on about 60 meV) and a broad structure at 90–170 meV due to optical H vibrations. The unusual width of this structure can be attributed to the broken symmetry of hydrogen sites in the titanium monoxides: because of the presence of vacancies in the titanium sublattice, the actual point symmetry of these sites appears to be lower than octahedral. Proton NMR measurements have revealed that both hydrogenated compounds are metallic; no signs of hydrogen diffusive motion in TiO0.72H0.30 and TiO0.96H0.14 at the frequency scale of about 105 s-1 have been found up to 370 K.

Ferromagnetic martensitic transformation and large magnetocaloric effect in Ni35Co15−xFexMn35Ti15 (x = 2, 4, 6, 8) alloys

The crystal structure, ferromagnetic martensitic transformation, and magnetocaloric effect of Ni35Co15−xFexMn35Ti15 (x = 2, 4, 6, 8) alloys have been investigated. All the alloys crystallize in a B2-type cubic structure at room temperature. The Curie temperature of austenite between paramagnetic and ferromagnetic states decreases from 349.0(7) K to 287.2(8) K with increasing Fe content x up to 8, whereas the corresponding martensitic transformation temperature from ferromagnetic austenite to weak-magnetic martensite increases continuously from 182 K to 230 K, and the accompanied thermal hysteresis decreases gradually. The maximum values of magnetic entropy change (ΔSM)/refrigeration capacity reach 9.5(8) J kg−1 K−1/79.4(5) J kg−1 (0–20 kOe) and 24.0(4) J kg−1 K−1/206.8(4) J kg−1 (0–50 kOe), respectively. The present results indicate that the Ni35Co15−xFexMn35Ti15 alloys are considerable for magnetic refrigeration.

Study of thermal and structural characteristics of mechanically milled nanostructured Al-Cu-Fe quasicrystals

Single phase nanostructured Al-Cu-Fe quascrystalline powders, with various grain sizes, were synthesized by low intensity mechanical milling of spray deposited icosahedral quasicrystals. Structural evolution as a function of temperature was studied in-situ using high energy synchrotron X-ray diffraction. Coefficient of thermal expansion was calculated using the change in d-spacing. The diffraction results revealed a change in the lattice parameter, grain size and quasi-lattice strain with increasing temperature. During heating, onset of appreciable grain growth in different nanostructured quascrystalline powders was observed at a certain critical temperature, which was closely related with the milling time. Quasicrystalline powders milled for 1 and 80 h exhibited appreciable grain coarsening at 923 and 573 K, respectively. For all the samples, the activation energy for grain growth (Egg) was ~30–35 kJ/mol. In addition, the β Al(Cu,Fe) phase with B2-type structure evolved during heating of powder samples milled for 10 h. Onset temperature for β phase formation showed an inverse relationship with the milling time.

Finding a New B1-Type Phase in Single Crystals of Fe–Al and Fe–Ga Soft Magnetic Alloys

The atomic structure of Fe–Al (7 and 9 at % Al) and Fe–Ga (18 at % Ga) alloys is studied by X‑ray diffraction using a laboratory four-circle diffractometer. After refining annealing, single-crystal alloy samples were annealed in the ferromagnetic state (T TC), was quenched in room temperature water. Earlier, the authors reported on the peculiarities of the ordering of alloying atoms in B2 and D03 phase structures in quenched and annealed samples of these alloys. Here, we present and discuss the results of our observations in these alloys of a new phase with a face-centered cubic (fcc) lattice (B1-type structure with NaCl prototype and unit cell parameter ~5.2 nm). The fcc phase appears in the Fe–Al alloy as the aluminum concentration increases from 7 to 9 at %; it is observed in the Fe–18 at % Ga alloy, and its volume fraction increases after annealing in the ferromagnetic state in comparison with a quenched alloy sample. In these alloys (9 at % Al) and (18 at % Ga), different ways of embedding fcc crystals in the bcc phase of single crystals are realized; i.e., the axes of the fcc lattice are directed in four different ways relative to the axes of the bcc lattice.

Crystallographic ordering in a series of Al-containing refractory high entropy alloys Ta–Nb–Mo–Cr–Ti–Al

High entropy alloys based on the Ta–Nb–Mo–Cr–Ti–Al system are expected to possess high creep and oxidation resistance as well as outstanding specific mechanical properties due to presumed high melting points and low densities. However, we recently reported that arc-melted and subsequently homogenized alloys within this system exhibit a lack of ductility up to 600 °C [H. Chen et al. in Metall. Mater. Trans. A 49 (2018) 772–781 and J. Alloys Cmpd. 661 (2016) 206–215]. Thermodynamic calculations suggest the formation of a B2-type ordered phase below the homogenization temperature. In the present article, we provide results of a detailed microstructural characterization of a series of Ta–Nb–Mo–Cr–Ti–Al derivatives and evaluate if B2-type ordering could be the origin for the observed lack of ductility. Backscatter electron (BSE) imaging, energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT) were used to verify uniform elemental distribution after homogenization. X-ray diffraction (XRD) patterns indicate both, A2 or B2-type crystal structure, whereas transmission electron microscopy (TEM) diffraction experiments unambiguously confirm B2-type order in the as-homogenized state of all investigated alloys. In MoCrTiAl, planar defects that show antiphase boundary contrast with a {100}-type habit plane were detected by TEM dark field (DF) imaging. They are wetted by a Cr-enriched and Ti-depleted layer as confirmed by scanning transmission electron microscopy (STEM)-EDX line scans as well as APT analyses. The planar defects arise from a disorder-order solid-state phase transformation during cooling, as indicated by differential scanning calorimetry (DSC).

New M(2 + δ1)X(2 + δ2) Superstructure of Ordered Nanocrystalline Titanium Monoxide and the Problem of Transition States

The results of experiments on the vacancy ordering in nanocrystalline nonstoichiometric titanium monoxide TiOy (space group Fm $$\bar {3}$$ m), which is accompanied by the formation of the orthorhombic Ti9O10 phase (space group Immm), are considered. The vacancy distribution in the ordered phase is described by a two-sublattice-ordering M(2 + δ1)X(2 + δ2) superstructure. The disorder–order (TiOy–Ti(2 + δ1)O(2 + δ2)) phase transition is accompanied by symmetry distortions along non-Lifshitz star {k4}. When ordering, vacancies concentrate in every third (110) plane of the basic B1-type structure. The deviation of stoichiometry is caused by different vacancy concentrations in the defect planes in the metallic and nonmetallic sublattices (parameters δ1 and δ2, respectively). The new class of M(n + δ1)X(n + δ2) superstructures is shown to generate numerous order–disorder transition state models, which cannot be described in terms of the mean-field approximation. At δ1 = δ2 = 0.5, the orthorhombic M(2 + δ1)X(2 + δ2) structure is a particular case of the partly disordered monoclinic M5X5 superstructure (space group C2/m(A2/m)), which is characteristic of a coarse-grained state.

Structural evolution and electronic properties of CaS: An ab initio study

CaS crystallizes in cubic NaCl (B1) type structure with symmetry Fm3¯m. In this work, the structural and electronic properties of CaS were investigated by considering the Density Functional calculations within the framework of Generalized Gradient Approximation (GGA) under high pressure. The structural change was found at the B1 type structure of CaS. B1 type structure transformed into another cubic CsCl (B2) type structure with symmetry Pm3¯m at 36.6 GPa. An intermediate state with symmetry R3¯m was predicted during this transition. Besides, the effects of the pressure on the electronic properties of CaS were also studied. Both the B1 and B2 type structures exhibited semiconducting behaviors with direct band gaps at the Γ-point and R-point, respectively. Intermediate state was searched during this phase change first time in detail. The obtained results were compared with experimental and theoretical ones in the literature.

Theoretical study of elastic and thermodynamic properties of CuSc intermetallic compound under high pressure

The structural, mechanical and thermodynamic properties of copper scandium CuSc intermetallic compound under temperature and pressure have been investigated using the plane wave (PW) - pseudopotential (PP) approach in the framework of the density functional theory (DFT). The structural parameters at equilibrium, the elastic moduli, the mechanical stability criteria and the sound velocity are studied in the pressure range 0–12 GPa. In addition, the heat capacity, the Grüneisen parameter, the Debye temperature, the entropy, and the thermal expansion coefficient are studied for temperatures ranging from 0 up to 1000 K. The equilibrium lattice parameter found is around 3.261 Å. It is in good agreement with the experimental one of 3.25 Å reported in the literature. According to the generalized elastic stability criteria, we predict the occurrence of a phase transition of the B2-type structure at 25.5 GPa. At room temperature and zero-pressure, the isothermal bulk modulus and the Grüneisen parameter found were 80.86 GPa and 2.04 respectively.

Realization of Magnetostructural Transition and Magnetocaloric Properties of Ni–Mn–Mo–Sn Heusler Alloys

As magnetic cooling refrigerant, Ni–Mn–Sn Heusler alloys have gained significant potentials for a wide range of practical applications as compared with other expensive Ga-doped Ni–Mn based Heusler alloys, toxic As and P doped Mn-based alloys and rare-earth elements like Gd-based alloys. Consequently, an inclusive investigation on magnetostructural transition and magnetocaloric features of Ni43Mn47−xMoxSn10 alloys is carried out. The alloys are assembled through arc-melting technique and their X-ray diffraction analysis confirms their B2-type crystal structure at room temperature. As the Mo contents are increased, the magnetostructural transition temperature increases for a temperature range between Curie temperature of austenite and martensite phases. Obvious metamagnetic behavior and magnetic hysteresis are observed in isothermal magnetization curves, suggesting that the temperature and magnetic field can be used as robust tools for tuning the structural transformation in Heusler alloys. The noticeable magnetocaloric effect suggests that the prepared alloys can be an attractive choice for magnetic refrigeration applications.

A new tetragonal phase in CoFeCrGe Heusler alloy

CoFeCrGe ribbons were prepared by melt spinning and then annealed at 300°C for 4h and 500°C for 4h, respectively. The as-spun ribbons of CoFeCrGe have the B2 type structure with a=0.287nm. The CoFeCrGe ribbons annealed at 300°C are composed mainly of the L21 type structure with a=0.578nm in partial disorder and the B2 type structure with a=0.289nm in further disorder case. The ribbons annealed at 500°C are composed of three compounds with average compositions of Co26.0Fe26.5Cr23.5Ge24.0, Co3.8Fe6.4Cr68.5Ge21.3 and Co53.4Fe30.4Cr8.6Ge7.6, listed in a sequence of volume percentage from major to minor. The primary compound Co26.0Fe26.5Cr23.5Ge24.0 is the L21-type structure with a=0.571nm in partial disorder and the B2 type structure with a=0.286nm in further disorder case. The secondary compound Co3.8Fe6.4Cr68.5Ge21.3 has a cubic Cr3Ge structure with a=0.461nm. The tertiary compound Co53.4Fe30.4Cr8.6Ge7.6 is a new tetragonal structure with lattice parameters a=0.76nm, c=0.284nm, which were determined by using tilt-series electron diffraction technique in this work. The tertiary (Co, Fe)-rich phase is usually embedded in the matrix of the secondary Cr-rich phase, but there is no special orientation between the two phases. The difference in the magnetic hysteresis loops of the samples annealed at 300°C and 500°C has been interpreted as the appearance of the new tetragonal crystalline phase. Element doping is the possible way for further developing the single tetragonal phase or increasing the amount of this phase.

First Principles Thermodynamics of Minerals at HP–HT Conditions: MgO as a Prototypical Material

Ab initio thermodynamic properties, equation of state and phase stability of periclase (MgO, B1-type structure) have been investigated in a broad P–T range (0–160 GPa; 0–3000 K) in order to set a model reference system for phase equilibria simulations under deep Earth conditions. Phonon dispersion calculations performed on large supercells using the finite displacement method and in the framework of quasi-harmonic approximation highlight the performance of the Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid density functional in predicting accurate thermodynamic functions (heat capacity, entropy, thermal expansivity, isothermal bulk modulus) and phase reaction boundaries at high pressure and temperature. A first principles Mie–Grüneisen equation of state based on lattice vibrations directly provides a physically-consistent description of thermal pressure and P–V–T relations without any need to rely on empirical parameters or other phenomenological formalisms that could give spurious anomalies or uncontrolled extrapolations at HP–HT. The post-spinel phase transformation, Mg2SiO4 (ringwoodite) = MgO (periclase) + MgSiO3 (bridgmanite), is taken as a computational example to illustrate how first principles theory combined with the use of hybrid functionals is able to provide sound results on the Clapeyron slope, density change and P–T location of equilibrium mineral reactions relevant to mantle dynamics.

Effect of Sb-doping on martensitic transformation and magnetocaloric effect in Mn-rich alloys**Project supported by the National Natural Science Foundation of China (Grant Nos. 51271093, 51571121, 11604148, and 51601092), the Fundamental Research Funds for the Central Universities, China (Grant Nos. 30920140111010, 30916011344, and 30916011345), Jiangsu Natural Science Foundation for Distinguished Young Scholars, China (Grant No. BK20140035), China Postdoctoral Science Foundation (Grant

We investigate the influence of Sb-doping on the martensitic transformation and magnetocaloric effect in Mn50Ni40Sn Sbx (x = 1, 2, 3, and 4) alloys. All the prepared samples exhibit a B2-type structure with the space group at room temperature. The substitution of Sb increases the valence electron concentration and decreases the unit cell volume. As a result, the magnetostructural transformation shifts rapidly towards higher temperatures as x increases. The changes in magnetic entropy under different magnetic field variations are explored around this transformation. The isothermal magnetization curves exhibit typical metamagnetic behavior, indicating that the magnetostructural transformation can be induced by a magnetic field. The tunable martensitic transformation and magnetic entropy changes suggest that Mn50Ni40Sn Sbx alloys are attractive candidates for applications in solid-state refrigeration.

Ab Initio Study of Structural Phase Transition and Electronic Properties in Samarium (Sm) Compounds

The electronic structure and high pressure structural phase transition of SmTe and SmPo have been studied by using tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). The total energy as a function of volume is obtained and it is found that these compounds are stable in NaCl-type (B1-phase) structure and transform to CsCl-type (B2-type) structure. The transition pressure of SmTe and SmPo are found to be 6.6 GPa and 8.6 GPa respectively. We have also calculated lattice parameter (a0), bulk modulus (B0), band structure (BS) and density of states (DOS). From energy band diagram, we observed that these compounds exhibit weakly metallic behaviour. The calculated values of lattice parameter and bulk modulus agree well with the available data.