Body-centered Tetragonal Phase Research Articles

Boundary-induced phase in epitaxial iron layers

We report on the discovery of a boundary-induced body-centered tetragonal iron phase in thin films deposited on MgAl2O4 (001) substrates. We present evidence for this phase using detailed x-ray analysis and density functional theory calculations. A lower magnetic moment and a rotation of the easy magnetization direction are observed, as compared with body-centered cubic iron. Our findings expand the range of known crystal and magnetic phases of iron, providing valuable insights for the development of heterostructure devices using ultrathin iron layers. Published by the American Physical Society 2024

Orientation-dependent deformation mechanisms of alpha-uranium single crystals under shock compression

Large-scale non-equilibrium molecular dynamics (NEMD) simulations were employed to investigate the dynamic deformations of alpha-uranium (α-U) single crystals subjected to varying shock strengths along low-index crystallographic orientations. The pronounced anisotropy of α-U gives rise to a complex microstructural evolution under shock loading. In-depth microstructural analysis of post-shock specimens reveals the identification of multiple dynamic deformation mechanisms. Notably, when the shock loading direction aligns with the a-axis, dynamic deformation of the α-U single crystals is primarily dominated by lattice instability, which attributes to a crystalline-to-amorphous transition serving as the dominant shear stress relaxation pathway. On the other hand, shock loading along the b-axis results in an abundance of deformation twins, with twinning planes identified as (130) and (13¯0). During the twinning event, the α-U matrix undergoes a transition to a metastable intermediate phase, subsequently decomposing into a composite structure comprising α-U twins and matrix. This unconventional twinning mechanism significantly deviates from classical theories. Furthermore, upon loading along the c-axis, twinning and a phase transition from α-U to body-centered tetragonal phase (bct-U) occur in α-U single crystal samples. Given that the pressure threshold of this phase transition predicted by ab initio calculations is as high as ∼270 GPa, the phase transition from α-U to bct-U might be implausible. An alternative interatomic potential of uranium with the higher pressure threshold was employed to reinvestigate the shock response of α-U single crystals along the c-axis. The phase transition of α-U to bct-U disappears, and twinning dominates the plastic deformation, with the twinning orientation conforming to the {112} twinning. The strong anisotropy of the α-U lattice triggers a wealth of orientation-dependent dynamic deformation mechanisms. The activation of the twinning system is evidently associated with the loading direction, constituting the potential cause for the discovery of multiple twinning variants during the deformation in polycrystalline uranium.

Microstructure, mechanical and corrosion properties of Al–Co–Cr–Ni near-eutectic medium entropy alloys

In this work, a series of Fe-free AlCoaCrbNic (AlCo1.31Cr0.5Ni2.4, AlCo1.71Cr0.5Ni2.0, AlCo1.8Cr0.6Ni1.8, and AlCo1.16CrNi2.0 denoted as E1, E2, E3, and E4, respectively) medium-entropy alloys (MEAs) were designed though the mathematical model. Among the four MEAs, E2 and E3 exhibit uniform and typical eutectic structural characteristics, consisting of face-centered cubic (FCC) and ordered body-centered cubic (B2) phases, with ordered FCC (L12) nanoprecipitates uniformly distributed in the FCC matrix. The E4 alloy is composed of primary B2 phase and eutectic FCC + B2 phases, and the high-density Co- and Cr-rich BCC-structured spherical precipitates are distributed in the B2 phase. Importantly, the E1 alloy is characterized by a phase structure comprising both FCC and body-centered tetragonal (BCT) phases, a combination that is notably infrequent among high-entropy alloys. There are high-density twinning and stacking faults (SFs) in the BCT phase. All four alloys exhibit excellent compressive properties, with fracture strength surpassing 2000 MPa and compression ratio exceeding 30 %. The potentiodynamic polarization curves implied that the E4 MEA presented excellent corrosion resistance with the lowest corrosion current density of 4.05 × 10−7 A/cm2 and the lowest passive current density of 7.01 × 10−6 A/cm2 in 3.5 wt% NaCl solution. Moreover, the electrochemical impedance spectroscopy (EIS) test results also indicating that the E4 MEA has the best corrosion resistance among the four alloys because it has the highest electrode resistance (Rp = 171.76 × 103 Ω cm2). The ordered FCC/BCT phase structure in E1 alloy provides effective exploration for the development of new structured MEAs.

Synchrotron based investigation of anisotropy and microstructure of wire arc additive manufactured Grade 91 steel

Additive manufacturing (AM) rapidly produces complex shapes crucial for energy technologies and engineering designs. In this work, the microstructure and mechanical properties of additive manufactured Grade 91 (modified 9Cr–1Mo) ferritic/martensitic (FM) steel were investigated. Computational thermodynamics, synchrotron X-ray diffraction, scanning electron microscopy, and microhardness testing were utilized to quantify microstructure (crystallographic phases, microstrain), lattice parameters, and mechanical properties as functions of the build directions and post-build heat-treatment. The as-built microstructures were found to contain a large fraction of microstrain due to two-dimensional defects, and a low-carbon martensite phase (maximum value 38.4 vol%), as quantified from the X-ray diffraction analysis. It was shown that a post-build heat-treatment can effectively remove this martensitic phase and promote the formation of desired microstructures with dispersed M23C6 carbides which was consistent with Thermo-Calc predictions. From the X-ray diffraction analysis, an ∼82.5% reduction in microstrain, a 61.5% increase in the coherent grain size and complete elimination of body-centered tetragonal (BCT) phase demonstrated that an appropriate heat-treatment of AM built FM steel yields microstructures and hardness comparable to conventionally processed steel. However, based on lattice parameter and hardness map statistics, heat-treatment on the as-manufactured steel did not eliminate anisotropy.

Theoretical studies on phase stability, electronic, optical, mechanical and thermal properties of chalcopyrite semiconductors HgXN2 (X=Si, Ge and sn): A comprehensive DFT analysis

In this study, the ground state physical properties of Hg-based chalcopyrite semiconductors, HgXN2 (X = Si, Ge and Sn), are investigated using the density functional theory (DFT), which is based on full potential linearized augmented plane wave (FP-LAPW) method. The structural, electronic, optical, mechanical and thermal properties of the body-centered tetragonal (BTC) phase, HgXN2 (X = Si, Ge and Sn), have been calculated using the modified Becke-Johnson exchange-correlation potential (TB-mBJ). The obtained lattice parameters and volume are in good agreement with the values reported earlier, indicating that the results are reliable and reproducible. The phase stability of the titled compounds is elaborately studied and it is confirmed that all are thermodynamically, dynamically and mechanically stable. The electronic band structure demonstrated a direct band gap of 1.70 eV, 0.99 eV, and 0.94 eV for the HgXN2 (X = Si, Ge and Sn) compounds, respectively. Both HgGeN2 and HgSnN2 compounds have a notably lighter carrier effective mass, resulting in higher carrier mobility and conductivity than HgSiN2. The order of the mechanical hardness is as follows: HgSiN2> HgGeN2> HgSnN2. The studied materials are also anisotropic in mechanically and optically. All the title compounds exhibit ductile behavior. The charge density contour plots indicates the dominant covalent nature of the bonds in HgXN2 (X = Si, Ge and Sn). All these compounds have a high absorption coefficient and low optical reflectivity in the visible light range. Due to the favorable band gap and high absorption coefficient, these compounds could be suitable for optoelectronic or photovoltaic device applications. Moreover, the thermal expansion coefficient, thermal conductivity, melting point and fracture toughness behavior have been calculated, which also showed promise for thermal barrier coating (TBC) applications.

Temperature-Controlled Reversible Formation and Phase Transformation of 3D Nanocrystal Superlattices Through In Situ Small-Angle X-ray Scattering

For decades, the spontaneous organization of nanocrystals into superlattices has captivated the scientific community. However, achieving direct control over the formation of the superlattice and its phase transformations has proven to be a grand challenge, often resulting in the generation of multiple symmetries under the same experimental conditions. Here, we achieve direct control over the formation of the superlattice and its phase transformations by modulating the thermal energy of a nanocrystal dispersion without relying on solvent evaporation. We follow the temperature-dependent dynamics of the self-assembly process using synchrotron-based small-angle X-ray scattering. When cooled below -24.5 °C, lead sulfide nanocrystals form micrometer-sized three-dimensional phase-pure body-centered cubic superlattices. When cooled below -35.1 °C, these superlattices undergo a collective diffusionless phase transformation that yields denser body-centered tetragonal phases. These structural changes can be reversed by increasing the temperature of the dispersion and may lead to the direct modulation of the optical properties of these artificial solids.

Shape-induced crystallization of binary DNA-functionalized nanocubes.

Leveraging the anisotropic shape of DNA-functionalized nanoparticles holds potential for shape-directed crystallization of a wide collection of superlattice structures. Using coarse-grained molecular dynamics simulations, we study the self-assembly of a binary mixture of cubic gold nanoparticles, which are functionalized by complementary DNA strands. We observe the spontaneous self-assembly of simple cubic (SC), plastic body-centered tetragonal (pBCT), and compositionally disordered plastic body-centered tetragonal (d-pBCT) phases due to hybridization of the DNA strands. We systematically investigate the effect of length, grafting density, as well as rigidity of the DNA strands on the self-assembly behavior of cubic nanoparticles. We measure the potential of mean force between DNA-functionalized nanocubes for varying rigidity of the DNA strands and DNA lengths. Using free-energy calculations, we find that longer and flexible DNA strands can lead to a phase transformation from SC to the pBCT phase due to a gain in entropy arising from the orientational degrees of freedom of the nanocubes in the pBCT phase. Our results may serve as a guide for self-assembly experiments on DNA-functionalized cubic nanoparticles.

Temperature dependent phase transition and negative thermal expansion of Hg2Cl2 compound: insights from first-principle DFT and Born-Oppenheimer on the fly molecular dynamics calculations

ABSTRACT The paper is aimed to understand the key phonon modes that are responsible for the temperature dependent structural phase transition and negative thermal expansion of Hg2Cl2 compound for the first time with the aid of density functional theory and Born-Oppenheimer on the fly molecular dynamics calculations. The phonon dispersion spectra, phonon density of states and Grüneisen parameters for the body-centered tetragonal and base-centered orthorhombic phases of the compound have been explored in detail. The order parameter associated with the phase transition and its nature has also been reported herewith. We believe that the present study will not only help for futuristic designs of improved functionalized systems with Hg2Cl2 compound but also can augment their applications in thermoelectric conversion systems, fibre-optic communications, thermal expansion compensators and in fuel cells.

Twinning-induced plasticity (TWIP) effect in multi-phase metastable beta Zr-Nb alloys

• Twinning-induced plasticity (TWIP) strategy was first applied in Zr alloys for strain-hardening and ductility enhancements. • Zr-13.8Nb-0.7Hf and Zr-15.5Nb-0.7Hf alloys designed by electron-to-atom ratio exhibit multi-phase microstructures consisting of acicular body-centered tetragonal phase and monoclinic phase in beta grains. • TWIP-enhanced Zr-13.8Nb-0.7Hf alloy resulted in superior strain-hardening rate and ductility when comparing to Zr-15.5Nb-0.7Hf without mechanical twinning. • In-situ traction tests in TEM and SEM-EBSD were conducted to clarify the {332}<113> twinning operation and its interaction with body-centered tetragonal phase. The deformation mechanisms were studied in metastable Zr-13.8Nb-0.7Hf and Zr-15.5Nb-0.7Hf (wt.%) alloys exhibiting complex microstructures due to quenched-in precipitates (acicular body-centered tetragonal phase and globular monoclinic phase) in beta grains. Twinning-induced plasticity (TWIP) effect was observed in Zr-13.8Nb-0.7Hf via {332}<113> twinning operation, resulting in higher strain-hardening rate and uniform elongation than those of Zr-15.5Nb-0.7Hf deforming via only dislocation glide. In-situ characterizations, by applying tensile deformation during electron backscattered diffraction (EBSD) mapping and transmission electron microscopy (TEM) observation, were conducted to clarify the twinning process and the role of quenched-in precipitates when interacting with deformation twins.

Effect of scanning strategy on microstructure and texture evolution in a selective laser melted Al-33Cu eutectic alloy

Laser-based powder-bed fusion (LPBF) / Selective laser melting (SLM) was used to fabricate a eutectic composition of Al-33Cu alloys in-situ. Different scan strategies (1 × 1, 2 × 2, 3 × 3, 4 × 4, and 60°) were adopted to understand its effect on porosity, phase formation, microstructural and texture evolution, and mechanical properties. The 1 × 1 specimen showed the highest porosity among the hatch variation, and the 60° scan strategy showed the least. X-ray diffraction from the specimens prepared using different scan strategies showed a mixture of Al (face-centered cubic) and Al2Cu (body-centered tetragonal) phases. The microstructure of all the specimens revealed eutectic lamellar-type morphology; however, the interlamellar spacing was observed to be different at the melt and laser track-overlapped regions. The nano-lamellar eutectic microstructure exhibited a compressive strength exceeding 1 GPa. Nano-indentation in two distinct regions indicated an Al-rich softer zone and an Al2Cu rich harder zone. Bulk texture analysis was also done for both phases for all the hatch-varied specimens, and it shows a particular texture component intensifies from weak fiber as the scan strategy is altered.

Preparation of α′-Fe(N)/α′-Fe8N by γ′-Fe4N precursor

An approach by using the γ′-Fe4N precursor was proposed to prepare a bulk magnet with high α″-Fe16N2 content, and the feasibility for high N content α′-Fe(N)/α′-Fe8N was demonstrated in ribbon samples. γ′-Fe4N decomposed when it was heated to a high temperature, and then, a supersaturated γ-Fe(N) phase formed because of N atoms diffusing out from the γ′-Fe4N lattice. The short-lived supersaturated γ-Fe(N) had the face-centered cubic lattice, and its nitrogen content is beyond 3.0 wt. %. The supersaturated γ-Fe(N) acts as a precursor of the martensite transformation for a body-centered tetragonal phase. After the cryo-treatment, which was essential to complete the martensite transformation, a mixture of α′-Fe(N) and α′-Fe8N was obtained. The N content of more than 2.5 wt. % in the samples was achieved.

Synthesis, characterization and first hydrogen absorption/desorption of the Mg35Al15Ti25V10Zn15 high entropy alloy

In this work, a novel Mg-containing high entropy alloy (HEA), namely Mg35Al15Ti25V10Zn15, was synthesized, characterized, and the phases formed after synthesis and after first hydrogen absorption/desorption were analyzed. The alloy composition was conceptualized aiming at increasing the atomic fraction of lightweight elements (35 at.% Mg and 15 at.% Al) to increase its gravimetric hydrogen storage capacity. Ti and V were selected to increase the hydrogen affinity of the alloy. Finally, Zn was selected as an alloying element because of its negative enthalpy of mixing with Mg, which could favor the formation of solid solution and avoid Mg segregation. The Mg35Al15Ti25V10Zn15 HEA was synthesized by high-energy ball milling (HEBM) in two different routes: under argon atmosphere (mechanical alloying – MA) and under hydrogen pressure (reactive milling – RM). The sample produced by MA was composed of a body-centered cubic (BCC) phase with an amount of unmixed Mg. When hydrogenated at 375 °C and under 4.0 MPa of H2, the MA sample absorbed about 2.5 wt% of H2 by forming a mixture of MgH2, a face-centered cubic (FCC) hydride, a body-centered tetragonal (BCT) phase and MgZn2. The desorption sequence upon heating was investigated and it was shown that the MgH2 is the first to desorb. The sequence of desorption is completed by the decomposition of the FCC hydride and BCT phase leading to the formation of the BCC phase. On the other hand, the alloy produced by RM was composed of a mixture of an FCC hydride and MgH2. During desorption upon heating, the MgH2 is also the first to desorb. At lower temperatures (below 350 °C), the FCC hydride decomposes partially forming the BCT phase. At higher temperatures, both FCC hydride and BCT phase decomposes forming the BCC phase. The desorption capacity of the RM sample was 2.75 wt% H2.

Phase transitions and properties of lanthanum under high pressures

Lanthanum (La), the first member of the lanthanide elements, recently aroused interests due to the discovery of high-T c superconductor LaH10 under high pressures and its unique superconducting properties. Here, we study the phase transitions, superconductivity and mechanical properties of metallic La under high pressures by first-principle calculations. The known face-centered cubic (fcc) phase with space group still exists above 100 GPa. And it transforms into an unprecedented body-centered tetragonal (bct) phase with space group I4/mmm above 180 GPa, which expands the high pressure phase transition sequence. Further calculations show that the superconducting transition temperature T c of fcc phase decreases with increasing pressure with the rate of −0.13 K GPa−1, in good agreement with the experimental results. For the bct phase, the estimated superconducting transition temperature is very low with T c of 0.7 K at 200 GPa. The calculations of mechanical properties show that both of fcc and bct phases are compressible and brittle.

Pressure tuned incommensurability and guest structure transition in compressed scandium from machine learning atomic simulation.

Scandium (Sc) is the lightest non-main-group element and transforms to a host-guest (H-G) incommensurate structure under gigapascal (GPa) pressures. While the host structure is stable over a wide pressure range, the guest structure may exist in multiple forms, featuring different incommensurate ratios, and mixing up to generate long-range "disordered" guest structures. Here, we employed the recently developed global neural network (g-NN) potential and the stochastic surface walking (SSW) global optimization algorithm to explore the global potential energy surface of Sc under various pressures. We probe the global minima structure in a system made of hundreds of atoms and revealed that the solid-phase transition between Sc-I and H-G Sc-II phases is fully reconstructive in nature. Above 62.5 GPa, the pressure will further destabilize the face-centered tetragonal (fct, Sc-IIa) guest structure to a body-centered tetragonal phase (bct, Sc-IIb), while sustaining the host structure. The structural transition mechanism of this work will shed light on the nature of the complex H-G structural modifications in compressed metals.

High-resolution transmission electron microscopy study of body-centered tetragonal Zr2Fe nanoparticle phase in Zircaloy-4 alloy and its typical crystalline defect structures

High-resolution transmission electron microscopy study of body-centered tetragonal Zr2Fe nanoparticle phase in Zircaloy-4 alloy and its typical crystalline defect structures

Anodic pulse electrodeposition of mesoporous manganese dioxide nanostructures for high performance supercapacitors

A pseudocapacitive electrode based on mesoporous manganese dioxide (MnO2) was fabricated via the anodic pulse electrodeposition method. Pulse current (PC) has many parameters that affect the physicochemical and electrochemical properties of the material. PC parameters are generally divided into independent and dependent types. Independent pulse parameters consist of the time of applying current (ton), interval time with zero current (toff), and peak current density (Ip). Dependent pulse parameters include duty cycle (θ), frequency (f), and average current density (Ia). In this work, the changes of two independent parameters (ton and toff) at the constant Ip = 2 mA cm-2 on the anodic pulse electrodeposition of MnO2 were investigated, which also caused changes in the independent parameters. The effects of changes in pulse parameters on physicochemical properties of MnO2 were studied by field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction technique, and nitrogen gas adsorption-desorption isotherms. Cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy techniques were applied to investigate the electrochemical behavior of the prepared MnO2 samples. The pulse conditions in the best sample were ton = 10 ms, toff = 30 ms, θ = 25% and f = 25 Hz. This sample was the body-centered tetragonal phase of α-MnO2 with a significant specific surface area of 154.03 m2 g-1. The specific capacitances of the optimal sample were 526.44 and 120 F g-1 at the current densities of 0.5 and 10 A g-1, respectively. The two-electrode symmetric supercapacitor cell assembled using the best sample was showed the specific capacitance of 300.63 F g-1 at a current density of 0.5 A g-1. The capacitance maintenance of the fabricated device was 87.10% after 5000 cycles at a current density of 4 A g-1, which confirmed the good cyclic stability of this symmetric supercapacitor. The maximum energy and power densities were 81.84 Wh kg-1 and 17.49 kW kg-1, respectively, which indicated the good pseudocapacitive performance of the MnO2-based symmetric supercapacitor.

Theoretical evidence for the Peierls transition in NbO2

We show by advanced electronic structure calculations that ${\mathrm{NbO}}_{2}$ essentially is a Peierls-type material. After simulating the rutile as well as the body-centered tetragonal phase with the Bethe-Salpeter equation, we are able to reproduce the experimental values for the electronic properties without adding correlations. Our calculation includes only excitonic corrections and no further interactions. The principal indirect gap is between $N$ and $\mathrm{\ensuremath{\Gamma}}$ and is found to be 0.98 eV, the direct gap at the $\mathrm{\ensuremath{\Gamma}}$ point amounts to 1.35 eV. We found the rutile structure to be anisotropic, with nesting vectors in the Fermi surfaces in the $\mathrm{\ensuremath{\Gamma}}MAZ$ and $\mathrm{\ensuremath{\Gamma}}XRZ$ planes.

Effect of Fe and Al on hydrogen storage properties of 75 V-Ti-Cr alloys

V-based hydrogen storage alloys are considered promising materials with high hydrogen capacity under ambient conditions. 75 V-Ti-15Cr alloys with different Fe and Al addition (1–3 at%) have been investigated. All the alloys are confirmed to show body-centered tetragonal (BCT) structure after dehydrogenated at room temperature. The PCT tests manifest that the hydrogen storage capacity decreases and desorption plateau pressure rises with the content of Fe or Al. The relationship between the hydrogen storage property and lattice parameters of BCT phase is discussed. Also, the relationship between hydrogen capacity and electron density is analyzed. Among the studied alloys, the 75 V-Ti-15Cr-1Al-1Fe alloy shows optimal hydrogen storage capacity (2.26 wt%) and plateau pressure (0.5 MPa) at room temperature. Cycle test proves that the optimized 75 V-Ti-15Cr-1Al-1Fe alloy possesses excellent durability and the retention rate of hydrogen capacity can achieve 97% after 100 cycles. The slight capacity attenuation possibly results from the derivation of lattice defects and small contraction of the hydrogen absorption phases.

Structure and Phase Transition Features of Monoclinic and Tetragonal Phases in U–Mo Alloys

Using molecular dynamics simulations, we studied the structural properties of orthorhombic, monoclinic, and body-centered tetragonal (bct) phases of U–Mo alloys. A sequence of shear transformations between metastable phases takes place upon doping of uranium with molybdenum from pure α -U: orthorhombic α ′ → monoclinic α ″ → bct γ 0 → body-centered cubic (bcc) with doubled lattice constant γ s → bcc γ . The effects of alloy content on the structure of these phases have been investigated. It has been shown that increase in molybdenum concentration leads to an increase in the monoclinic angle and is more similar to the γ 0 -phase. In turn, tetragonal distortion of the γ 0 -phase lattice with displacement of a central atom in the basic cell along the &lt;001&gt; direction makes it more like the α ″ -phase. Both of these effects reduce the necessary shift in atomic positions for the α ″ → γ 0 -phase transition.

Pressure engineering of the Dirac fermions in quasi-one-dimensional Tl2Mo6Se6

Topological band dispersions other than the standard Dirac or Weyl fermions have garnered the increasing interest in materials science. Among them, the cubic Dirac fermions were recently proposed in the family of quasi-one-dimensional (q-1D) conductors A2Mo6X6 (A = Na, K, In, Tl; X = S, Se, Te), where the band crossing is characterized by a linear dispersion in one k-space direction but the cubic dispersion in the plane perpendicular to it. It is not yet clear, however, how the external perturbations can alter these nontrivial carriers and ultimately induce a new distinct quantum phase. Here we study the evolution of Dirac fermions, in particular the cubic Dirac crossing, under external pressure in the representative q-1D Tl2Mo6Se6 via the first-principles calculations. Specifically, it is found that the topological properties, including the bulk Dirac crossings and the topological surface states, change progressively under pressure up to 50 GPa where it undergoes a structural transition from the hexagonal phase to body-centered tetragonal phase. Above 50 GPa, the system is more likely to be topologically trivial. Further, we also investigate its phonon spectra, which reveals a gradual depletion of the negative phonon modes with pressure, consistent with the more three-dimensional Fermi surface in the high-pressure phase. Our work may provide a useful guideline for further experimental search and the band engineering of the topologically nontrivial fermions in this intriguing state of matter.