Isomer shifts in Zr2Fe and Zr3Fe

The⁵⁷Fe isomer shifts in the crystalline compounds Zr₂Fe and Zr₃Fe are calculated. We use the standard first-principles linear-muffin-tin-orbital formalism in the atomic sphere approximation (LMTO-ASA) to obtain the electronic structure of the compounds. The electronic structure of pure Fe was also calculated with the same formalism, where the local spin density approximation for the exchange correlation term is used. We found that the calculated isomer shift values are in good agreement with the experimental values for Zr₂Fe and Zr₃Fe found in the literature.     

Investigation of Zr3Fe hydride phases

Detailed Mössbauer studies of a series of hydrogenated Zr₃Fe samples have demonstrated the existence of several hydride phases with hydrogen concentration up to Zr₃FeH₈. Despite the overall similarity of the spectra, assessment of their parameters has identified separate groupings, each linked with a particular phase. At low temperatures magnetic ordering is observed for Zr₃FeH_5.3 and Zr₃FeH_6.9.     

Hydrogen absorption effects in the Zr(Fe0.5Cr0.5)2 compound

Zr(Fe_0.5Cr_0.5)₂ Laves phase samples have been made to absorb different hydrogen amounts up to 3.3 H/f.u. XRD showed all ZrFeCrH _y samples to have the C14 structure and, for 0 < y < 2.6, to be composed of a H-poor α-phase and a H-rich β-phase having different c/a ratios. Both phases could be distinguished in the Mössbauer spectra. The β-phase fraction was determined both from X-ray and Mössbauer analysis, with excellent agreement. β-phase lattice constants and hyperfine parameters vs. y exhibited a small jump at y ≈ 1.75, suggesting a phase transition. Mössbauer spectra of ZrFeCrH_3.3 in the 78–350 K range revealed that, for Fe at the 6h site, quadrupole splitting and linewidth increased for decreasing temperature. These effects were attributed to H atomic jump diffusion, and an activation energy of ≈70 meV was estimated.     

Hydrogen desorption processes in Li-Mg-N-H systems

Li-Mg-N-H systems composed of Mg(NH₂)₂ and LiH with various ratios can reversibly store a large amount of hydrogen under the temperature condition above 150 °C. These composites with 3:6, 3:8 and 3:12 ratio of Mg(NH₂)₂ and LiH have been independently reported by four groups as promising candidates of high performance hydrogen storage materials possessing the reversibility and the high capacity. In any cases, an interaction between NH₃ and LiH plays an important role for the progress of hydrogen desorbing and absorbing reactions. For the hydrogen desorption process, the NH₃ molecule generated from Mg(NH₂)₂ reacts with LiH, forming LiNH₂ and H₂. Especially, under an equilibrium condition, in situ diffraction results indicated that the single phase of LiNH₂·MgNH (LiMgN₂H₃) could be generated other than the separated two phases. As a next step, the NH₃ molecule generated from LiNH₂ reacts with LiH, desorbing H₂. As a result, the dehydrogenated phase was evaluated to be Li₂NH·MgNH (Li₂MgN₂H₂) or separated two phases, in which the final phase should depend on the experimental conditions. Thus, if the amount of LiH is enough to react with NH₃, the hydrogen desorption processes are described by the NH₃ generation from the corresponding amides and the imide.     

The effect of a nonmagnetic Zr layer on the magnetic and magneto-optical properties of Fe/Zr and Fe/Zr/Fe thin-film systems

The magnetic and magneto-optical properties of nanocrystalline Fe/Zr and Fe/Zr/Fe thin-film systems have been studied using the magneto-optical method. The strong effect of Zr layer thickness t _Zr on the magnetic properties of Fe/Zr samples was discovered. It was found that the value of the saturation field of the Fe/Zr/Fe systems oscillates as a function of t _Zr, which is explained by the oscillating character of the exchange interaction between ferromagnetic layers via a Zr spacer with the change in t _Zr. It was established that the values of the transverse Kerr effect depend on the thicknesses of both magnetic and nonmagnetic layers.     

Hydrogen desorption effect on cathodoluminescence of ZnO

The effects of the low-energy electron beam on ZnO single crystals were investigated by cathodoluminescence (CL), Time-of-Flight Electron Stimulated Desorption (TOF-ESD) and Temperature Programmed Desorption (TPD). Under e-beam irradiation, the ultraviolet (UV) emission decreases exponentially for O-face, while it increases and then decreases for Zn-face. Meanwhile, a large desorption of H⁺ is observed by TOF-ESD for both O- and Zn-faces. On the other hand, an increase of H in the irradiated volume is observed after stopping the e-beam irradiation. Similar H₂ desorption is found by TPD for both faces. These data suggest that the decrease of the UV intensity is related to H desorption.     

Photorefractive and optical scattering properties of Zr:Fe:LiNbO3 crystals

The LiNbO₃ crystal co-doped with ZrO₄ and Fe₂O₃ has been grown with [Li]/[Nb]=0.85 and 1.20, respectively, by the Czochralski method in air atmosphere. The incident exposure energy flux threshold for the light-induced scattering was characterized to investigate the scattering properties of the crystals. Applying the results of the incident exposure energy flux threshold effect, the photorefractive properties at different laser wavelengths (473 nm and 532 nm) were also measured by using the typical two-wave coupling experiments. The results show that Zr:Fe:LiNbO₃ crystal has a larger refractive index change, higher recording sensitivity and larger two-wave coupling gain coefficient at 473 nm wavelength than those obtained at 532 nm wavelength under the same experimental conditions. Moreover, the photorefractive properties decrease with the increasing [Li]/[Nb] ratios. The material of Zr:Fe:LiNbO₃ crystal is a promising candidate for blue photorefractive holographic recording.     

Investigation of optical photorefractive properties of Zr:Fe:LiNbO3 crystals

A series of Zr:Fe:LiNbO₃ crystals with various levels of ZrO₂ doping were grown by Czochraski technique. The optical damage resistance and photorefractive properties were deeply explored. The results showed that the ability optical damage resistance increased remarkably when the concentration of ZrO₂ is over threshold concentration, but which is lower than that of traditional damage resistant additive MgO. While, the holographic storage properties can be greatly enhanced by proper level of ZrO₂ doping in Fe:LiNbO₃. In terms of ions' site occupation model, the photo-damage resistant ability enhancement and the change of the photorefractive properties were discussed.     

Hydrogen absorption in Zr(AlxB1−x)2 (BFe,Co) laves phase compounds

The hydrogen absorption capacity of the systems Zr(Al_xFe_1?x)₂ and Zr(Al_xCo_1?x)₂ (0? x ?1) was measured at hydrogen pressure of 70 atm and room temperature and at 40 atm and liquid nitrogen temperature. The two systems present very interesting and unexpected results.A dramatic rise in the hydrogen capacity occurs for small x values similar to previous results for the systems Zr(A_xB_1?x)₂ (A  V, Cr, Mn; B  Fe, Co; 0?x?1). The maximum hydrogen content in both systems is achieved for x ≈ $\text{2}\text{12}$ at 40 atm and 80 K. Further increase of the Al content leads, however, to a steep decrease in the hydrogen capacity. This general behaviour is well described by a phenomenological model, recently proposed by us, and thus supporting the importance of short-range neighbouring effects for the hydrogen absorption capacity. The influence of Al on the hydrogen sorption properties in different intermetallic compounds is discussed.     

Solid state amorphization reaction in Fe−Zr multilayers

The solid state reaction of thin multilayered Fe?Zr films has been studied by means of Conversion Electron Mössbauer Spectroscopy and Rutherford Backscattering. The results for the samples with the overall composition Fe_0.67Zr_0.33, Fe_0.50Zr_0.50 and Fe_0.33Zr_0.67 reveal the formation of an amorphous phase. The growth of this phase and the crystallization of the intermetallic compound FeZr₃ for the rich zirconium sample are discussed.     

Hydrogen dynamics in Ce2Fe17H5: inelastic and quasielastic neutron scattering studies

The vibrational spectrum of hydrogen and the parameters of H jump motion in the rhombohedral Th(2)Zn(17)-type compound Ce(2)Fe(17)H(5) have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying interstitial Ce(2)Fe(2) sites participate in the fast localized jump motion over the hexagons formed by these tetrahedral sites. The H jump rate τ(-1) of this localized motion is found to change from 3.9 × 10(9) s(-1) at T = 140 K to 4.9 × 10(11) s(-1) at T = 350 K, and the temperature dependence of τ(-1) in the range 140-350 K is well described by the Arrhenius law with the activation energy of 103±3 meV. Our results suggest that the hydrogen jump rate in Th(2)Zn(17)-type compounds strongly increases with decreasing nearest-neighbor distance between the tetrahedral sites within the hexagons. Since each such hexagon in Ce(2)Fe(17)H(5) is populated by two hydrogen atoms, the jump motions of H atoms on the same hexagon should be correlated.     

Hydrogen associative desorption from Ru(1010)

The associative desorption of hydrogen from the Ru(10 0) surface has been studied on the atomic level by means of density-functional calculations with various exchange-correlation functionals and kinetic Monte Carlo simulations. The simulations reproduce forming structures of the layers and main features of TPD spectra for a wide range of hydrogen coverages. In particular, it has been shown that the decrease of binding energies with coverage due to lateral repulsion is correlated with the appearance of low-temperature peaks in the spectra.     

Hydrogen induced change from superconducting to magnetically ordered state in Th7Fe3

The Pauli paramagnet Th₇Fe₃ becomes a superconductor at temperatures below 1.86 K.¹ This compound readily hydrogenates to Th₇Fe₃H₃₀, giving one of the densest hydrogen media known. Magnetization measurements on the hydride show that it is magnetically ordered. It has a Curie temperature above 300 K and a saturated moment measured at 4.2 K of 1.4 μ_B/Fe. The differing behavior in the host metal and the hydride is ascribed, in the latter, to competition between H and Fe for the electrons provided by Th. In the host metal the Fe d-band is filled by electron transfer and the Fe moment is quenched. In the hydride some of the electrons provided by Th are absorbed by H, leaving vacancies in the Fe d-band. These, in turn, give rise to the magnetic order. The present instance in which a superconductor is converted into a ferromagnet appears to be unique as regards the effects of hydrogenation on the properties of metallic systems.     

Hydrogen as origin of thermal fixing in LiNbO3: Fe

Protons are identified as the mobile ions, which neutralize the electronic space charge fields during thermal fixing of holograms in LiNbO₃:Fe. The protons form OH^? centers and can be observed via vibrational absorption bands. In crystals with reduced OH^? content holograms cannot be fixed thermally. In crystals containing fixed patterns the OH^? concentration shows a spatial variation corresponding to the electric charge distribution.     

Ion-beam-mixing induced amorphization of Fe/Zr multilayers

The Ar-ion-beam-mixing of the Fe/Zr multilayers is studied by conversion electron Mössbauer spectroscopy. The dependence of the amorphization process on ion dose is studied in detail for two sample thicknesses with an Fe to Zr ratio of 1 and modulation wavelength of 20 and 60 nm. Experimental results are compared with the predictions of the ballistic cascade and thermal spike models of mixing.     

Exchange-Driven Magnetic Anomalies in Fe–Zr–B-Based Nanocomposites

Hyperfine Interactions - The oxidation of Fe(II)–Fe(III) hydroxycarbonate green rust GR(CO3 2?) in the presence of phosphate ions known as corrosion inhibitor is monitored by...     

Enhancement of blue holographic properties in Zr4+-doped near stoichiometric Fe:LiNbO3 crystals

The near stoichiometric LiNbO₃ crystals co-doped with ZrO₂ and Fe₂O₃ have been grown from a Li-rich melt (Li/Nb=1.38, atomic ratio) by the Czochralski method in air atmosphere at the first time. The OH^? absorption and UV–vis absorption spectra were characterized to investigate the defect structure of the crystals. The appearances of the 3479 cm^?1 absorption peak and 358 nm absorption edge manifest that the composition of the grown crystal is close to the stoichiometric ratio. The blue holographic properties were also measured by the two-wave coupling experiments. As a result, in the near stoichiometric Zr:Fe:LiNbO₃ crystals, photorefractive response speed, recording sensitivity, and two-wave coupling gain coefficient are significantly enhanced. Meanwhile, the high saturation diffraction efficiency is still maintained. These findings prove that the material of near stoichiometric Zr:Fe:LiNbO₃ crystals are a promising candidate for blue photorefractive holographic recording.     

Hydrogen desorption from tantalum with segregated oxide surface layers

The combination of thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES) was used to examine the correlation between elementary processes on an oxidized tantalum surface and the acceleration of the dehydriding rates at increasing temperatures. We present the TDS of TaD_x foils (foil thickness ~ 25μm) for various initial concentrations (0.038 ⩽ x ⩽ 0.108; α-phase), various heating rates and for high and ultra high vacuum conditions. Activation energies for desorption were derived from the TDS spectra on the assumption that the rate determining process is the recombination of hydrogen atoms to hydrogen molecules at the surface. We find two different activation energies for the low and for the high temperature region. The AES measurements show that the corresponding change in the desorption process is correlated with the dissolution of the segregated oxide surface layer.     

Hydrogen adsorption and desorption in carbon nanotube systems and its mechanisms

The hydrogen physisorption properties in single-walled carbon nanotube (SWNT) based materials were characterized. The SWNTs were highly purified and three useful pores for hydrogen physisorption were activated. Hydrogen was physisorbed in intra-tube pores at room temperature and the capacity was estimated to be about 0.3–0.4 wt.% at room temperature. The adsorption capacity can be explained by the Langmuir model. The intra-tube pores have large adsorption potential and this induces hydrogen physisorption at comparatively higher temperatures. This fact indicates the importance of fabricating sub-nanometer ordered pores for this phenomena. PACS 51.30.+i; 51.90.+r; 81.05.Tp; 81.07.De     

Amorphization of the intermetallic compounds Co2Zr and Fe2Zr under mechanical grinding

The crystalline intermetallic compounds Co₂Zr and Fe₂Zr were produced in the stoichiometric composition and milled in a planetary ball mill for different milling periods. The samples were investigated in respect to the question if a crystal-to-glass transition occurs due to the milling process. Three different experimental methods were used for this study: X-ray diffraction, Mößbauer spectroscopy, and measurements of the specific heat capacityc _p . The intermetallic compound Fe₂Zr is very suitable for this study since it is ferromagnetic at room temperature. Thus it shows characteristic features in the Mößbauer spectrum and in the measurement of the specific heat capacityc _p . The investigation shows that the intermetallic compounds Co₂Zr and Fe₂Zr undergo a crystal-to-glass transition under mechanical grinding but the X-ray diffraction patterns show that the transformation is not complete. Even after long milling periods always an amount of a crystalline phase is present in the milled samples. In comparison the mechanically ground samples show the same properties as mechanically alloyed powder mixtures of the two elements of the same chemical composition. A probable explanation for the development of an amorphous phase by mechanical grinding of the crystalline compounds Co₂Zr and Fe₂Zr is the accumulation of internal strain in the crystalline grains. Another possible explanation, the addition of iron impurities to the crystalline compounds due to the wear debris of the milling equipment, seems to be improbable since the intermetallic phases Co₂Zr and Fe₂Zr show extended existance ranges in the equilibrium phase diagrams and hence are stable in respect to a variation in the composition.