A15 Structure Research Articles

Real-time study of the interplay between phase formation and stress evolution at the W1−xSix/a−Si interface

Nanoscale W1−xSix layers with different Si content were deposited by magnetron sputtering on amorphous silicon layers. The structure and stress evolution during deposition were monitored and in real time. Thus, it was possible to disentangle different origins of stress built-up (interface formation, phase formation, grain growth, and texture) on the nanoscale. It was found that the bcc phase with a composition-dependent texture forms at low Si content (less than 10% Si), but only above a critical thickness. At intermediate Si content (13.9% and 16.5% Si), or low Si content and low thicknesses (≲4.5 nm), the β phase with A15 structure and coexisting phases (bcc or amorphous) form, while a single, amorphous phase is observed at high Si content (≳22% Si). An A15 formation driven by impurities (O,C,N) was excluded by combined x-ray photoelectron spectroscopy and x-ray diffraction measurements on a second sample series. calculations of substitutional bcc and A15 W1−xSix alloys support the formation of A15 at higher Si content. The A15-containing interlayer should be taken into account when discussing the superconductivity of W/Si multilayers. The stress evolution during deposition of W1−xSix was correlated with the microstructure evolution, and compared to similar observations for Mo1−xSix. Due to the crystalline phase (bcc/A15) and bcc texture ([111] and [110]) competition, the structure formation of W1−xSix shows a higher complexity. This explains the wide range of stress states (from tensile to compressive) observed after deposition of W1−xSix. Nanoscale W-Si based stress-compensation layers could be employed for tailoring the stress state of nonepitaxial semiconductor devices. Published by the American Physical Society 2024

Investigation of superconductivity of A15 type cubic Nb3Os material by using density functional theory

In this study, the structural, elastic, electronic, phonon and electron-phonon interaction properties of A15 type cubic Nb3Os compound were investigated by density functional theory. The lattice constant of this compound was obtained in agreement with experiments, and the investigation of elastic properties revealed its ductile nature. From electronic calculations for Nb3Os, the density of states at Fermi level (N(EF)) was found to be 5.31 states/eV with the largest contribution of Nb 4d orbitals. According to the results obtained from phonon calculations, Nb3Os was found to be dynamically stable in the cubic A15 structure and phonon modes were largely formed by the vibrations of Nb atoms. Using the electronic and phonon properties, we determined the Eliashberg spectral function, which illustrates the electron-phonon interaction. The electron-phonon interaction parameter and average phonon frequency values for Nb3Os were found to be 0.41 and 222.97 K, respectively. The calculated superconducting transition temperature of 1.05 K is in excellent agreement with the experimental value of 1.02 K.

Hydrogen in Compounds and Alloys with A15 Structure

In the present work, a theoretical study of atomic ordering in the A3BCx alloy is carried out. The mutual influence of the ordering and solubility of impurity C in the A3B alloy is studied. The dependences of solubility on the composition of the alloy, temperature, degree of long-range order are found and studied. In addition, the criteria for the manifestation of extremity in the concentration and temperature dependences of solubility are obtained. The atomic ordering is studied using the average-energies’ method; the features of the C impurity solubility in the A3B alloy are elucidated using the configuration method. Experiments confirming the results of the theory are currently unknown to authors. However, the available experimental data on determining the temperatures of martensitic transformation (Tm) and superconducting transition (Tc) for the Nb3SnHx alloy allow us to hope and assert a possible agreement between the data of theory and experiment.

Effects of interstitial impurities (H, O, and He) on the structure and electronic properties of [formula omitted]–W: A density functional theory-based study

The A15 structure of W, the β–W, exhibits better properties for spintronics applications than bcc α–W. Previous works found that impurities could stabilize β–W. It is therefore important to identify the role and effects of impurities in the material. Hence, we performed DFT-based calculations and investigated the effects of interstitial impurities (12.50% and 1.56% atomic impurity concentrations of H, O, and He) on the structural and electronic properties of β–W. We also investigated α–W for comparison. We found that the binding energy of O is more favored in β–W than in α–W. However, the cohesive energy per atom of β–W is higher than α–W only when there is 12.50% atomic concentration of O in the system. This suggests that O has the tendency to induce α–W-to-β–W transition at this concentration of O. We found that the binding energies of H in β–W are energetically more favorable than in α–W, but the cohesive energies per atom are higher in α–W. These results imply that it will require a larger amount of energy to form β–W with H as an impurity. We found no stable site for He. Interaction between impurities and W atoms induces structural changes and an increase in the electron density of states at the Fermi level which highlights the effects of impurities in β–W.

Superconductivity in the cobalt-doped V3Si A15 intermetallic compound

The A15 structure of superconductors is a prototypical type-II superconductor that has generated considerable interest since the early history of superconducting materials. This paper discusses the superconducting properties of previously unreported V3−x Co x Si (0 ⩽ x ⩽ 0.30) alloys. It is found that the lattice parameter decreases with increasing cobalt-doped content and leads to an decreased residual resistivity ratio value of the V3−x Co x Si system. Meanwhile, the superconducting transition temperature (Tc ) also decreases with increasing cobalt-doped content. Furthermore, the fitted data show that the increase of cobalt-doped content also reduces the lower/upper critical fields of the V3−x Co x Si system. Type-II superconductivity is demonstrated in all V3−x Co x Si samples. With higher Co-doped content, V3−x Co x Si (0.15 ⩽ x ⩽ 0.30) alloys may have superconducting and structural phase transitions in low-temperature regions. As the electron/atom ratio increases, the Tc variation trend of V3Si is as pronounced as in crystalline alloys and monotonically follows the trend observed for amorphous superconductors.

Bayesian modeling of pattern formation from one snapshot of pattern.

Partial differential equations(PDEs) have been widely used to reproduce patterns in nature and to give insight into the mechanism underlying pattern formation. Although many PDE models have been proposed, they rely on the pre-request knowledge of physical laws and symmetries, and developing a model to reproduce a given desired pattern remains difficult. We propose a method, referred to as Bayesian modeling of PDEs (BM-PDEs), to estimate the best dynamical PDE for one snapshot of a objective pattern under the stationary state without ground truth. We apply BM-PDEs to nontrivial patterns, such as quasicrystals (QCs), a double gyroid, and Frank-Kasper structures. We also generate three-dimensional dodecagonal QCs from a PDE model. This is done by using the estimated parameters for the Frank-Kasper A15 structure, which closely approximates the local structures of QCs. Our method works for noisy patterns and the pattern synthesized without the ground-truth parameters, which are required for the application toward experimental data.

Comparative studies on superconductivity in Cr3Ru compounds with bcc and A15 structures

Chromium (Cr) is a transition metal element with 3d orbital electrons. In most compounds containing Cr, due to the correlation effect, twofold features, namely localization and itinerancy are expected. The localization gives rise to a magnetic moment, while the latter exhibits as the effective coherent weight for conductivity. Here we report the physical properties of Cr3Ru compounds with body-centered cubic (bcc) and A15 structures by using multiple experimental tools. The resistivity measurements show sharp superconducting transitions at = 2.77 K and = 3.37 K for the bcc and A15 structures, respectively. A high residual resistivity exists in both phases. Magnetization measurements also show rather narrow superconducting transitions, with a clear hump feature in the intermediate temperature region (about 150 K), which may be ascribed to the remaining antiferromagnetic spin fluctuations. A pronounced second peak effect has been observed in magnetization hysteresis loops in the superconducting state only for samples with bcc structure. The specific heat coefficient reveals a clear jump at critical temperatures (). We find that s-wave gaps can be adopted to fit the low temperature specific heat data of both samples yielding ratios of about 3.6, indicating a moderate pairing strength. Interestingly, the Wilson ratios are 3.81 and 3.62 for the bcc and A15 phases, suggesting a moderate correlation effect of conducting electrons in the normal state. Besides, for samples with A15 structure, another specific heat anomaly occurs at about 0.85 K and is sensitive to magnetic fields. In addition, by applying high pressures, both systems will exhibit an enhancement of with a rate of about 0.019 K GPa−1 and 0.013 K GPa−1 for the bcc and A15 phases, respectively. Our combinatory results point to unusual behavior of both superconducting and normal states in these two Cr based alloys.

Superconducting properties of the A15 structure compound V[formula omitted]Ge

Applications of A15 superconductors have received considerable attention, resulting in a large body of reported research work on A15 superconductors. Nonetheless, studies on this material focusing on its superconducting parameters are scarce. This motivates new efforts toward bulk measurements in typical A15 material V3Ge, which is also the isomorph of V3Si with a superconducting transition temperature at 17 K. In this study, we determined the superconducting parameters of single-phase V3Ge crystals, which crystallize in the A15 structure, through measurements of magnetization, magnetic susceptibility, electrical resistivity, and heat capacity. Our results show that the temperature (Tc) of its superconducting transition is 6.2±0.4 K. Moreover, we conducted electrical resistivity and magnetic susceptibility measurements and found the lower and higher critical fields to be 559 Oe and 39.8 kOe, respectively. The Ginzburg–Landau parameter κGL is determined to be 9.78, which is much greater than 1/2, implying V3Ge Type-II superconductor. The Sommerfeld coefficient γ in the normal state is determined to be 31.2 mJ/K2 mol, which is 50∼100 folds higher compared to that in the normal metal, thereby indicating the enhancement of the density of state at the Fermi energy. The Debye temperature Θ is estimated to be 430 K, and the value of the drastic increase in the specific heat at Tc is approximately 1.33, which is 7% smaller than the weak coupling limit of 1.43 predicted by the Bardeen–Cooper–Schrieffer (BCS) theory. The electron–phonon coupling constant λe−ph is 0.61. These values suggest that V3Ge is a conventional superconductor with s-wave symmetry in the BCS framework, which implies that the high Debye temperature and large density of states at the Fermi energy contribute to the increase in Tc in V3Ge. The superconducting parameters determined in this study provide guidance for the development of a new metal superconductor with high-Tc.

Microstructure, phase composition and hardness of Ti–Au cladding deposited on Ti–6Al–4V substrate by electron beam powder bed fusion method

In this study, a Ti–Au cladding was deposited on a substrate from the Ti–6Al–4V alloy by the electron beam powder bed fusion method in a vacuum. The main goal was to assess the possibility of using titanium powders and gold foils as a feedstock for additive manufacturing of such dental products. The microstructure, chemical element distributions, phase composition and hardness of the formed Ti–Au alloy were studied using optical microscopy, energy dispersive X-ray analysis, as well as X-ray diffraction and nanoindentation tests. Gold-containing intermetallic compounds were observed through the entire cladding thickness. The α-Ti, α-Au, AuTi and AuTi3 phases were found, in addition to the Ti3Au one, which provided hardness values greater than those of conventional titanium alloys. It was shown by results of the crystal-geometric and X-ray phase analysis that the AuTi3 phase possessed the most densely packed A15 structure. This fact correlated with the obtained data on the deviation of the atomic volume per ion from Zen's law and the high hardness levels. Metallurgical patterns of the microstructure formation that affected the functional properties of such claddings were discussed and a further research direction was proposed.

Theoretical investigations of hydrogen absorption in the A15 intermetallics Ti3Sb and Ti3Ir

Abstract Ti3Sb and Ti3Ir adopt the A15 (Cr3Si type) structure and are reported to incorporate hydrogen atoms to an extent, respectively, of Ti3SbH∼3 and Ti3IrH3.8. First-principles electronic structure calculations were performed to identify factors contributing to the difference in maximum hydrogen composition for these two intermetallic compounds. Relative energies and changes in energy densities of states and crystal orbital Hamilton populations upon H insertion in the intermetallic compounds were examined. In both compounds, hydrogen atoms are attracted to [Ti4] tetrahedral interstitial sites over any others. The natures of metal-hydrogen and metalloid-hydrogen bonding and the effects of hydrogen insertion on metal-metal and metal-metalloid bonding have an influence on the maximum hydrogen contents for Ti3Sb and Ti3Ir.

Origin of the enhanced Nb3Sn performance by combined Hf and Ta doping

In recent years there has been an increasing effort in improving the performance of Nb3Sn for high-field applications, in particular for the fabrication of conductors suitable for the realization of the Future Circular Collider (FCC) at CERN. This challenging task has led to the investigation of new routes to advance the high-field pinning properties, the irreversibility and the upper critical fields (HIrr and Hc2, respectively). The effect of hafnium addition to the standard Nb-4Ta alloy has been recently demonstrated to be particularly promising and, in this paper, we investigate the origins of the observed improvements of the superconducting properties. Electron microscopy, Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) and Atom Probe Tomography (APT) characterization clearly show that, in presence of oxygen, both fine Nb3Sn grains and HfO2 nanoparticles form. Although EXAFS is unable to detect significant amounts of Hf in the A15 structure, APT does indeed reveal some residual intragrain metallic Hf. To investigate the layer properties in more detail, we created a microbridge from a thin lamella extracted by Focused Ion Beam (FIB) and measured the transport properties of Ta-Hf-doped Nb3Sn. Hc2(0) is enhanced to 30.8 T by the introduction of Hf, ~ 1 T higher than those of only Ta-doped Nb3Sn, and, even more importantly the position of the pinning force maximum exceeds 6 T, against the typical ~ 4.5–4.7 T of the only Ta-doped material. These results show that the improvements generated by Hf addition can significantly enhance the high-field performance, bringing Nb3Sn closer to the requirements necessary for FCC realization.

Superconducting germanium hydride with A15 structure

The possibility of the formation of A15 germanium hydride is investigated theoretically by the density functional method. It is shown that A15 germanium hydride Ge2H6 is stable in a certain pressure range of about 125 GPa. The results of calculations of the structural, phonon and electronic properties, energy characteristics of the normal phase are presented. The critical temperature of the superconducting transition is estimated.

Large Magnetoresistance and Nontrivial Berry Phase in Nb3Sb Crystals with A15 StructureSupported by the National Key R&D Program of China (Grant No. 2016YFA0300402), the National Natural Science Foundation of China (Grant Nos. 12074335, and 11974095), and the Fundamental Research Funds for the Central Universities.

Compounds with the A15 structure have attracted extensive attention due to their superconductivity and nontrivial topological band structures. We have successfully grown Nb3Sb single crystals with the A15 structure and systematically measured the longitudinal resistivity, Hall resistivity and quantum oscillations in magnetization. Similar to other topological trivial/nontrivial semimetals, Nb3Sb exhibits large magnetoresistance (MR) at low temperatures (717%, 2 K and 9 T), unsaturating quadratic field dependence of MR and up-turn behavior in ρxx (T) curves under magnetic field, which is considered to result from a perfect hole-electron compensation, as evidenced by the Hall resistivity measurements. The nonzero Berry phase obtained from the de-Hass van Alphen (dHvA) oscillations demonstrates that Nb3Sb is topologically nontrivial. These results indicate that Nb3Sb superconductor is also a semimetal with large MR and nontrivial Berry phase. This indicates that Nb3Sb may be another platform to search for the Majorana zero-energy mode.

Structural and superconducting properties of Nb3Sn film for superconducting radio-frequency cavities synthesized via bronze route

Superconducting Nb3Sn films were synthesized via the ex-situ annealing of Nb-coated bronze precursors. The precursors were prepared by sputtering 2 [Formula: see text]m of niobium film onto a bronze surface. The annealing was conducted in a vacuum tubular furnace at a temperature not higher than 700[Formula: see text]C. By changing the annealing time and temperature, a series of Nb3Sn samples with critical temperatures ([Formula: see text] > 16 K were obtained. X-ray diffraction confirmed the existence of the cubic Nb3Sn phase (A15 structure), whereas magnetic moment measurements indicated that critical temperature of the studied samples ranged from 16.2 to 16.5 K. Therefore, this study explores a new technique appropriate for creating copper-based Nb3Sn thin-film-coated superconducting radio-frequency (TFSRF) cavities.

A15 Nb3Si: a ‘high’ T c superconductor synthesized at a pressure of one megabar and metastable at ambient conditions

A15 Nb3Si is, until now, the only ‘high’ temperature superconductor produced at high pressure (∼110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb3Si and its production from tetragonal Nb3Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb3Si material to trace T c as a function of pressure. T c is suppressed to ∼5.2 K at 88 GPa. Then, using these T c (P) data for A15 Nb3Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb3Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e. no indications of the superconductivity characteristic of A15 Nb3Si. This is in contrast to the explosive compression (up to P ∼ 110 GPa) of tetragonal Nb3Si, which produced 50%–70% A15 material, T c = 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. This implies that the accompanying high temperature (1000 °C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal → A15 Nb3Si structural transformation. Our theoretical calculations show that A15 Nb3Si has an enthalpy vs the tetragonal structure that is 70 meV atom−1 smaller at 100 GPa, while at ambient pressure the tetragonal phase enthalpy is lower than that of the A15 phase by 90 meV atom−1. The fact that ‘annealing’ the A15 explosively compressed material at room temperature for 39 years has no effect shows that slow kinetics can stabilize high pressure metastable phases at ambient conditions over long times even for large driving forces of 90 meV atom−1.

Electrochemical and thermal synthesis of Nb3Sn coatings on Nb substrates

Electrochemical and thermal synthesis of superconducting Nb3Sn films were studied in this paper. The Nb3Sn films were obtained by 700 °C ex-situ vacuum annealing of electrochemically deposited bronze layer on Nb substrates. Precursors were prepared via electrochemical deposition performed from aqueous solutions at room temperature. Samples were then precisely electropolished to remove the a-few-micrometres of remnant bronze phase on top of the surface and then characterized. Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) allowed accurate measurement of the thickness of the Nb3Sn layer, whose average thickness was 1.2 μm. X-ray diffraction (XRD) patterns confirmed the existence of a cubic Nb3Sn phase (A15 structure). The highest Tc observed on these samples was 17.6 K. Owing to the superior film properties and the absence of high temperature treatment, this synthesis technique opens a new route towards copper-based Thin Film coated Superconducting Radio-Frequency (TFSRF) cavities.

Superconducting and antiferromagnetic properties of dual-phase V3Ga

The binary compound V3Ga can exhibit two near-equilibrium phases, the A15 structure that is superconducting and the Heusler D03 structure that is semiconducting and antiferromagnetic. Density functional theory calculations show that these two phases are nearly degenerate, being separated in energy by only ±10 meV/atom. Our magnetization measurements on bulk-grown samples show antiferromagnetism and superconducting behavior below 14 K. These results indicate the possibility of using V3Ga for quantum technology devices exploiting the co-existence of superconductivity and antiferromagnetism in a dual-phase material.

Insight into structural, electronic, elastic and thermal properties of A15-type Nb3X (X = Si, Ge, Sn and Pb) compounds

Niobium-based compounds with the A15 structure have been widely applied in fusion engineering test reactor and superconducting materials. Hence, for a better understanding of the physical properties of A15-type Nb3X (X = Si, Ge, Sn and Pb) compounds, their structural, electronic, elastic and thermodynamic properties were investigated using the first-principles calculations. The calculated formation enthalpies indicated that these Nb3X compounds are energetically favorable and the sequence of thermodynamic stability is Nb3Si > Nb3Ge > Nb3Sn > Nb3Pb. Meanwhile, the phonon dispersions verified that these Nb3X compounds are dynamically stable. The electronic properties, such as the density of state, band structure, electron density difference and Mulliken population, suggested that these compounds are metallic, and Nb-Nb and Nb-X chemical bonds are ionic-covalent mixed bonds. The single-crystal and polycrystalline elastic properties were calculated. Moreover, the elastic anisotropy was discussed by elastic anisotropic indexes, three-dimensional surface construction and planar projections of elastic modulus, and the sequence of elastic anisotropy is Nb3Si > Nb3Ge > Nb3Sn > Nb3Pb. Finally, thermal properties such as Debye temperatures and sound velocities were analyzed based on elastic moduli.

Elastic constants of beta tungsten thin films studied by picosecond ultrasonics and density functional theory

Tungsten thin films are used for various applications and sometimes exhibit an A15 structure (β-W). They have some superior properties in comparison to the bcc structure (α-W), such as a higher superconducting transition temperature and larger spin Hall angle. However, elastic constants of β-W are unclear, which restricts mechanical applications and reliable density-functional-theory calculations. In this study, we synthesized α-W, β-W, and mixed-phase W films and determined their elastic constants by picosecond ultrasonics. We also calculated the elastic constants based on density functional theory and reveal that β-W has a larger elastic anisotropy and smaller shear modulus. Our calculation further indicates a stable stacking faulted β-W, which leads to a monoclinic structure.

Superconducting Zirconium Polyhydrides at Moderate Pressures.

Highly compressed hydrides have been at the forefront of the search for high-Tc superconductivity. The recent discovery of record-high Tc's in H3S and LaH10±x under high pressure fuels the enthusiasm for finding good superconductors in similar hydride groups. Guided by first-principles structure prediction, we successfully synthesized ZrH3 and Zr4H15 at modest pressures (30-50 GPa) in diamond anvil cells by two different reaction routes: ZrH2 + H2 at room temperature and Zr + H2 at ∼1500 K by laser heating. From the synchrotron X-ray diffraction patterns, ZrH3 is found to have a Pm3̅n structure corresponding to the familiar A15 structure, and Zr4H15 has an I4̅3d structure isostructural to Th4H15. Electrical resistance measurement and the dependence of Tc on the applied magnetic field of the sample showed the emergence of two superconducting transitions at 6.4 and 4.0 K at 40 GPa, which correspond to the two Tc's for ZrH3 and Zr4H15.