Magnetic characterization of a hydrogen phase trapped inside deep dislocation cores in a hydrogen-cycled PdH x (x ≈ 4.5 × 10−4) single crystal

The magnetic characterization of Pd single crystals deformed by cycling in a hydrogen atmosphere has been performed. Based on evidence obtained from thermal desorption analysis, it is shown that the condensed hydrogen phase formed inside deep dislocation cores in PdH _x (x = H/Pd ≈ 4.5 × 10⁻⁴) is tightly bound with a Pd matrix. The activation energy of hydrogen desorption from these cores was found to be as high as e = 1.6eV/H-atom, suggesting the occurrence of a strong band overlapping between Pd and H atoms. SQUID measurements carried out in a weak magnetic field (H < 5.0 Oe) showed an anomalous diamagnetic contribution to the DC and AC magnetic susceptibilities of the PdH _x sample at T < 30 K resulting in the presence of the hydrogen phase. It is suggested that the anomalous diamagnetic response in PdH _x is caused by the presence of a hydrogen dominant phase, tightly bound with a Pd matrix inside the dislocation cores (nanotubes). The text was submitted by the authors in English.     

Electrical resistivity of PdHx: I. Residual resistivity

Results are presented for the electrical resistivity of PdH_x alloys at several fixed temperatures. The residual resistivity goes through a maximum at x = 0.73 and then drops sharply as x approaches 1. From this we conclude that the residual resistivity for 0.8 < x < 1 is due primarily to vacancies in the hydrogen lattice and that the hydrogen atoms occupy only the octahedral sites of the Pd lattice. The absolute values of the residual resistivity are in reasonable agreement with theoretical expectations.     

Comparison between the magnetic susceptibilities of Pd1−xAgx and single phase PdHn

The rapid decrease of the isothermal magnetic susceptibility χ of single phase PdH_n with n ( = atomic ratio H/Pd) above the critical temperature T_c = 564 differs from that of the magnetically similar single phase system Pd_1−xAg_x (x: silver mole fraction) at the same temperature and at equal valence electron concentrations (n = x), i.e. χ (PdH_n)−χ(Pd_1−xAg_x) <0. By use of a semiphenomenological susceptibility ansatz related to the nonideal solution behaviour of H (Ag) in Pd the susceptibility difference is interpreted as an electronic excess effect.The analysis of the steep descent of the magnetic susceptibility also applies to the Pd-rich side of PdH_n in the subcritical temperature region (so-called α-phase) and can be supported by ¹⁰⁵Pd Knight shift data at 348 K.     

Role of hydrogen in the absorption of ionizing radiation energy by a metal-hydrogen system

Ab initio calculations of the electronic structure of pure Pd, pure Ti, and PdH_x and TiH_x (x = 1, 2, 3) systems are performed within the local density approximation. It is found that the electronic subsystem of metals containing dissolved hydrogen increases their capacity to absorb the energy of electromagnetic radiation and accumulate it for a longer time than pure metals. These two factors promote the nonequilibrium migration of hydrogen atoms and their release from metals upon exposure to ionizing radiation.     

Where do the H atoms reside in PdH x systems?

We present an ab initio density-functional theory study of PdH _x systems. We evaluated the total energy of PdH _x systems with the H atoms occupying interstitial (octahedral and tetrahedral) sites of a Pd supercell, allowing for the relaxation of the coordinates and supercell dimensions. The majority of our calculations were based on supercells consisting of four Pd atoms, and up to four H atoms, covering the range from x = 0.25 to x = 1. In addition some larger calculations are reported. In order to compare the relative stability of systems at different values of x (at fixed pressure and temperature T = P = 0), we computed the enthalpy of formation ΔH _f (x) of the (non)stoichiometric systems. In the regime x = 0 → 1, the ΔH _f (x) decrease in a manner indicative of the existence of attractive interactions between the dissolved H atoms. Ideal-solution theory cannot be applied to this system. Furthermore, we find that tetrahedral occupation is favoured over octahedral occupation at high x, leading to the formation of a zincblende structure at x = 1. A preliminary vibrational analysis of normal modes has been performed. Inclusion of vibrational zero-point energies in a harmonic approximation leads us to conclude, tentatively, that the observed stability of octahedral site occupation is due to more favourable zero-point energies of the H atoms in those sites. The results indicate that a proper understanding of this system must take into account the quantum nature of the dissolved hydrogen.     

Hydrogen mobility in crystalline and amorphous Zr2PdHx

NMR measurements of proton rotating-frame relaxation times were made to deduce hydrogen diffusion parameters for crystalline and amorphous Zr₂PdH_x. Enhanced proton mobility was observed in glassy a-Zr₂PdH_2.9 relative to diffusion behavior in any crystalline Zr₂PdH_x sample. Both host crystal structure and proton site occupancy make significant contributions to the diffusion process where smaller activation energies correspond to jumps involving octahedral sites.     

Color-neutral switchable mirrors based on magnesium-titanium thin films

In an investigation of smart-window applications of switchable mirror thin films, Pd-capped magnesium-titanium thin films were prepared by dc magnetron sputtering. Their optical properties, switching durability and crystalline structures have been investigated. We show that Pd/Mg-Ti thin films with specific thicknesses are completely color-neutral in the transparent state and their optical switching properties are suitable for building and automobile window glass applications. The ternary hydrides of Mg_(1-x)Ti_x thin films with Pd overlayers are identified by in situ X-ray diffraction measurements during a hydrogen gas loading of 4%. Pd/Mg_(1-x)Ti_x thin film switchable mirrors show fast hydriding and dehydriding kinetics as compared to a Pd-capped pure Mg thin film due to the catalytic role of doped metallic Ti. PACS 75.20.En; 78.20.-e; 78.70.Ck; 42.70.-a     

Electronic states and Fermi surface properties of α-phase PdHx

Calculations based on a muffin tin model for the electronic structure of the random interstitial alloy PdH_x are presented together with the first high resolution de Haas-van Alphen data for this system in the α-phase. In the dilute limit good agreement is obtained for the concentration dependence of the areas of several extremal orbits. The behavior of the PdH_x Fermi surface is unique in that the Γ-centered electron orbits are found to grow at approximately the rate predicted by the rigid band model while the X-centered hole pockets shrink at only half the rigid band rate.     

Experimental and theoretical investigations of the Knight shift of Pd and Ag in the alloy system Ag x Pd1−x

The Knight shift of Pd in Ag _x Pd_1–x has been determined for concentrationsx0.2. In full accordance with the expectations based on the behaviour of the magnetic susceptibility, it was found that the Knight shift of Pd is rapidly reduced in magnitude by adding Ag to Pd. To allow for a detailed interpretation of this finding, we have performed Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band structure calculations for Ag _x Pd_1–x . These calculations clearly demonstrate that the decrease in spin susceptibility with increasingx is accompanied with a decrease in core polarization. In contrast to Pd, the negative Knight shift of Ag on the Pd-rich side of the system is caused by the valence band contribution, as it is demonstrated by our calculations. This is caused by an intersite effect in analogy to the transferred hyperfine field found for non-magnetic elements dissolved in a magnetic host.     

Model of instabilities in PdHx and PdDx

The instability and inhomogeneity of the PdH_x and PdD_x systems is discussed. A simple model based on the existence of more than one local electronic and or ionic configuration is proposed, and the results of this model are shown to be consistent with several important lattice properties of these systems including a difference between the Pd-H and Pd-D force constants.     

Ab initio calculation of the geometric,electronic and magnetic properties of neutral and anionic Au n Pd (n = 1–9) clusters

The ab initio method based on density functional theory at the B3PW91 level has been applied to study the geometric, electronic, and magnetic properties of neutral and anionic Au _n Pd (n?=?1–9) clusters. The results show that the most stable geometric structures adopt a three-dimensional structure for neutral Au₇Pd and Au₈Pd clusters, but for anionic clusters, no three-dimensional lowest-energy structures were obtained. The relative stabilities of neutral and anionic Au _n Pd clusters were analysed by means of the dependent relationships between the binding energies per atom, the dissociation energies, the second-order difference of energies, the HOMO–LUMO energy gaps and the cluster size n, and a local odd–even alternation phenomenon was found. Natural population analysis indicates the sequential transfer from the Pd atom to the Au _n frame in Au_1,2,3,5Pd and Au_2,3Pd^? clusters, and from the Au _n frame to the Pd atom in other clusters. Much to our surprise, irrespective of whether it is the total magnetic moment or the local magnetic moment, the magnetic moment presents an odd–even alternation phenomenon as a function of the cluster size n. The magnetic effects are mainly localized on the various atoms (Au or Pd) for different cluster size n.     

A density functional theory study of the electronic structures and magnetic properties of Fe（1-x）Cox alloy nanowires encapsulated in （10,0） carbon nanotubes

Under the generalized gradient approximation, the electronic structures and magnetic properties of Fe_(1-x)Co_x alloy nanowires encapsulated inside zigzag (10,0) carbon nanotubes (CNTs) are investigated systematically using firstprinciple density functional theory calculations. For the fully relaxed Fe_(1-x)Co_x/CNT structures, all the C atoms relax outwards, and thus the diameters of the CNTs are slightly increased. Formation energy analysis shows that the combining processes of all Fe_(1-x)Co_x/CNT systems are exothermic, and therefore the Fe_(1-x)Co_xalloy nanowires can be encapsulated into semiconducting zigzag (10,0) CNTs and form stable hybrid structures. The charges are transferred from the Fe_(1-x)Co_xnanowires to the more electronegative CNTs, and the Fe-C/Co-C bonds formed have polar covalent bond characteristics. Both the spin polarization and total magnetic moment of the Fe_(1-x)Co_x/CNT system are smaller than those of the corresponding freestanding Fe_(1-x)Co_xnanowire, and the magnetic moment of the Fe_(1-x)Co_x/CNT system decreases monotonously with increasing Co concentration, but the Fe_(1-x)Co_x/CNT systems still have a large magnetic moment, implying that they can be utilized in high-density magnetic recording devices.     

“Inorganic Proton Conductor-Hydrogenated Metal” new ionic heterostructures

Hydrogenated metal-proton conductor heterostructures are of great applied and fundamental interest in connection with the possibility of forming proton heterojunctions of PdH _x | KOH · nH₂O type. Here, original compositions, such as Pd|(NaOH + KOH)|Pd, Pd|CsHSO₄|Pd, Ti|KOH · H₂O|C, and Ti|KOH · H₂O|Ti, are synthesized and studied in the temperature range 320–430 K. After electrochemical activation a stable potential difference from 0.8 to 1.4 V (depending on physicochemical conditions) arises between the electrodes of the heterostructures. Part of the potential difference (≈0.8 V for Pd and 0.1–0.2 V for Ti) is due to proton heterojunctions, which are found to be kinetically reversible.     

Reduction of hysteresis loss in LaFe11.7Si1.3Hx hydrides with significant magnetocaloric effects

Magnetic properties and magnetocaloric effects (MCEs) have been investigated in hydrogenated LaFe_11.7 Si_1.3H _x (x=0,1.37, and 2.07) compounds. It is found that the Curie temperature, T _C, can be tuned from 192 to 338 K by adjusting the hydrogen content from 0 to 2.07. It is attractive that both thermal and magnetic hysteresis are remarkably reduced because of the weakness of the itinerant-electron metamagnetic transition after hydrogenation. The maximal hysteresis loss at T _C decreases from 33.4 to 8.8 J/kg as x increases from 0 to 2.07. For the samples with x=0,1.37, and 2.07, the maximal values of the isothermal magnetic entropy change, ΔS _M, are 20.9, 15.1, and 15.83 J/kg K for the increasing field and 20.76 J/kg K, 14.53 J/kg K and 15.61 J/kg K for the decreasing field at T _C, with efficient refrigeration capacities of 439, 330, and 304 J/kg for a field change of 0–5 T, respectively. Large reversible MCE and small hysteresis with considerable refrigeration capacity indicate the potential of LaFe_11.7Si_1.3H _x hydride as a candidate magnetic refrigerant around room temperature.     

原子层沉积技术沉积的FeOx包裹层显著提高Pd/C催化剂在甲酸分解放氢反应中的催化活性

Hydrogen generation from formic acid (FA) has received significant attention.The challenge is to obtain a highly active catalyst under mild conditions for practical applications.Here atomic layer deposition (ALD) of FeO_x was performed to deposit an ultrathin oxide coating layer to a Pd/C catalyst,therein the FeO_x coverage was precisely controlled by ALD cycles.Transmission electron microscopy and powder X-ray diffraction measurements suggest that the FeO_x coating layer improved the thermal stability of Pd nanoparticles (NPs).X-ray photoelectron spectroscopy measurement showed that deposition of FeO_x on the Pd NPs caused a positive shift of Pd3d binding energy.In the FA dehydrogenation reaction,the ultrathin FeO_x layer on the Pd/C could considerably improve the catalytic activity,and Pd/C coated with 8 cycles of FeO_x showed an optimized activity with turnover frequency being about 2 times higher than the uncoated one.The improved activities were in a volcanoshape as a function of the number of FeO_x ALD cycles,indicating the coverage of FeO_x is critical for the optimized activity.In summary,simultaneous improvements of activity and thermal stability of Pd/C catalyst by ultra-thin FeO_x overlayer suggest to be an effective way to design active catalysts for the FA dehydrogenation reaction.     

钯氢的热中子非弹性散射

在铍过滤探测器谱仪上,对含氢量为x=0.17和0.67的两种钯氢(PdH_x)样品分别测定了定域模(或光学支),观察到x=0.67的定域模峰位置相对于x=0.17的峰位置向低能移动约5毫电子伏,定性地讨论了定域模软化与超导性的关系。 关键词：     

Monte Carlo simulation of magnetic and magnetocaloric properties of binary alloy Gd1−xCx

The Potts-like model is utilized to describe an alloy Gd_1−xC_x with x=0, 0.025, 0.06, 0.09, and the magnetic and magnetocaloric properties are calculated by Monte Carlo method. The effect of the local distortion of the lattice due to adulterated C atom on the exchange interaction between Gd atoms can be considered. The spontaneous magnetization, specific heat, and magnetic susceptibility are calculated. It is found that the magnetization at low temperature decreases but phase transition temperature from ferromagnetic to paramagnetic increases, as the concentration of the C atom in the system increases. Moreover, the specific heat and the susceptibility exhibit peaks at the transition temperature. For two external magnetic field h/J=0.25 and 10.0, the magnitude of the isothermal magnetic entropy change in binary alloy is more than in pure Gd system. Furthermore, the range of temperature of half peak in the curve of the magnetic entropy change becomes wide and the refrigerant capacity increases in the alloy.     

Magnetoresistive Fe19Ni81/Tb-Co medium with an internal magnetic bias

The magnetization reversal and magnetoresistance of two-layer exchange-coupled Fe19Ni81/Tb-Co films are studied. Amorphous Tb _x Co_{100 − x} layers with 30 < x < 35 are found to have a uniaxial magnetic anisotropy and a rather high coercive force, which ensures magnetic biasing of the adjacent permalloy layers. In addition, the permalloy layers subjected to selective annealing exhibit a significant anisotropic magnetoresistance and a small magnetic hysteresis. These properties make it possible to consider the developed film structure as an effective magnetoresistive medium. This structure is used to form magnetic sensor samples that have an odd transfer function in the absence of external magnetic biasing.     

Hydrogen storage capabilities of the most stable isomers of NanBm (m+n=6) clusters

The most stable isomers of NanBm(m+n=6) clusters and their hydrogen storage properties are investigated by means of density functional theory with a 6-311+G(d) basis set. To study the hydrogen storage properties,all of the stable structures of Na n BmHx (m+n=6) clusters have been optimized. It shows that boron atoms of Na n B m are separated from the other boron atoms,and form satellite BHx (x=3,4) clusters around the centre,which attach to the system by a bridging bond of a hydrogen atom or an Na atom. Compared with the hydrogen storage capabilities,the Na3B3 has the highest hydrogen storage capacity among Na n B m clusters. The binding energies,interaction energies of hydrogen atom with Na n B m clusters and second difference in energy of Na3B3Hx clusters have been calculated. The results show that the stability of the Na n B m H x clusters present an odd-even oscillatory effect,as the number of H atoms increases.     

CoxCu1-x复相纳米线阵列的制备及其磁性的研究

利用电化学沉积方法在同一种富Co²⁺溶液Co²⁺/Cu²⁺=10∶1中,利用不同的沉积电位成功地制备了一系列不同成分(x=0.38—0.87)和复合相结构的Co_xCu_1-x纳米线阵列.发现随着纳米线中Cu含量的变化,Co_xCu_1-x纳米线的复相结构随之发生规律的变化,最终导致纳米线的磁性也随之规律的变化.随着纳米线中Cu含量的不断增加,一部分Cu与Co形成面心立方结构(fcc)的CoCu固溶体,减弱了磁晶各向异性与形状各向异性的竞争,从而提高样品的方形度;一部分Cu以fcc结构的Cu单质的形式存在于纳米线中,并随着Cu颗粒大小的不同分别起到破坏磁晶各向异性和钉扎畴壁的作用,从而增加纳米线的方形度和矫顽力.对比不同成分的样品,发现Co_xCu_1-x纳米线的方形度和矫顽力的最大值分别出现在Co₇₅Cu₂₅和Co₆₀Cu₄₀中,并且由于其特殊的复相结构致使它们的值要好于相同直径的单相结构的结果. 关键词： 纳米线 电化学沉积 磁性