Mössbauer study of LaNiSn and NdNiSn compounds and their deuterides

Summary LaNiSn and NdNiSn compounds and their deuterides have been studied by variable temperature ¹¹⁹Sn M?ssbauer spectroscopy. The hyperfine parameters obtained experimentally are in good agreement with those derived from first principle calculations. The enlargement of quadrupole splitting observed for LaNiSn after deuteration confirms the lower symmetry of electron density around tin atoms indicated by the calculation of partial Sn-p density of states (DOS). Magnetic ordering is observed at low temperature in deuterided NdNiSn.     

Crystal chemistry and thermodynamic properties of anisotropic Ce2Ni7H4.7 hydride

A new intermetallic deuteride Ce₂Ni₇D_4.7 with an anomalous volume expansion has been studied. Its structure was solved on the basis of in situ neutron diffraction data. Expansion proceeds along the c-axis and within the CeNi₂ slabs only. All D atoms are located inside these slabs and on the border between CeNi₂ and CeNi₅. Ordering of D atoms in the bulk of CeNi₂ is accompanied by substantial deformation of these slabs thus lowering the hexagonal symmetry to orthorhombic [space group Pmcn (No. 62); a=4.9251(3) Å, b=8.4933(4) Å, c=29.773(1) Å]. Inside the CeNi₂ layer the hydrogen sublattice is completely ordered; all D-D distances exceed 2.0 Å. Local coordination of Ni by D inside the CeNi₂ blocks is of “open”, saddle-like type. Hydrogen ordering is mainly determined by Ce-H and H-H interactions. The pressure-composition-temperature measurements yielded the following thermodynamic parameters of the formation of the hydride: ΔH=−22.4 kJ/mol_H, ΔS=−59.9 J/(K mol_H).     

Features of the Hydrogenation of Magnesium with a Ni-Graphene Coating

Russian Journal of Physical Chemistry A - The kinetics of the hydrogenation of magnesium composites with graphene-like material (GLM) on which nickel particles are deposited (Ni/GLM) and...     

Higher order symmetrizer and application to an unusual transport equation

We study a class of stationary transport equation with nonlocal low-order tems We obtain the existence and uniqueness of a solution in sobolev spaces     

Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La2Ni7-H(D)2 system

The present work is focused on studies of the influence of magnesium on the hydrogenation behaviour of the (La,Mg)₂Ni₇ alloys. Substitution of La in La₂Ni₇ by Mg to form La_1.5Mg_0.5Ni₇ preserves the initial Ce₂Ni₇ type of the hexagonal P6₃/mmc structure and leads to contraction of the unit cell. The system La_1.5Mg_0.5Ni₇-H₂ (D₂) was studied using in situ synchrotron X-ray and neutron powder diffraction in H₂/D₂ gas and pressure-composition-temperature measurements. La replacement by Mg was found to proceed in an ordered way, only within the Laves-type parts of the hybrid crystal structure, yielding formation of LaMgNi₄ slabs with statistic and equal occupation of one site by La and Mg atoms. Mg alters structural features of the hydrogenation process. Instead of a strong unilateral anisotropic expansion which takes place on hydrogenation of La₂Ni₇, the unit cell of La_1.5Mg_0.5Ni₇D_9.1 is formed by nearly equal hydrogen-induced expansions proceeding in the basal plane (Δa/a=7.37%) and along [001] (Δc/c=9.67%). In contrast with La₂Ni₇D_6.5 where only LaNi₂ layers absorb hydrogen atoms, in La_1.5Mg_0.5Ni₇D_9.1 both LaNi₅ and LaMgNi₄ layers become occupied. Nine types of sites were found to be filled by D in total, including tetrahedral (La,Mg)₂Ni₂, (La,Mg)Ni₃, Ni₄, tetragonal pyramidal La₂Ni₃ and trigonal bipyramidal (La,Mg)₃Ni₂ interstices. The hydrogen sublattice around the La/Mg site shows formation of two co-ordination spheres of D atoms: an octahedron MgD₆ and a 16-vertex polyhedron LaD₁₆ around La. The interatomic distances are in the following ranges: La-D (2.28-2.71), Mg-D (2.02-2.08), Ni-D (1.48-1.86 Å). All D-D distances exceed 1.9 Å. Thermodynamic PCT studies yielded the following values for the ΔH and ΔS of hydrogenation/decomposition; ΔH_H=−15.7±0.9 kJ (mol_H)⁻¹ and ΔS_H=−46.0±3.7 J (K mol_H)⁻¹ for H₂ absorption, and ΔH_H=16.8±0.4 kJ (mol_H)⁻¹ and ΔS_H=48.1±1.5 J (K mol_H)⁻¹ for H₂ desorption.     

Double-bridge bonding of aluminium and hydrogen in the crystal structure of gamma-AlH3

Aluminum trihydride (alane) is one of the most promising among the prospective solid hydrogen-storage materials, with a high gravimetric and volumetric density of hydrogen. In the present work, the alane, crystallizing in the gamma-AlH3 polymorphic modification, was synthesized and then structurally characterized by means of synchrotron X-ray powder diffraction. This study revealed that gamma-AlH3 crystallizes with an orthorhombic unit cell (space group Pnnm, a = 5.3806(1) A, b = 7.3555(2) A, c = 5.77509(5) A). The crystal structure of gamma-AlH3 contains two types of AlH6 octahedra as the building blocks. The Al-H bond distances in the structure vary in the range of 1.66-1.79 A. A prominent feature of the crystal structure is the formation of the bifurcated double-bridge bonds, Al-2H-Al, in addition to the normal bridge bonds, Al-H-Al. This former feature has not been previously reported for Al-containing hydrides so far. The geometry of the double-bridge bond shows formation of short Al-Al (2.606 A) and Al-H (1.68-1.70 A) bonds compared to the Al-Al distances in Al metal (2.86 A) and Al-H distances for Al atoms involved in the formation of normal bridge bonds (1.769-1.784 A). The crystal structure of gamma-AlH3 contains large cavities between the AlH6 octahedra. As a consequence, the density is 11% less than for alpha-AlH3.     

求解复杂湍流的非线性涡黏性系数模型和代数应力模型

非线性涡黏性系数模型和代数应力模型联系了线性涡黏性系数湍流模型和完整的微分 雷诺应力模型.随着它们受到日益关注,其形式也越来越多样化.本篇综述的目的是对这些模 型加以总结并比较它们之间的共同点及不同之处,指出它们与完整微分雷诺应力模型之间的 关系,以及相对于线性涡黏性系数模型而言它们在预报流场上所具有的优势.     

Hydrogen in La2MgNi9D13: the role of magnesium

Reversible hydrogen storage capacity of the La(3-x)Mg(x)Ni(9) alloys, charged by gaseous hydrogen or by electrochemical methods, reaches its maximum at composition La(2)MgNi(9). As (La,Mg)Ni(3-3.5) alloys are the materials used in advanced metal hydride electrodes in Ni-MH batteries, this raises interest in the study of the structure-properties interrelation in the system La(2)MgNi(9)-H(2) (D(2)). In the present work, this system has been investigated by use of in situ synchrotron X-ray and neutron powder diffraction in H(2)/D(2) gas and by performing pressure-composition-temperature measurements. The saturated La(2)MgNi(9)D(13.1) hydride forms via an isotropic expansion and crystallizes with a trigonal unit cell (space group R3m (No.166); a = 5.4151(1) ?; c = 26.584(2) ?; V = 675.10(6) ?(3)). The studied hybrid structure is composed of a stacking of two layers resembling existing intermetallic compounds LaNi(5) (CaCu(5) type) and LaMgNi(4) (Laves type). These are occupied by D to form LaNi(5)D(5.2) and LaMgNi(4)D(7.9). The LaNi(5)D(5.2) slab has a typical structure observed for all reported LaNi(5)-containing hybrid structures of the AB(5) + Laves phase types. However, the Laves type slab LaMgNi(4)D(7.9) is different from the characterized individual LaMgNi(4)D(4.85) hydride. This results from the filling of a greater variety of interstitial sites in the La(2)MgNi(9)D(13)/LaMgNi(4)D(7.9), including MgNi(2), Ni(4), (La/Mg)(2)Ni(2), and (La/Mg)Ni(3), in contrast with individual LaMgNi(4)D(4.85) where only La(2)MgNi(2) and Ni(4) interstitials are occupied. Despite a random distribution of La and Mg in the structure, a local hydrogen ordering takes place with H atoms favoring occupation of two Mg-surrounded sites, triangles MgNi(2) and tetrahedra LaMgNi(2). A directional bonding between Ni, Mg, and hydrogen is observed and is manifested by a formation of the NiH(4) tetrahedra and MgH(6) octahedra, which are connected to each other by sharing H vertexes to form a spatial framework.