Structural and valence changes of europium hydride induced by application of high-pressure H₂

Europium hydride EuH(x), when exposed to high-pressure H?, has been found to exhibit the following structural and valence changes: Pnma(x = 2, divalent) → P6?/mmc(x = 2, 7.2-8.7 GPa) → I4/m(x > 2, 8.7-9.7 GPa) → I4/mmm(x > 2, 9.7 GPa-,trivalent). With a trivalent character and a distorted cubic fcc structure, the I4/mmm structure is the β phase commonly observed for other rare-earth metal hydrides. Our study clearly demonstrates that EuH(x) is no longer an irregular member of the rare-earth metal hydrides.     

Superconductivity induced by longitudinal ferromagnetic fluctuations in UCoGe

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)～2.5 K and T(SC)～0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.     

Interaction between Intermetallic Compounds <Emphasis Type="Italic">R</Emphasis>Ni<Subscript>3</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Gd,Dy) and Hydrogen at Low Temperatures

The hydride phases of the intermetallic compounds GdNi₃ and DyNi₃ are synthesized at room temperature and at 273 K under a hydrogen pressure of 1–30 bar. The phase composition of the obtained samples is established using the X-ray diffraction method and the crystal-lattice parameters of the hydride phase are determined. The crystal phases are synthesized at room temperature under a hydrogen pressure of about 1 bar. At 273 K and under a pressure of 30 bar, amorphous samples are formed. The desorption of hydrogen from amorphous hydrides at 573 K leads to the formation of well-crystallized samples of the initial intermetallides. The amorphous samples are formed due to the ordering of hydrogen atoms in the metallic matrix of the hydride at low temperatures.     

Superconductivity on the border of weak itinerant ferromagnetism in UCoGe

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T(C)=3 K and a small ordered moment m(0)=0.03 micro(B). Superconductivity is observed with a resistive transition temperature T(s)=0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T(s), and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.     

Hydrogen accumulation and distribution during the saturation of a VT1-0 titanium alloy by an electrolytic method and from a gas atmosphere

The accumulation, distribution, and thermally stimulated release of hydrogen in a VT1-0 titanium alloy during electrolytic saturation and gas-phase saturation are studied. After electrolytic saturation, a 0.4-μm-thick surface layer consisting of δ hydrides with a binding energy of 108 kJ/mol forms in the alloy. The hydride dissociation after electrolytic saturation in heating occurs in the temperature range 320–370°C. After saturation from a gas atmosphere, δ hydrides with a binding energy of 102 kJ/mol form throughout the alloy volume. The dissociation of the hydrides formed during gas-phase saturation in heating occurs in the temperature range 520–530°C. A further increase in the temperature is accompanied by the transformation of titanium from the α into the β modification. At 690–720°C, the phase transformation is completed, and another hydrogen desorption peak appears in a thermally stimulated hydrogen desorption spectrum.     

Electronic structure of Ce and Sm in hydrides and the 4f collapse in YbHx

The population of the 4f, 5d, and 6s shells of rare-earth atoms in RH_x hydrides (R=Ce, Sm, Yb; x≈2–3) has been studied by the x-ray line-shift method. The population of the 5d and 6s shells of Ce and Sm atoms, and the charge on them in metals and hydrides, were determined from experiment and calculated within the Hartree-Fock-Dirac (Koopmans) model. The decrease of the charge on Ce and Sm revealed upon transition from the metal to the hydride argues unambiguously for the anionic (hydride) model. In YbH_x with x⩾2, the structural transition is accompanied by a strongly pronounced electronic transition from divalent to a noninteger-valence state . Fiz. Tverd. Tela (St. Petersburg) 40, 1393–1396 (August 1998)     

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.     

Bulk properties of the UCoGe Kondo-like system

We report on extensive experimental investigations of a single crystal of the orthorhombic uranium compound UCoGe. Bulk measurements on as-grown and annealed single crystals, recording magnetization, magnetic susceptibility, electrical resistivity, magnetoresistivity, thermopower, thermal conductivity and heat capacity data do not reproduce the previously reported coexistence of ferromagnetism with superconductivity. The latter phenomenon was only observed for the annealed sample at T _SC = 0.65 K. New observations show a crossover at around 13 K, visible in thermal and transport measurements as well as the coherent state around 50 K, signaled by a wide knee in ρ(T). Above this temperature, UCoGe exhibits a single-ion Kondo-like effect. The magnetoresistivity of the annealed single crystal increases negatively down to 4.2 K, reaching as a large value about ?27% at a field of 8 T. The latter may be interpreted in terms of fairly strong magnetic fluctuations existing in UCoGe at low temperatures.     

Hydrogen desorption in Zr3Fe H5.5

Hydrogen desorption during heat treatments of (crystalline) Zr₃FeH_5.5 was studied by Mössbauer spectroscopy (MS), X-ray (powder) diffraction (XRD) and thermal gravimetry (TG). In case of sufficiently slow heating rate all hydrogen effuses below 650°K and no phase segregation occurs. At higher temperatures retained hydrogen induce phase separation into Fe₂Zr (or more likely Fe₂ZrH_x) and zirconium hydride. The existence of a highly disordered f.c. tetragonal ternary hydride phase was also detected both by XRD (a_o=4.76 Å, c=4.96 Å) and MS .     

Magnetic spin ladder (C5H12N)2CuBr4: high-field magnetization and scaling near quantum criticality

The magnetization, M(H< or =30 T,0.7< or =T< or =300 K), of (C5H12N)2CuBr4 has been used to identify this system as an S = 1/2 Heisenberg two-leg ladder in the strong-coupling limit, J( perpendicular) = 13.3 K and J( parallel) = 3.8 K, with H(c1) = 6.6 T and H(c2) = 14.6 T. An inflection point in M(H,T = 0.7 K) at half saturation, M(s)/2, is described by an effective XXZ chain. The data exhibit universal scaling behavior in the vicinity of H(c1) and H(c2), indicating that the system is near a quantum critical point.     

Systematic effects of carbon doping on the superconducting properties of Mg(B1-xCx)(2)

The upper critical field, H(c2), of Mg(B1-xCx)(2) has been measured in order to probe the maximum magnetic field range for superconductivity that can be attained by C doping. Carbon doped MgB2 filaments were prepared, and for carbon levels below 4% the transition temperatures are depressed by about 1 K/% C and H(c2)(T=0) rises by about 5 T/% C. This means that 3.8% C substitution will depress T(c) from 39.2 to 36.2 K and raise H(c2)(T=0) from 16.0 to 32.5 T. These rises in H(c2) are accompanied by a rise in resistivity at 40 K from about 0.5 to about 10 microOmega cm.     

A statistical thermodynamic approach to sonochemical reactions

The calculation of the equilibrium constants K of the sonolysis reactions of CO2 into CO and O atom, the recombination of O atoms into O2 and the formation of H2O starting with H and O atoms, has been studied by means of statistical thermodynamic. The constants have been calculated at 300 kHz versus the pressure and the temperature according to the extreme conditions expected in a cavitation bubble, e.g. in the range from ambient temperature to 15200 K and from ambient pressure to 300 bar. The decomposition of CO2 appears to be thermodynamically favored at 15200 K and 1 bar with a constant K1=1.52 x 10(6), whereas the formation of O2 is not expected to occur (K2=1.8 x10(-8) maximum value at 15200 K and 300 bar) in comparison to the formation of water (K3=3.4 x 10(47) at 298 K and 300 bar). The most thermodynamic favorable location of each reactions is then proposed, the surrounding shell region for the thermic decomposition of CO2 and the wall of the cavitation bubble for the formation of water. Starting from a work of Henglein on the sonolysis of CO2 in water at 300 kHz, the experimental amount of CO formed (7.2 x 10(20)molecules L(-1)) is compared to the theoretical CO amount (1.4 x 10(27)molecules L(-1)) which can be produced by the sonolysis of the same starting amount CO2. With the help of the literature data, the number of cavitation bubble has been evaluated to 6.2 x 10(15) bubbles L(-1) at 300 kHz, in 15 min. This means that about 1 bubble on 1900000 is efficient for undergoing the sonolysis of CO2.     

高压下氢基高温超导体研究的新进展

富氢材料被认为是室温超导体的最佳候选体系,是物理学、材料科学等多学科的热点研究领域之一。理论和实验研究发现的新型共价氢化物H3S和笼状氢化物LaH10的超导转变温度(T_c)均超过200 K,进一步推动了对富氢化合物超导电性的探索。最近,通过高压实验合成的碳质硫氢化物在288 K的室温下实现了零电阻,让人们看到了室温超导的曙光。本文结合课题组在此领域的主要成果,介绍了3类典型富氢化合物的结构及超导特性,包括近期首次在层状氢化物中发现的具有类五角石墨烯结构的富氢超导体HfH₁₀,其超导转变温度高达213~234 K。     

Ab initio study of H and He migrations in β-phase Sc, Y, and Er hydrides

Ab initio calculations based on the density functional theory have been performed to investigate the migrations of hydrogen(H) and helium(He) atoms in β-phase scandium(Sc),yttrium(Y),and erbium(Er) hydrides with three different ratios of H to metal.The results show that the migration mechanisms of H and He atoms mainly depend on the crystal structures of hydrides,but their energy barriers are affected by the host-lattice in metal hydrides.The formation energies of octahedral-occupancy H(H oct) and tetrahedral vacancy(V tet) pairs are almost the same(about 1.2 eV).It is of interest to note that the migration barriers of H increase with increasing host-lattice atomic number.In addition,the results show that the favorable migration mechanism of He depends slightly on the V tet in the Sc hydride,but strongly on that in the Y and Er hydrides,which may account for different behaviours of initial He release from ScT2 and ErT2.     

BPR氧化减色法测定过氧化氢亨利常数

利用冷阱收集气-液平衡管出来的气相过氧化氢,以及氯化血红素(Hemin)催化下溴邻苯三酚红(bromopyrogallolred,BPR)被过氧化氢氧化减色的分光光度法检测,测定了温度依赖的过氧化氢亨利常数。在10~35℃的范围内,过氧化氢亨利常数KH(mol·L-1·atm-1)表达为lnKH=a/T-b,其中a=7269±22,b=13·26±0·08,T为温度(K)。过氧化氢溶解焓ΔH为60·43±0·18kJ·K-1·mol-1。     

Evidence for the coexistence of an anisotropic superconducting gap and nonlocal effects in the nonmagnetic superconductor LuNi2B2C

A study of the dependence of the heat capacity C(p)(alpha) on the field angle in LuNi2B2C reveals an anomalous disorder effect. For pure samples, C(p)(alpha) exhibits a fourfold variation as the field H (alpha=0). A slightly disordered sample, however, develops anomalous secondary minima along <110> for mu(0)H>1 T, leading to an eightfold pattern at 2 K and 1.5 T. The anomalous pattern is discussed in terms of coexisting superconducting gap anisotropy and nonlocal effects.     

Violation of the minimum h-h separation "Rule" for metal hydrides

Using gradient-corrected, full-potential, density-functional calculations, including structural relaxations, it is found that the metal hydrides RTInH1.333 (R=La, Ce, Pr, or Nd; T= Ni, Pd, or Pt) possess unusually short H-H separations. The most extreme value (1.454 A) ever obtained for metal hydrides occurs for LaPtInH1.333. This finding violates the empirical rule for metal hydrides, which states that the minimum H-H separation is 2 A. The paired, localized, and bosonic nature of the electron distribution at the H site are polarized towards La and In which reduces the repulsive interaction between negatively charged H atoms. Also, R-R interactions contribute to shielding of the repulsive interactions between the H atoms.     

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.     

NMR evidence for gapped spin excitations in metallic carbon nanotubes

We report on the spin dynamics of 13C isotope enriched inner walls in double-wall carbon nanotubes using 13C nuclear magnetic resonance. Contrary to expectations, we find that our data set implies that the spin-lattice relaxation time (T1) has the same temperature (T) and magnetic field (H) dependence for most of the inner-wall nanotubes detected by NMR. In the high-temperature regime (T approximately > or = 150 K), we find that the T and H dependence of 1/T1T is consistent with a 1D metallic chain. For T approximately < or = 150 K we find a significant increase in 1/T1T with decreasing T, followed by a sharp drop below approximately = 20 K. The data clearly indicate the formation of a gap in the spin excitation spectrum, where the gap value 2delta approximately = 40 K (congruent to 3.7 meV) is H independent.     

Hydrogen adsorption on palladium and palladium hydride at 1 bar

The dissociative sticking probability for H₂ on Pd films supported on sputtered Highly Ordered Pyrolytic Graphite (HOPG) has been derived from measurements of the rate of the H–D exchange reaction at 1 bar. The sticking probability for H₂, S, is higher on Pd hydride than on Pd (a factor of 1.4 at 140 °C), but the apparent desorption energy derived from S is the same on Pd and Pd hydride within the uncertainty of the experiment. Density Functional Theory (DFT) calculations for the (1 1 1) surfaces of Pd and Pd hydride show that, at a surface H coverage of a full mono layer, H binds less strongly to Pd hydride than to Pd. The activation barrier for desorption at a H coverage of one mono layer is slightly lower on Pd hydride, whereas the activation energy for adsorption is similar on Pd and Pd hydride. It is concluded that the higher sticking probability on Pd hydride is most likely caused by a slightly lower equilibrium coverage of H, which is a consequence of the lower heat of adsorption for H on Pd hydride.