A57Fe Mössbauer study of the high pressure hydride phases of Fe and Co

Hydrides of iron and cobalt prepared at pressures between 4.0 and 9.5 GPa were studied by⁵⁷Fe Mössbauer spectroscopy at 4.2 K. Iron hydride was found to be nearly stoichiometric FeH. The two iron sites in its dhep lattice have hyperfine fields of 33.8 and 28.8 T. Practically the same results were found for the deuteride. In hep ε-CoH_x, the hyperfine fields decrease with hydrogen content by about 6% betweenx=0 andx=0.5. In all studied hydrides the electron densities at the⁵⁷Fe nuclei are smaller than in the pure metals.     

X-ray investigation of the mean square thermal displacements in VDx, NbHx and TaHx interstitial alloys

X-Ray measurements of the integrated Bragg intensities from V, Nb and Ta single crystals as a function of hydrogen (D) concentration and temperature have been carried out. Two different methods were applied: (i) the usual angular dispersive method using MoK_α1 characteristic radiation; and (ii) the energy dispersive method using the white spectrum of the Mo tube and an intrinsic Ge detector. From the results the thermal Debye-Waller factor for the three metals and its change with hydrogen concentration are determined. The mean square of the thermal displacements (u²) decreases with hydrogen (D) concentration. For NbH_x(D_x) this agrees well with (u²) values as determined from measured phonon density of states spectra. The results are also given in terms of a Debye temperature depending on H(D) concentration.     

Experimental study of the electronic and lattice contributions to the VO2 transition

The contributions of electronic and crystallographic components to the semiconductor → metal transition in VO₂ have been estimated from resistivity, E.P.R., and calorimetric measurements of the electronic and thermodynamic properties of Ga_xV_?xO₂, where 0 < x < 0.0130. E.P.R. and resistivity measurements indicate a decrease in the metallic character of the high-temperature R phase with increasing x, and calorimetric measurements of the energetics of the transition show a decrease in the enthalpy and entropy of the transition with increasing levels of doping. This concomitant decrease in enthalpy and metallic character with increasing x implies a strong contribution of the electronic entropy to the transition. An extrapolation of the combined electronic and calorimetric data for Ga_xV_?xO₂ to pure VO₂ suggest that the electronic entropy comprises about 60% of the total entropy of transition in VO₂.     

Theory of superconductivity for transition-metal alloys

A theory of superconductivity for transition-metal alloys is presented by using the coherent potential approximation. Expressions for the superconducting order parameter and transition temperature of transition-metal alloys A_xB_1?x in terms of the parameters characterizing the pure metals are given.     

Theory of band gap bowing of disordered substitutional II–VI and III–V semiconductor alloys

For a wide class of technologically relevant compound III?CV and II?CVI semiconductor materials AC and BC mixed crystals (alloys) of the type A _x B_1?x C can be realized. As the electronic properties like the bulk band gap vary continuously with x, any band gap in between that of the pure AC and BC systems can be obtained by choosing the appropriate concentration x, granted that the respective ratio is miscible and thermodynamically stable. In most cases the band gap does not vary linearly with x, but a pronounced bowing behavior as a function of the concentration is observed. In this paper we show that the electronic properties of such A _x B_1?x C semiconductors and, in particular, the band gap bowing can well be described and understood starting from empirical tight-binding models for the pure AC and BC systems. The electronic properties of the A _x B_1?x C system can be described by choosing the tight-binding parameters of the AC or BC system with probabilities x and 1 ? x, respectively. We demonstrate this by exact diagonalization of finite but large supercells and by means of calculations within the established coherent potential approximation (CPA) We apply this treatment to the II?CVI system Cd _x Zn_1?x Se, to the III?CV system In _x Ga_1?x As and to the III-nitride system Ga _x Al_1?x N.     

Electronic conductivity in La1−xBaxF3−x crystals

The electronic conductivities of solid solutions La_1?xBa_xF_3?x (0 ? x ? 0.0952) were investigated up to 533 K using the Hebb-Wagner dc polarization technique. The electrochemical cell (-)La|La_1?xBa_xF_3?x|Pt(+) has been utilized with Pt as the ion-blocking electrode. Under steady-state conditions the La_1?xBa_xF_3?x solid solutions exhibit electronic conductivity. The electronic conductivity vanishes in pure LaF₃. Together with ac conductivity measurements it appears that the ionic transference number for La_1?xBa_xF_3?x (0 ? x ? 0.0952) is essentially unity over the temperature range studied.     

Evidence for fcc-like short range order in Fe-Ni-B metallic glasses

Soft X-ray appearance potential spectra have been measured for iron and nickel in (Fe_0.5Ni_0.5)_100?xB_x metallic glasses with x = 16, 18, 20, 22, 24, 26. For x = 16 both iron and nickel spectra exhibit structural features characteristic for the electronic structure of fcc transition metals. It is concluded that in the amorphous state Ni and Fe are arranged in fcc-like geometry beyond nearest neighbours.     

Ionic conduction in BaxCe0.8Er0.2O3−α

Ba_xCe_0.8Er_0.2O_3−α (x=0.98–1.03) solid electrolytes were synthesized by high temperature solid-state reaction. X-ray diffraction (XRD) patterns showed that the specimens were a single perovskite-type orthorhombic phase. The conduction properties of the specimens were studied electrochemically in the temperature range of 600–1000 °C. The influence of nonstoichiometry in the specimens with x≠1 on conduction properties was investigated and compared with that in the specimen with x=1. These three specimens showed a good protonic conduction under wet hydrogen, a mixed conduction of oxide-ion and electronic hole under dry higher oxygen partial pressure, and a mixed conduction of proton, oxide-ion and electronic hole under wet higher oxygen partial pressure. Both of the protonic and oxide-ionic conductivities increased with increasing barium content in the specimens under wet hydrogen and dry air, respectively.     

Analysis of hydrogen adsorption in the bulk and on the surface of magnesium nanoparticles

The stability of magnesium hydride (MgH _x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ～ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH _x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH₂. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH₂ are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH₂. To attain high diffusion rates, the structures of Mg₅₃Sc and Mg₅₃Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg₅₃ScH₁₀₈ near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH₂, while the frequency of jumps in Mg₅₃TiH₁₀₈ near Ti atoms is higher by approximately a factor of 2.5 × 10⁶. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles.     

Magnetic properties of titanium vanadium hydrides

Measurements have been made of the magnetic susceptibility X in ternary titanium-vanadium hydrides Ti_1?yV_yH_x in the range 0<y< 0.5 and with a hydrogen to metal ratio from 1.6 to 1.92; they cover the temperature interval from 80 to 440 K. The x^?T curves of some of the ternary hydrides possess a maximum similar to that seen in binary hydrides. Its occurrence depends upon both hydrogen and vanadium concentration. Comparison with the lattice constants shows that this dependence is related to the transition from the cubic γ-phase to the tetragonal δ-phase. The x^?T curves of those samples showing the phase transition were extrapolated from the cubic phase down to lower temperatures. Conclusions were drawn from the measured and extrapolated susceptibility values, as to the influence of both hydrogen and vanadium concentrations on the electronic structure of these hydrides. The dependence of the magnetic susceptibility on the vanadium concentration shows that, in the cubic phase, the fermi energy lies on the increasing side of a maximum in the density of states curve. The degree of occupation of the conduction band does not depend on the hydrogen concentration. The rigid band model cannot be used in the hydrides studied here to account for the effects of varying their hydrogen concentration.     

High-temperature electronic transport properties of (YCa)BaCo4O7 compounds

Y_1−xCa_xBaCo₄O₇ (0.0≤x≤1.0) samples were prepared by the solid-state reaction method and their high-temperature electronic transport properties were investigated in nitrogen and oxygen respectively. Phase structure of Y_1−xCa_xBaCo₄O₇ transforms from hexagonal symmetry for x ≤0.6 samples to orthorhombic symmetry for x≥0.8 samples. In nitrogen, Y_1−xCa_xBaCo₄O₇ samples evolve three kinds of electronic transport behaviors with the increase of Ca content: thermal activation conduction, small polaron hopping conduction, and a possible mixed conduction. Ca doping increases the hole concentration and thus decreases Seebeck coefficients. In oxygen, the temperature dependence of electrical resistivity and Seebeck coefficients of Y_1−xCa_xBaCo₄O₇ samples displays similar change to their respective thermogravimetric curve, showing their electronic transport behavior under the control of their oxygen adsoption/desorption process.     

Radiation-stimulated modification of C60 films on Si-oxide surfaces

The electronic structure of C₆₀/SiO_x/Si(1 0 0) interface was studied by photoemission (valence-bands, C 1s, and Si 2p core levels) and near-edge X-ray absorption fine structure (C 1s threshold) spectroscopies. It was concluded that the SiO_x/Si surface is non-reactive with respect to the interaction with C₆₀. The exposure of the C₆₀/SiO_x/Si system under non-monochromatic synchrotron radiation causes modification of the electronic structure of this system. It is explained by polymerization of the C₆₀ molecules and arising a strong ionic-like interaction of the polymerized C₆₀ with the SiO_x surface. Annealing of this system up to temperatures of 550–625 °C leads to complete desorption of the C₆₀ molecules from the non-irradiated sample areas while the modified by radiation fullerenes remain attached to the substrate.     

Surface and bulk phenomena in the process of hydride formation in thin films of rare earth metals: Comparison between terbium and europium

Work function changes ΔΦ caused by H₂ interaction with thin terbium and europium films deposited on glass under UHV conditions were correlated with hydrogen uptake and electrical resistance R, measured in situ. For both metals, the course of ΔΦ(H/Me) at room temperature confirms the change in charge-transfer direction on the surface during hydride formation. As a result, the hydrogen adsorbate's nature is changed from positively polarized (precursor state) to negatively polarized adspecies. The hydrogen behavior is significantly different at low temperature due to the formation of the surface ordered low-temperature phase (α′) with positively polarized hydrogen adspecies. This phase, strongly inhibiting penetration of hydrogen into the bulk, is stable up to 100 K for terbium and 160 K for europium. Increasing temperature above these values resulted in additional large absorption of hydrogen. Moreover, differences in the course of R(H/Me) are clearly noticeable between the investigated metals. The resistance of thin TbH_x (x ∼ 3) films in our experiments did not exceed 1 kΩ, however transition of thin metallic europium film into EuH_y (y ∼ 2) increased the resistance up to 10 MΩ. This dissimilarity in electrical behavior can be explained by the coexistence of two factors which are different for the two metals in question: the phase relation of hydrides and the response of thin film to stress generated during hydride formation.     

Molecular dynamics simulations on local structure and diffusion in liquid TixAl1−x alloys

The microscopic structure and dynamics of liquid Ti_xAl_1-x alloys together with pure liquid Ti and Al metals were investigated by means of molecular dynamics simulations. This work gives the structural properties, including pair-correlation function, bond-angle distribution function, HA and Voronoi indices, and their composition dependence. The dynamical properties have also been studied. The calculated pair-correlation function, bond-angle distribution function, and HA and Voronoi indices suggest that the stoichiometric composition Ti_0.75Al_0.25 exhibits a different local structure order compared with other concentrations, which help us understand the appearance of the minimum diffusion coefficient at this composition. These results indicate that the mobility of atoms strongly depends on their atomic local structure.     

Laser synthesis of nitrides on the surface of refractory metals immersed in liquid nitrogen

In this paper we describe an approach for the formation of composite layers on the surface of refractory metals. We show that laser radiation on refractory metals (Ti, V, Zr, Mo, Hf, Ta, and W) immersed in liquid nitrogen can provide a chemical synthesis of nitride phases on the surface of metals. The metals were subjected to pulsed laser radiation with a wavelength of 1.06 μm. The power density ranged from 10⁴ to 10⁹ W cm⁻². The synthesis of nitrides began with the formation of Me_xN_y (x > y) phases with low contents of nitrogen. When the melting point was reached at the metal surface, the quantity of MeN phases increased sharply. Study of the melting zone showed that it contained a non-uniform distribution of nitride phases. The quantity of nitrides was a maximum on the surface and decreased with the increase of the depth of melting zone. Due to the high-cooling rates, titanium nitride crystallized in the form of columns. Maximum microhardness in the Ti surface layer was up to 20,000 MPa.     

Electrical resistivity of microinhomogeneous PdMnxFe1−x alloys

The electronic band-structure calculations of the PdFe ferromagnet and the PdMn antiferromagnet performed in this work permit one to conclude that the specific features of the electrical resistivity observed in the ternary PdMn_xFe_1−x alloy system [the deviation from the Nordheim-Kurnakov rule ρ₀(x)∼x(1−x), which is accompanied by a high maximum of residual resistivity (not typical of metals) ρ ₀ ^m ∼220 μΩ cm at x _C∼0.8 and a negative temperature resistivity coefficient in the interval 0.5≤x≤1] are due to the microinhomogeneous (multiphase) state of the alloys and a variation in the band-gap parameter d spectrum caused by antiferromagnetic ordering of a PdMn-type phase. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 2, 2002, pp. 193–197. Original Russian Text Copyright ? 2002 by Kourov, Korotin, Volkova.     

Neutron diffraction study of phase transitions in highly concentrated hydrogen solid solutions ZrV2Dx (4 < x < 5)

Structural phase transitions in highly concentrated hydrogen solid solutions ZrV₂D_x (4 < x < 5) were studied in the wide temperature range 1.5–310 K. Phase transitions associated with hydrogen ordering in the lattice were found to occur in the concentration range 4.0 < x < 4.5. Contrary to the previously studied low-concentrated solid solutions (with x < 4), ordering of hydrogen atoms in this case is accompanied by a variation in their coordination during the formation of a superstructure. It is established that this effect is due to the short-range interaction between hydrogen atoms. A new type of hydride superstructure with stoichiometric composition ZrV₂D₅ was found. Solid solutions with x > 4.5 were established to decompose at low temperatures. The phase diagram is constructed.     

Electronic structure of GaAs1−xNx under pressure

The electronic band structures of GaAs_1−xN_x for x=0.009, 0.016, 0.031 and 0.062 are calculated ab initio using a supercell approach in connection with the full-potential linear muffin-tin orbital method. Corrections for the ‘LDA gap errors’ are made by adding external potentials which are adjusted to yield correct gaps in pure GaAs. Even small amounts of nitrogen modify significantly the conduction bands, which become strongly non-parabolic. The effective mass in the lowest conduction band thus exhibits strong k-vector dependence. Calculated variations of gaps and effective masses with x and externally applied pressure are presented and compared to a variety of experimental data. There are significant error bars on our results due to the use of the supercell approach. These are estimated by examining the effects of varying the geometrical arrangement of the N-atoms substituting As. However, the calculations show that the electron mass for x>0.009 is much larger than that of pure GaAs, and that it decreases with x.     

Structural and magnetic changes in hydrogenated TiFe1−xNix alloys

A series of crystalline B2 and amorphous TiFe_1−xNi_x samples with different compositions and hydrogen loading ratios was characterized by XRD, VSM, TG, mass-spectrometry, and ¹H NMR in order to investigate possible correlations between the sample magnetization and hydrogen content. In crystalline samples, a reversible second-order phase transition from α-phase (B2 with soluble hydrogen) through a mixture of α- and ß-phase (B19 TiFe_1−xNi_xH_y) into a pure ß-phase was found. An increase of magnetization upon hydrogen loading was observed, depending on the concentration of iron in the samples. No unambiguous correlation between the hydrogen content and the saturation magnetization was found. The saturation magnetization of as-prepared amorphous samples was found to be significantly higher than in as-prepared crystalline ones. For both types of samples, magnetization remained high after dehydrogenation as well. Thermal desorption mass-spectra was found to strongly depend on composition, whereas structure, charging pressure, and H-content did not have tremendous effect. By ¹H NMR, enhanced relaxation due to strong magnetic moments was observed; however, by TEM the existence of Fe-clusters in hydrogenated samples could not be verified.     

Pseudopotential in metals and alloys

In this communication, the pseudopotential investigation of, the various properties of non-transition metals and alloys, is discussed. Various one parametric model pseudopotentials, derived from well known spherical functions s_l(x), are employed in the calculations. Many recurrence relations of the s_l(x) function have been described. The effects of exchange and correlation on conduction electrons are also considered separately by using different dielectric screenings in various properties. The ion-ion interaction, force constants, phonon spectrum, temperature coefficient of Knight shift and electronic transport coefficients of certain metals and alloys are evaluated. The results are compared with available experimental values. Generally good agreement is achieved. The screening charge density of certain metals in low and high density region are also determined.