Fragility and thermal stability of Pt- and Pd-based bulk glass forming liquids and their correlation with deformability

The fragility parameter m of (Pt or Pd)–(Ni, Cu)–P bulk metallic glasses (BMGs) was characterized and compared to other systems. The results indicate that the Pt₆₅Ni₁₅P₂₅ alloy is the most fragile and thermally stable at the supercooled liquid state, thus is the most workable among the presently developed BMGs. To express the degree of workability, a deformability is introduced then correlated with m and the reduced thermal stability . A master curve in term of and has been established for various BMGs, d^* thus can be uniquely related to .     

High strain rate superplasticity in an Al–Mg–Sc–Zr alloy subjected to simple thermomechanical processing

Superior superplastic ductility of 2300% was achieved at 520 °C and  s⁻¹ in an Al–5%Mg–0.2%Sc alloy produced by traditional chill casting followed by cold rolling with a total reduction of 80%.     

The effect of substitution of La for Tb on some physical properties of Tb1−xLaxNi2 solid solutions

The standard Gibbs energy of formation, , of MgLa in the temperature range from near absolute 0 to 525 K were determined by calorimetry. The heat capacities, C_p, from 2 K to 525 K were measured by the relaxation method and DSC. Also, a thermal anomaly at 5.9 K, which appeared to be a superconductive phase transition, was found in the obtained C_p values. The values were determined by combining the C_p data with the standard enthalpy of formation at 298 K which was measured by the Calvet-type calorimeter using hydrochloric acid solution. From 2 to 300 K, the increases gradually, and it can be evaluated as a linear function of temperature above 300 K as follows:

This result is expected to be useful as basic thermodynamic data of Mg-based alloys.     

The dielectric function of Mgy NiHx thin films ()

Mg_y Ni (2≤y≤10) thin films covered with a Pd cap layer are hydrogenated in 10⁵  Pa H₂ between room temperature and 80 ° C and their dielectric function is determined from reflection and transmission measurements. The hydrogenated Mg_y NiH_x thin films show a continuous shift of the optical absorption towards higher photon energies with increasing y. Comparison of the obtained dielectric functions with predictions from an effective medium theory show that a considerable doping of the Mg₂ NiH₄ host takes place at least for y≤3.5 while no signature of MgH₂ is observed in that composition range in the optical spectra. This is in contrast to the predictions from the bulk phase diagram where a mixture of semiconducting Mg₂ NiH₄ (energy gap E_g=1.6  eV) and MgH₂ (E_g=5.6  eV) is expected.     

An atomistic analysis of the interface mobility in a massive transformation

A new multi-lattice kinetic Monte Carlo method has been used for an atomistic study on the interpretation of the interface mobility parameter for a massive face-centred cubic (fcc) to body-centred cubic (bcc) transformation in a single element system. For lateral growth of bcc in a system with an fcc(1 1 1)//bcc(1 1 0) and fcc[1 1 ]//bcc[0 0 ] interface orientation the overall activation energy for the interface mobility parameter is governed by energetically unfavourable atomic jumps. The atoms on the fcc lattice often cannot jump directly to bcc lattice sites because neighbouring atoms block the empty bcc sites. By single unfavourable jumps and by groups of unfavourable jumps a path from fcc to bcc is created. The necessity of these unfavourable jumps leads to an overall activation energy considerably larger than the activation energy barrier for a single atomic jump.     

Crystal structure, thermal expansion and magnetic properties of new compound Dy1.2Fe4Si9.8

The new ternary compound Dy_1.2Fe₄Si_9.8 have been prepared and studied by means of X-ray powder diffraction technique and vibrating sample magnetometer. The ternary compound Dy_1.2Fe₄Si_9.8 crystallizes in the hexagonal Er_1.2Fe₄Si_9.8-type structure, space group P6₃/mmc (no. 194) with lattice parameters a = 0.39415(1) nm and c = 1.52771(3) nm. The crystal structural refinement of the compound Dy_1.2Fe₄Si_9.8 has been performed by using Rietveld method. Lattice thermal expansion studies on the compound were carried out in the temperature range from 298 to 1013 K. The variation of the unit cell parameters shows that the unit cell parameters increase with the increase in temperature. The coefficients of average lattice thermal expansion along various axes in the temperature range from 298 to 1013 K are , and . The temperature dependence of the magnetization for the compound was also investigated in the range from 90 to 300 K. The experimentally determined magnetic effective paramagnetic moment is μ_eff = 11.3μ_B per formula unit (10.3μ_B per Dy atom).     

Structure at abrupt copper–alumina interfaces: An ab initio study

The atomistic structure and the energetics of Cu(1 1 1)/-Al₂O₃(0 0 0 1) interfaces for two experimentally observed interface orientation relationships were investigated from first-principles by means of the mixed-basis pseudopotential method. In the first orientation relationship, the direction of Cu is parallel to the direction of -Al₂O₃, and in the second the Cu is rotated with respect to the -Al₂O₃ by 90° around the interface normal [0 0 0 1]. Numerous candidate systems were considered for each case, covering all high-symmetry interface configurations, and with either Al or O termination of the -Al₂O₃(0 0 0 1) surfaces. For each of the most stable interfaces, full relaxations of the positions of all atoms were performed. It is found that despite the different atomistic structures of the two interface types, their interface stabilities, in terms of the work of separation, and their local atomistic structures are similar.     

Modification to the composition of nanocrystalline RuO2 through reactive milling under O2

Crystalline ruthenium dioxide (RuO₂) has been ball-milled in an O₂ atmosphere and the changes in the physico-chemical properties induced by different milling atmospheres (Ar and O₂) or milling sequences have been established. Cyclic voltammetry and BET measurements were used to evaluate the electrochemically active surface charge (, expressed in C g⁻¹) and the specific surface area (expressed in m² g⁻¹), respectively. The extent of oxygen uptake in the processed samples was determined by X-ray photoelectron spectroscopy (XPS). The value of RuO₂ milled under O₂ for 30 h (sample S4) is 120.7 C g⁻¹, a factor of two higher than the value of RuO₂ milled under Ar for the same period of time (sample S1, 60.3 C g⁻¹). A still higher value is obtained when the O₂ atmosphere of the crucible is frequently replenished (sample S5, 138.7 C g⁻¹) or when the milling operation is prolonged up to 81 h (sample S6, 160.5 C g⁻¹). These changes are paralleled by a variation of the BET surface area, which increases from 24 m² g⁻¹ for sample S1 to 51.5 m² g⁻¹ for sample S5. The concentration ratio [O_{bound to Ru}]/[Ru] determined by XPS increases steadily from sample S1 to sample S6, indicating that the amount [O_{bound to Ru}] increases with the exposure of RuO₂ to oxygen during the milling process. All these changes are explained by the fact that freshly exposed RuO₂ surfaces created during the milling process react with O₂ molecules, thereby lowering the surface energies and the tendency of the milled material to cluster into larger aggregates.     

Synthesis characterization of magnetic properties in La0.7−x Lnx Pb0.3 MnO3 (, Nd, Gd, Dy, Sm and Y) perovskite compounds

The influence of La substitution by rare-earth elements on the magnetization behaviors of perovskite oxides, La_0.7−x Ln_x Pb_0.3 MnO₃ (Ln=Pr, Nd, Gd, Dy, Sm and Y), is investigated. The replacement of La ion by Pr, Nd, Gd, Dy, Sm or Y results in a considerable decrease in the ferromagnetic ordering temperature T_C and clearly irreversible behavior in the FC–ZFC curves showing a short-range spin order phase. The fact is in agreement with the smaller ionic radii of Pr, Nd, Gd, Dy, Sm and Y ions in contrast to La ion and the corresponding larger distortion of perovskite structures. The saturation magnetization M_S increases as Pr or Nd content increases while M_S decreases as Sm or Y content increases. Moreover, the saturation magnetization M_S increases and then decreases as Gd or Dy content increases. These results can be explained in terms of the competition between the increase of ferromagnetically interacting spins due to the introduction of magnetic ions with f-shell electrons and suppression of ferromagnetism due to structure tuning induced by the small ionic radius of the interpolated cation into the La-site.     

Two polymorphs of Ba3Sn2P4: Single crystal and electronic structures

Two polymorphs (I and II) of Ba₃Sn₂P₄ have been found in the same preparative batch. Both compounds crystallize in the centrosymmetric monoclinic space group P2₁/c (#14, a = 7.8669(2) Å, b = 19.2378(5) Å, c = 7.8472(2) Å, β = 112.77(1)°, V = 1095.06(5) Å³, Z = 4, and R/wR = 0.0303/0.0710 for I; a = 7.8771(3) Å, b = 19.4099(7) Å, c = 7.7040(3) Å, β = 112.44(1)°, V = 1088.67(7) Å³, Z = 4, and R/wR = 0.0224/0.0415 for II). Both structures consist of one-dimensional chains separated by Ba²⁺ cations. The isolated chain consists of condensed ethane-like [Sn₂P₆] units. In polymorphs I and II, the condensation and connectivity of the [Sn₂P₆] units are quite different. While [Sn₂P₆] units form four- and six-membered rings in I, they form the five-membered rings in II. The electronic structure calculations indicate that semiconducting behavior is expected for both compounds.     

Dependence of dielectric properties of manganese-doped strontium titanate ceramics on sintering atmosphere

It is shown in this paper that the microscopic mechanism of the dielectric relaxation in Mn-doped strontium titanate (SrTiO₃–ST) ceramics is associated with the off-center displacement of ions. This was accomplished by studying the dielectric properties and electron spin resonance spectroscopy in combination with X-ray powder diffractometry and scanning electron microscopy techniques in Sr_1−xMn_xTiO₃ ceramic samples sintered in different atmospheres (air, oxygen and nitrogen) at 1500 °C. First, it is shown that manganese is incorporated into the perovskite lattice of ST, preferably as Mn²⁺ at Sr sites. However, a small amount of Mn⁴⁺ at Ti sites is also observed when fired in air or oxygen flow. The concentration of is the highest for sintering in oxygen, but firing in a reducing atmosphere (nitrogen) results solely in the incorporation of Mn²⁺ at Sr sites. Correspondingly, the dielectric relaxation observed in Sr_1−xMn_xTiO₃ markedly increases in intensity and slightly shifts towards a higher temperature for ceramics sintered in nitrogen compared with those fired in air or oxygen. All these facts are consistent with a suggestion that the off-center displacement of Mn²⁺ ions at the Sr sites of the highly polarizable ST lattice is the source of the observed relaxation behavior.     

Extrinsic and intrinsic magnetic properties of Co1−x Fex Sb3

We report magnetic properties of iron in Co_1−x Fe_x Sb₃ for x in the range 0<x<0.2, since x=0.2 is found to be the limit of solubility of iron in the skutterudite lattice. The magnetic ions diluted in the matrix carry a small magnetic moment reduced to that of the spin-only S=1/2 value of the Fe³⁺ in the low spin d⁵ configuration in presence of a strong crystal field that screens the orbital momentum. The magnetic properties give evidence that a small fraction of iron is spin-frozen in magnetite ferrimagnetic clusters, and antiferromagnetic FeO clusters. Because both types of clusters represent only very minor phases, their detection by the usual analytical means such as X-rays is not possible. The remaining part is diluted in the matrix to form a semimagnetic semiconductor characterized by a Fe–Fe nearest-neighbor exchange interaction J that is antiferromagnetic, with |J|/k_B19.6  K.     

Synthesis, structural, magnetic and electrical properties of La1−xCdxMnO3 manganites (0.1 ≤ x ≤ 0.5)

Cd-substituted LaMnO₃ compounds have been prepared using sol–gel method at a relatively low temperature of 900 °C. The crystal structure examined by X-ray powder diffraction indicates that the samples were single phase and crystallize in a rhombohedral () structure with increase the Cd content. The Rietveld refinement of the structure shows that the substitution of La by Cd in the La site changes the structure parameters, such as MnO bond length and MnOMn angles. Magnetic measurements reveal that the magnetization of these compounds exhibit a ferromagnetic to paramagnetic transition with an increasing in the Curie temperature T_C when x increases from 0.1 to 0.5. The resistivity measurements reveal that the samples exhibit a metallic to semiconductor transition and the electrical temperature transition T_P present a similar trend of T_C, with increasing the Cd content.     

Structural and magnetic characterization of martensitic Ni–Mn–Ga thin films deposited on Mo foil

Three martensitic Ni_51.4Mn_28.3Ga_20.3 thin films sputter-deposited on a Mo foil were investigated with regard to their crystal and magnetic domain structures, as well as their magnetic and magnetostrain properties. The film thicknesses, d, were 0.1, 0.4 and 1.0μm. X-ray and electron diffraction patterns revealed a tetragonal modulated martensitic phase (10 M) in the films. The surface topography and micromagnetic structure were studied by scanning probe microscopy. A maze magnetic domain structure featuring a large out-of-plane magnetization component was found in all films. The domain width, δ, depends on the film thickness as . The thickness dependencies of the saturation magnetization, saturation magnetic field and magnetic anisotropy were clarified. Beam cantilever tests on the Ni–Mn–Ga/Mo composite as a function of magnetic field showed reversible strains, which are larger than ordinary magnetostriction.     

First-principles investigations of the MMgH3 (, Na, K, Rb, Cs) series

The structural stability of the MMgH₃ (M=Li, Na, K, Rb, Cs) series has been investigated using the density-functional projector-augmented-wave method within the generalized-gradient approximation. Among the 24 structural arrangements used as inputs for the structural optimization calculations, the experimentally known frameworks are successfully reproduced, and positional and unit-cell parameters are found to be in good agreement with the experimental findings. The crystal structure of LiMgH₃ has been predicted, the most stable arrangement being the trigonal LiTaO₃-type (R3c) structure, which contains highly distorted octahedra. The formation energy for all members of the MMgH₃ series is investigated along different reaction pathways. The electronic density of states reveals that the MMgH₃ compounds are wide-band-gap insulators. Analyses of chemical bonding in terms of charge density, charge transfer, electron-localization function, Born effective charge, and Mulliken population show that these hydrides are basically saline hydrides similar to the parent alkali-/alkaline-earth mono-/di-hydrides.     

Crystal and magnetic structures of Laves phase compound NdCo2 in the temperature range between 9 and 300 K

The crystal and magnetic structures of the Laves phase compound NdCo₂ in the temperature range from 9 to 300 K are determined by Rietveld refinement technique, using high-resolution neutron powder diffraction data. The compound crystallizes in space group above the magnetic ordering temperature T_C (≈100 K), in space group I4₁/amd below T_C and in space group Imma below the tetragonal–orthorhombic structural/magnetic transitions at T_M ≈ 42 K. The assignment of the space groups to the crystal structures of NdCo₂ in different temperature ranges complies with the reported Mössbauer studies. Detailed information of the crystal and magnetic structures of NdCo₂ at different temperatures are reported.     

Factors controlling the time evolution of the corrosion potential of aluminum in alkaline solutions

The mechanism of corrosion of 99.99% purity aluminum in alkaline solutions was investigated, through detailed examination of open-circuit potential transients. These transients displayed a characteristic time dependence, in which the potential first decreased over a few seconds to a minimum of −1.7 to −1.9 V vs. Ag/AgCl, and then slowly increased over a period of hours. The value of the minimum potential of electropolished foils, along with its dependence on pH and aluminate ion concentration, indicated that it was determined by the Nernst potential for the oxidation of surface aluminum hydride (AlH₃). This finding supports the direct role of hydride in the dissolution process. The increase of anodic polarization after the minimum potential occurred in two stages, the first correlated with the buildup of surface hydride, and the second with surface enrichment of Cu and Fe impurities.     

Optical absorption and spectroscopic characteristics of Tm ions doped NaY(MoO4)2 crystal

The polarized absorption and emission spectra have been measured for the Tm³⁺ doped NaY(MoO₄)₂ crystal and spectral parameters have been estimated from the absorption data based on the Judd–Ofelt theory. The effective intensity parameters (t = 2, 4, 6) are 11.67 ×10⁻²⁰, 2.21 × 10⁻²⁰, 1.74 × 10⁻²⁰ cm², respectively. From the intensity parameters, the radiative transition probabilities, radiative lifetimes, branching ratios and the emission cross-section have been calculated. In comparison with other Tm³⁺ doped laser crystals, Tm³⁺:NaY(MoO₄)₂ crystal has potential as a promising laser crystal.     

Temperature dependent neutron powder diffraction study of the Laves phase compound TbCo2

The structure, magnetization and magnetostriction of Laves phase compound TbCo₂ are investigated by temperature dependent high resolution neutron powder diffraction. The compound crystallizes in the cubic Laves phase C15 structure above its Curie temperature T_C and exhibits a rhombohedral distortion (space group ) below T_C. By an appropriate extrapolation of the temperature factor of Co atom above T_C, the Rietveld refinement of the neutron powder diffraction data of the rhombohedral structure converges satisfactorily and reveals that the moments of Co1(3b) and Co2(9e) are almost equal. Tb moment follows well the Brillouin function. The total magnetic moment of TbCo₂ is about 5.8μ_B/f.u., the anisotropic magnetostriction constant λ₁₁₁ is about 4.6 × 10⁻³ and the volume magnetostriction ω_s is about 8.7 × 10⁻³ at 14 K.