A15-type Structure Research Articles

Design of high-temperature superconductors at moderate pressures by alloying AlH3 or GaH3

Since the discovery of hydride superconductors, a significant challenge has been to reduce the pressure required for their stabilization. In this context, we propose that alloying could be an effective strategy to achieve this. We focus on a series of alloyed hydrides with the AMH6 composition, which can be made via alloying A15 AH3 (A = Al or Ga) with M (M = a group ⅢB or IVB metal), and study their behavior under pressure. Seven of them are predicted to maintain the A15-type structure, similar to AH3 under pressure, providing a platform for studying the effects of alloying on the stability and superconductivity of AH3. Among these, the A15-type phases of AlZrH6 and AlHfH6 are found to be thermodynamically stable in the pressure ranges of 40–150 and 30–181 GPa, respectively. Furthermore, they remain dynamically stable at even lower pressures, as low as 13 GPa for AlZrH6 and 6 GPa for AlHfH6. These pressures are significantly lower than that required for stabilizing A15 AlH3. Additionally, the introduction of Zr or Hf increases the electronic density of states at the Fermi level compared with AlH3. This enhancement leads to higher critical temperatures (Tc) of 75 and 76 K for AlZrH6 and AlHfH6 at 20 and 10 GPa, respectively. In the case of GaMH6 alloys, where M represents Sc, Ti, Zr, or Hf, these metals reinforce the stability of the A15-type structure and reduce the lowest thermodynamically stable pressure for GaH3 from 160 GPa to 116, 95, 80, and 85 GPa, respectively. Particularly noteworthy are the A15-type GaMH6 alloys, which remain dynamically stable at low pressures of 97, 28, 5, and 6 GPa, simultaneously exhibiting high Tc of 88, 39, 70, and 49 K at 100, 35, 10, and 10 GPa, respectively. Overall, these findings enrich the family of A15-type superconductors and provide insights for the future exploration of high-temperature hydride superconductors that can be stabilized at lower pressures.

Designing ternary superconducting hydrides with A15-type structure at moderate pressures

Ever since hydrogen-based superconductors have been actively considered as candidates for room-temperature superconductors, binary hydrides have been almost completely explored theoretically. Now researchers are turning their interest to ternary hydrides. However, the enormous compositional and structural possibilities in the ternary hydride space limit the search efficiency. Here, using an advanced crystal structure search method combined with first-principles calculations, a series of ternary hydrides AXH6 (MgPH6, MgAsH6, CaSbH6, CaSnH6, NaSbH6 and ScSiH6) with the A15-type structure are predicted to be stable at around one million atmospheres. Among them, CaSnH6 presents the lowest thermodynamically stable pressure of 110 GPa. CaSbH6, CaSnH6 and ScSiH6 may be dynamically stable down to 41 (70), 71 (90) and 86 (70) GPa, respectively, without (with) anharmonic effects and temperature effects. Hydrogen atoms in these A15-type structures are found to form weak covalent bonds to X atoms. More interestingly, we find that ternary hydrides with the AXH6 stoichiometry may assume the A15-type structure under pressure when the sum of the chemical valences of the two non-hydrogen atoms is six or seven. Based on electron-phonon coupling (EPC) calculations, all these ternary hydrides are estimated to be superconductors. CaSbH6, CaSnH6 and ScSiH6 have superconducting transition temperatures (Tcs) of 65, 53 and 116 K at 45, 75 and 90 GPa, respectively, while MgPH6 has the highest Tc of 148 K at 135 GPa. Our study will provide guidance for designing high Tc ternary hydrides near ambient pressures.

Ab initio расчет зонной структуры и свойств модификаций соединения Ti-=SUB=-3-=/SUB=-Sb, допированного литием

Using the density functional theory (DFT) in the local electron spin density approximation (LSDA), 2×2×2 supercells based on the Ti3Sb compound have been studied. The supercells contained their own vacancies and doped lithium atoms replacing Ti and/or Sb. The DFT-LSDA method was used to calculate the structural, electronic, and magnetic properties, the enthalpy of formation, and the cohesion energy of supercells of two modifications of the Ti3Sb compound. We studied supercells with cubic system (A15 type structure; a = 5.217 Å) and tetragonal system (D8_m type structure; а = 10.457 Å, с = 5.258 Å). It has been established that the distribution of the density of states of the Ti3Sb cubic system has a more metallic character than for the D8_mTi3Sb modification. The enhancement of "metallicity" in the cubic modification A15 Ti3Sb is associated with an increase in the Ti–Sb interatomic distance in the crystal. Due to this, the degree of metallic bonding increases during the electronic interaction between Ti and Sb atoms near the Fermi level in Ti3Sb. In DFT calculations, the spin-polarized density of states was taken into account. It has been established that in both modifications (A15 and D8_m) of Ti3Sb near the Fermi level for s, p, d-states with spin "up" and spin "down" there is a spin imbalance in the population of energy levels. Defect-containing supercells based on Ti3Sb were studied by DFT-LSDA calculations. It is shown that Ti and/or Sb vacancies in the lattices of Ti3Sb crystals of both modifications (A15 and D8_m) increase the magnetic moment (M) as compared to the value of M (M_tot = 0.08 μ_B) of a “pure” Ti3Sb crystal. Considering that Al5 Ti3Sb is also interesting as a material for Li-ion batteries, we studied Ti3Sb–Li supercells doped with lithium. Li-doping and the creation of own Ti and/or Sb vacancies changes the distance between atoms in Ti3Sb. Correspondingly, the resulting local magnetic moments near the Ti or Sb vacancies also change. The enthalpy of formation and magnetic moment of Ti3Sb and Ti3Sb–Li based supercells are calculated. DFT calculations of the structural stability of binary compounds (phases) have been carried out, and the stability of conodes between phases in the Ti–Sb–Li system has been established. The isothermal section of the Ti–Sb–Li system was plotted at 298 K. It is shown that the introduction of various lithium concentrations (<6.25 at. % Li) into the Ti3Sb (Space group Pm3 ̅n; № 223; a = 5.217 Å) crystal lattice reduces the partial magnetic moment of Ti in the Ti3Sb–Li supercell.

Ab initio calculation of the band structure and properties of modifications of the Ti-=SUB=-3-=/SUB=-Sb compound doped with lithium

Using the density functional theory (DFT) in the local electron spin density approximation (LSDA), 2x2x2 supercells based on the Ti3Sb compound have been studied. The supercells contained their own vacancies and doped lithium atoms replacing Ti and/or Sb. The DFT-LSDA method was used to calculate the structural, electronic, and magnetic properties, the enthalpy of formation, and the cohesion energy of supercells of two modifications of the Ti3Sb compound. We studied supercells with cubic system (A15 type structure; a=5.217 Angstrem) and tetragonal system (D8m type structure; a=10.457 Angstrem, c=5.258 Angstrem). It has been established that the distribution of the density of states of the Ti3Sb cubic system has a more metallic character than for the D8m Ti3Sb modification. The enhancement of "metallicity" in the cubic modification A15 Ti3Sb is associated with an increase in the Ti-Sb interatomic distance in the crystal. Due to this, the degree of metallic bonding increases during the electronic interaction between Ti and Sb atoms near the Fermi level in Ti3Sb. In DFT calculations, the spin-polarized density of states was taken into account. It has been established that in both modifications (A15 and D8m) of Ti3Sb near the Fermi level for s-, p-, d-states with spin "up" and spin "down" there is a spin imbalance in the population of energy levels. Defect-containing supercells based on Ti3Sb were studied by DFT-LSDA calculations. It is shown that Ti and/or Sb vacancies in the lattices of Ti3Sb crystals of both modifications (A15 and D8m) increase the magnetic moment (M) as compared to the value of M (M=0.08 μB) of a "pure" Ti3Sb crystal. Considering that Al5 Ti3Sb is also interesting as a material for Li-ion batteries, we studied Ti3Sb-Li supercells doped with lithium. Li-doping and the creation of own Ti and/or Sb vacancies changes the distance between atoms in Ti3Sb. Correspondingly, the resulting local magnetic moments near the Ti or Sb vacancies also change. The enthalpy of formation and magnetic moment of Ti3Sb and Ti3Sb-Li based supercells are calculated. DFT calculations of the structural stability of binary compounds (phases) have been carried out, and the stability of conodes between phases in the Ti-Sb-Li system has been established. The isothermal section of the Ti-Sb-Li system was plotted at 298 K. It is shown that the introduction of various lithium concentrations (≤6.25 at.% Li) into the Ti3Sb (Space group Pm3 n; N 223; a=5.217 Angstrem) crystal lattice reduces the partial magnetic moment of Ti in the Ti3Sb-Li supercell. Keywords: Ti3Sb intermetallic compounds, A15 cubic modification, D8m tetragonal modification, doping with lithium, Ti3Sb-Li, DFT-LSDA calculations, supercell, electronic and magnetic properties, Ti-Sb-Li phase stability.

Structural, microstructural and mechanical properties of Cr3P2 chromium phosphide powders

Nanometric particles of a new chromium structure were produced by mechanical alloying of Cr3P2 under low nitrogen pressure. X-ray diffraction study has shown that the particles have an ordinary bcc structure of chromium (α-Cr) up to 1 h of milling but they have a new cubic chromium A15-type structure (δ-Cr), up to 2 h of milling, with a lattice parameter equal to 0.4703 ± 0.001 nm. The new structure has a Pm-3 space group with eight atoms distributed within the unit cell: 2 Cr atoms with occupancy equal to 1 in positions: 1a: 000 and 1 b: ½½½ in addition to 6 others Cr atoms with occupancy equal to 0.25 in positions: 6f: ¼ 0 ½; 6f: ½ ¼ 0; 6f: 0 ½ ¼; 6f: ¾ 0 ½; 6f: ½ ¾ 0; 6f: 0 ½ ¾ . As well as the new δ-Cr A15-type phase two chromium phosphides are formed in the nanometric Cr3P2 alloy: an orthorhombic CrP and the monoclinic CrP2 after 1 and 2 h of milling, respectively. Therefore, the formed phosphide records the highest density of dislocations and the lowest stacking fault probability after 2 h of milling. The mechanical properties show that the P and CrP are auxetic strong reentrant cellular structures with negative Poisson’s ratio which makes them specifically resistant.

Superconductivity in Cubic A15-type V–Nb–Mo–Ir–Pt High-Entropy Alloys

We report the crystal structure and superconducting properties of new V5+2xNb35−xMo35−xIr10Pt15 high-entropy alloys (HEAs) for x in the range of 0 ≤x≤ 10. These HEAs are found to crystallize in a cubic A15-type structure and have a weakly coupled, fully gapped superconducting state. A maximum Tc of 5.18 K and zero-temperature upper critical field Bc2(0) of 6.4 T are observed at x = 0, and both quantities decrease monotonically with the increase of V content x. In addition, Tc shows an increase with increasing valence electron concentration from 6.4 to 6.5, which is compared with other A15-type HEA and binary superconductors.

Ιntermetallic β-Ti3Au hard thin films prepared by magnetron sputtering

In the current study, we report on a deposition method to synthesize the intermetallic β-Ti3Au thin films by magnetron sputtering. We examine thin films crystallinity and structure-property relationships in terms of deposition conditions. The study is motivated by the vast application potential of β-Ti3Au, known to possess high hardness, interesting scratch and wear behaviors, and very good biocompatibility. The Ti1−xAux thin films (x=0–1) were deposited onto the Si(100) substrates by triple target magnetron sputtering under an Ar atmosphere and temperature varying from room temperature and to 650°C. The samples were characterized by X-ray diffraction (XRD) under grazing incidence and Bragg-Brentano geometries, scanning and transmission electron microscopy (SEM, TEM) equipped with an energy dispersive spectroscopy detector (EDS), and depth-sensing nanoindentation to investigate the dependence of the phase configuration, microstructure and mechanical properties on the Au/Ti ratio and deposition temperature. It was shown that upon varying the Au content the coatings develop a variety of single-phases or mixed structures composed of cubic, hexagonal and/or tetragonal phases of the binary Au-Ti system. At x=0.25, the compound Ti3Au developed a quasi-amorphous or defective nanocrystalline structures when deposited at room or low-temperatures, while a fully crystalline β-Ti3Au with an A15-type structure and lattice parameter of a=0.50925nm was obtained when deposited at 350–400 °C. The β-phase films exhibited a hardness of 10.4–12.1GPa, which is among the upper limits for intermetallic compounds, while the Young's modulus (173–201GPa) is fairly comparable to 209GPa predicted by first principles theoretical calculations. Thin film crystallinity and hardness strongly influenced both the deformation mode and the wear behavior under nanoscratch testing. The wear mechanisms changed from ploughing with ductile displacement, to microcutting with the formation of curly debris, to cracking with brittle fragmentation.

Phase transformation of the A15 metastable phase of Fe-Cr thin films prepared by ion-beam sputtering

Thermal stability of metastable A15 Fe-Cr phase is investigated through the study of its magnetic and structural properties. This phase presents very interesting mechanical properties suggesting that A15-structured films might be of great interest for tribological applications when considering the hardness H over Young’s modulus E ratio i.e., a description in terms of “elastic strain to failure” for wear resistance. Indeed, H is multiplied by a factor 2 with respect to the value measured for the bulk cubic centered α phase whereas E remains identical. Then, an improvement by a factor 8 of resistance to plastic deformation may be expected since predictive models stand that this quantity is proportional to the H3∕E2 ratio. However, heating problems due to sliding during tribological tests may lead to structural transformation in the film and then a loose of mechanical performance. The formation and the stability of the A15 cubic structure (δ phase) in centered-cubic refractory metals are generally attributed to the presence of oxygen atoms in the unit cell. For equiatomic Fe-Cr thin films elaborated by physical vapor deposition techniques, residual oxygen atoms present in the deposition chamber would be absorbed during the deposition process. In this work, the δ-phase transformation has been studied ex situ in the temperature range 400−650 °C; structural changes have been accurately investigated thanks to the combination of x-ray diffraction and Mössbauer spectroscopy techniques. Thin films were deposited onto quartz substrates and then annealed ex situ under secondary vacuum. From 400 °C, a “structural relaxation” occurring in the δ phase precedes and accompanies the beginning of the phase transformation. Finally, the partially ordered metastable δ-phase transforms into a stable α-phase presenting the precipitation phenomenon at temperature above 550 °C and the presence of a τ-carbide phase is clearly visible from 600 °C. The δ-phase transformation is completed at 650 °C. The stability of the A15 type structure up to 550 °C is a promising indication of good tribological performance of such coatings.

Effects of O<SUB>2</SUB> Gas on the Size and Structure of Cr Clusters Formed by Plasma-Gas-Condensation

Cr clusters have been produced by a plasma-gas-condensation type cluster deposition apparatus, and studied using a time-of-flight mass spectrometer and a transmission electron microscope. The Cr clusters formed in high pressure inert (Ar and/or He) gas atmosphere are of an A15-type structure. When an O2 gas is mixed with the inert gases in the source (sputtering) chamber, a bcc phase is formed together with Cr2O3. The O2 gas introduction leads to an increase in the gas temperature of the source chamber probably due to release of the formation enthalpy of the oxide. The A15 phase is annealed by such excess heat and becomes the equilibrium bcc phase. The sizes of bcc clusters are smaller than those of the A15-clusters, probably due to the heterogeneous nucleation promoted by the oxide formation.

A15 type structure of ultrafine chromium particles studied by UHV-HRTEM

We found that ultrafine chromium (Cr) particles prepared in UHV condition have an A15 type structure different from their bulk crystals by UHV high-resolution transmission electron microscopy (JEM 2000FXV). Cr (99.999% purity) was evaporated in situ in the UHV microscope by electron beam heating to various diameters ranging from 5 to 20 nm onto an amorphous carbon substrate kept at 600°C. Diffraction patterns of the Cr particles showed the (200), (210) and (211) spots expected from the A15 type structure, while they showed no diffraction spots from the bcc structure. From the high-resolution images of the Cr particles, showing the (110), (111), (200) and (220) lattice fringes of the A15 type structure, we confirmed that the stable phase of the clean ultrafine Cr particles has an A15 type structure.

The charge density map and electronic band structure of A15-type Cr calculated by the FLAPW method

The charge density and electronic band structure of A15-type Cr have been calculated using a self-consistent full-potential linearized augmented plane wave (FLAPW) method. The calculated charge density maps qualitatively reproduce well the maps obtained by a combination of the powder X-ray diffraction data and the maximum-entropy method (MEM). Both charge density maps show the covalent bonding charge between the adjacent 6c site Cr atoms within the infinite linear chain. The 3d bands are split into the bonding and antibonding states and the density of states exhibits a minimum near the Fermi level of 0.7461 Ryd. The valence electrons occupy only the bonding 3d bands and therefore it seems that the A15-type Cr is a stable structure from the viewpoint of the band structure. The results of the band structure are consistent with the facts that A15-type Cr shows a temperature-independent paramagnetism down to 77 K, and does not have a high superconducting transition temperature which A3B compounds with the A15-type structure have.

The electron-density distribution and chemical bonding of A15-type Cr obtained by the maximum-entropy method

The electron-density distribution of A15-type Cr was obtained by a combination of powder-X-ray-diffraction data and the maximum-entropy method (MEM). Fine powder, in which about 13 vol% BCC Cr was contained, prepared by evaporation of Cr in a low-pressure atmosphere of Ar, was used. Though the powder is composed of the two phases, the final R and RW factors were 0.21% and 0.29%, respectively, and therefore excellent MEM maps were obtained. The MEM maps indicate that the main interaction is a strong covalent bond between adjacent 6c-site Cr atoms within the infinite linear chains. The MEM maps are compared with the valence-electron-density maps of superconducting V3Si and non-superconducting Cr3Si having an A15-type structure. The structure factors for some forbidden reflections, which originate from asymmetric bonding-charge distribution, were also calculated from the MEM densities.

Phase Evolution by Annealing in V/Al Multilayers Prepared on Thermally Oxidized Si

Phase evolution by solid-state reactions in Al/V multilavers ion-plated on thermally grown SiO 2 substrates was investigated by means of X-ray diffraction and transmission electron microscopy,and the possibility for the formation of superconducting phases of V 3 Al and V 3 Si with the A15 type structure wasexamined. Annealing at a temperature below about 973 K caused interdiffusion between V and Al layers resulting mainly a solid-solution of Al in V

Phase equilibria in cobalt-rich portions of the Co-Si and Co-Ge systems

The cobalt-rich portions of the Co-Si and Co-Ge systems between 700 and 1100°C have been investigated by means of electron probe microanalysis. The equilibrium between α(f.c.c.) and ε(c.p.h.) phases has been detected mainly by the diffusion couple technique, and it has been clarified that the c.p.h. phase is stable up to temperatures greater than 1100 °C in both the systems. It has also been found that the Co 3Ge phase, probably having an A15-type structure, exists somewhere between about 770 and 636 °C, being in equilibrium with the ε and βCo 5Ge 3 phases.

The influence of Cr and Ni impurities on superconducting properties of Nb80Ga20 alloys with A15 type structure

The superconducting properties of the Nb80Ga20–xMex (Me = Cr, Ni and 0 ≦ x ≦ 7) alloys are presented. Measurements of the critical temperature Tc, the upper critical field, the magnetization curves, and the normal state magnetic susceptibility and an X-ray analysis are performed. The weak temperature dependence of the magnetic susceptibility as well as the Tc versus impurity concentration dependence suggest that Cr and Ni should be treated as nonmagnetic resonant d-states. [Russian Text Ignored].

The influence of Co impurities on superconducting properties of the Nb80Ga20 alloy with A15 type structure

The superconducting properties of the Nb80Ga20-xCox with 0 ≦ x ≦ 7 alloys are investigated. The small changes of magnetic susceptibility with temperature suggest that Co forms a nonmagnetic resonant d state in Nb80Ga20. The slope –dTc/dx|x0 is equal to 1.55 K/at% for the arc-melted samples and to 1.7 K/at% for the arc-melted and then annealed ones. The temperature dependence of the upper critical field Hc2, the magnetization curves, the first penetration field, and the resistance ratio are given as well. [Russian Text Ignored.]

Preparation of Nb3Ge films by chemical transport reaction and their critical properties

Niobium-germanium films have been deposited on sapphire substrates at 900 ‡C by a chemical transport reaction method. The highest superconducting transition onset temperature T C,on of 22.4 K is observed for a nearly stoichiometric Nb3Ge film with the A15-type structure (thickness ~ 93.5 Μm). Lattice constants for the Nb3Ge phase formed in the Nb-Ge films with both T C,on above 22 K and T C,midpoint above 21 K are found to extend from 5.143 to 5.153 ». Deposition rates for the obtained films are in the range of 2–10 Μm/min. Critical current densities for the Nb3Ge film with the highest T C,on value are observed to be relatively low (~ 103 A/cm2 at 19 K at self-field). This is due to the coarse grain structure of the film or the low density of effectual pinning centers in the film. Field variations of the pinning forces operating in this film in magnetic fields both parallel to the film surface and perpendicular to the film surface are found to follow closely b 1/2(1\s-b)2, to which the pinning force for flux pinning at the surface of normal regions, such as grain boundaries, film surfaces, etc., is proportional, and where b is the reduced magnetic induction (B/B C2).A small increase in J C at low fields is caused by the presence of a small amount of the Nb5Ge3 phase in a Nb3Ge film, and seems attributable to additional flux pinning on Nb5Ge3-phase particles in the film.

Particularities in the growth behaviour of vacuum-evaporated chromium thin films

New experimental data concerning the favourable effect of substrates of RbI, of RbCl with lead colloids and of KI with F centres on the growth of thin continuous chromium films containing the metastable A15-type structure are reported.

Linear-chain model to explain density of states and Tcchanges with atomic ordering

The effect of long-range atomic order on the electronic density of states has been recalculated for the A15-type structure within the linear-chain model. The author finds that a defect concentration c reduces the density of states at the Fermi level by a factor (1+c/c0)(c/c0)-3(ln(1+c/c0))3. This result is in qualitative agreement with experimental data on the specific heat, magnetic susceptibility and superconducting transition temperature of V3Au.

Chaleur spécifique et transformation martensitique dans le système Nb1− xSnx

Experimental data on the superconducting temperature, and low temperature specific heat, together with X-ray and neutron investigations are reported for two sets of Nb x Sn 1−x, samples with the A15 type structure. The first one, intended for a study of the effect of stoichiometry, consists of cast alloys with lattice constants ranging from 5.273 to 5.289 Å, γ's from 4.9 to 9 mJ K 2 g-at and T c 's from 6 to 17.9 K. The second one consists of nearly stoichiometric sintered alloys with lattice constants ranging from 5.288 to 5.291 Å, γ's from 5.6 to 13.5 mJ K 2 g-at and T c 's close to 18 K. The martensitic transformation ratio has been determined by neutron diffraction in the latter set and the values 32, 77 and 95% were obtained. The analysis of the specific heat jumps at T c shows that the superconducting T c of the tetragonal phase is 17.9 K and both T c and γ are reduced in the cubic phase. Phonon information inferred from specific heat data are indicative of a structure with a typical energy of 9 meV in the density of states. The agreement with spectra obtained by inelastic neutron scattering is good.