Energetics and electronic properties of Mg7TMH16 (TM=Sc, Ti, V, Y, Zr, Nb): An ab initio study

First-principles calculations have been performed on the face-centered cubic (FCC) magnesium-transition metal (TM) hydrides Mg₇TMH₁₆ (TM=Sc, Ti, V, Y, Zr, Nb). The cohesive energies are calculated to analyze the stability, and the obtained enthalpies of formation for hydrides Mg₇TMH₁₆ have been used to investigate the possible pathways of formation reaction. The calculated enthalpy changes show that the decomposition temperatures of Mg₇TMH₁₆ are lower than that of MgH₂. The electronic densities of states reveal that all the hydrides studied here exhibit metallic characteristics. The bonding nature of Mg₇TMH₁₆ is investigated, showing stronger covalent bonding between TM and H than between Mg and H.     

金属氢化物力学性能的第一性原理研究

采用第一性原理方法详细研究了氟化钙结构的多种金属氢化物MH₂ (M= La, Nd, Gd, Tb, Y, Dy, Ho, Er, Lu, Sc, Ti, Zr, Hf)的力学性质(弹性常数、体弹模量、剪切模量、杨氏模量).计算结果表明, MH₂ (M= La, Nd, Gd, Tb, Y, Dy, Ho, Er, Lu, Sc)在低温下具有稳定的氟化钙结构,其抵抗体积形变, 切应变和拉伸(或压缩)形变的能力从LaH₂, NdH₂, GdH₂, TbH₂, YH₂, DyH₂, HoH₂, ErH₂, LuH₂到ScH₂逐次递增, 而MH₂ (M= Ti, Zr, Hf)在低温下的氟化钙结构不稳定.通过对两种稳定的氢化物(TbH₂, ErH₂) 和两种不稳定的氢化物(TiH₂, HfH₂)的电子态密度以及差分电荷密度进行对比, 发现它们的稳定性与金属和氢之间的相互作用有密切关系.     

Magnetic properties and structure of hydrogen-amorphized intermetallic compounds RFe2Hx (R=Y, Gd, Tb, Dy, Ho, Er)

The magnetic properties and structure of hydrogen-amorphized RFe₂H_x compounds (R=Y, Gd, Tb, Dy, Ho, Er) have been studied. It is shown that amorphization of the RFe₂H_x hydrides results in an increase of Fe-Fe, and a decrease of R-Fe exchange interaction energy compared to their crystalline hydride counterparts. The magnetic structure of amorphous RFe₂H_x compounds, with the exception of those with R=Y and Gd is apparently noncollinear ferrimagnetic, as in the crystalline hydrides. A model of the heterophase state of amorphous RFe₂H_x hydrides based on x-ray diffraction and magnetic data is proposed. Fiz. Tverd. Tela (St. Petersburg) 39, 908–912 (May 1997)     

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.     

Ab initio study of formation, migration and binding properties of helium-vacancy clusters in aluminum

Ab initio calculations based on density functional theory have been performed to study the dissolution and migration of helium, and the stability of small helium-vacancy clusters He_nV_m (n, m=0-4) in aluminum. The results indicate that the octahedral configuration is more stable than the tetrahedral. Interstitial helium atoms are predicted to have attractive interactions and jump between two octahedral sites via an intermediate tetrahedral site with low migration energy. The binding energies of an interstitial He atom and an isolated vacancy to a He_nV_m cluster are also obtained from the calculated formation energies of the clusters. We find that the di- and tri-vacancy clusters are not stable, but He atoms can increase the stability of vacancy clusters.     

A 1H NMR study of the effects of Ni additions on the behavior of hydrogen in β-vanadium hydride

The behavior of hydrogen in the hydrides of vanadium metal containing small amounts of nickel was studied by ¹H NMR spectroscopy. FT spectra and spin-spin and spin-lattice relaxation times for VH_0.77, V_0.95Ni_0.05H_0.73, and V_0.90Ni_0.10H_0.65 were measured at temperatures between 77 and 400 K. On the addition of nickel the number of hydrogen atoms on O₂₂ sites decreases and the superstructure of hydrogen is altered. Different effects of nickel on hydrogen diffusion are observed above and below about 200 K, and, therefore, the mechanism of hydrogen diffusion is assumed to change at this temperature.     

Defect annealing in the hexagonal close packed heavy rare earth metals Gd,Tb, Dy,Ho, Er,Tm and Lu after electron irradiation at low temperatures

The recovery spectra of the heavy rare earths Gd, Tb, Dy, Ho, Er and Tm were determined after irradiation with electrons at low temperatures. In the cases of Er and Tm, strong thermal cycling phenomena were observed and attributed to hydrogen atoms changing configurations and interacting with the magnetic structure of the metal. After elimination of these effects in Er and Tm, all the spectra—together with those of the earlier treated Lu—had rather similar form: several close-pair peaks followed by a broad complex substage resembling the stage I _D in f.c.c metals and attributed to long-range interstitial migration. A correlation is found between the temperature of the maximum of this substage (normalized to the melting temperature) and the c/a-ratio of the metals.     

Nuclear quadrupole interactions of 111In/Cd solute atoms in a series of rare-earth palladium alloys

Nuclear quadrupole interactions were measured at ¹¹¹In/Cd probe atoms in rare-earth palladium phases RPd₃ having the L1₂ structure using the technique of perturbed angular correlation of gamma rays (PAC). Measurements were made for pairs of samples having compositions of the Pd-poorer and Pd-rich boundaries of the RPd₃ phase fields, typically 75 and 78 at.% Pd. Two signals were detected in most phases, corresponding to probe atoms on cubic R- and non-cubic Pd-sublattices. Site preferences of parent In-probe atoms were characterized by site-fractions of probes on the R- and Pd-sublattices. For all Pd-rich samples, probes exclusively occupied the R-sublattice, consistent with a heuristic rule that solute atoms tend to occupy the sublattice of an element in which there is a deficiency. For Pd-poorer alloys with R = Tb, Er, Yb, Lu, Y and Sc, probes exclusively occupied the Pd-sublattice. For Pd-poorer alloys with R = Ce, Pr, Nd, Sm and Eu, probes occupied both sublattices, with site fractions varying as a function of temperature. In contrast, probes only occupied the R-sublattice in Pd-poorer LaPd₃. Jump frequencies on the Pd-sublattice of daughter Cd-probes were determined from nuclear relaxation caused by fluctuating electric field gradients. Activation enthalpies for diffusional jumps were determined from temperature dependences and found to increase linearly as the lattice parameter decreases along the series Pr, Nd, Eu and Sm. Jump frequencies are believed to have been even higher in CePd₃ than in PrPd₃, but were too low to be detectable in Tb, Er, Yb, Lu, Y and Sc palladides. A correlation between site preferences and jump frequencies is noted and discussed. This paper provides a complete account of measurements of jump frequencies of Cd-probe atoms and comparisons with similar measurements made on other series of L1₂ phases.     

Structural and magnetic properties of hydrides R3Fe29−xVxHy (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic investigation of crystallographic and intrinsic magnetic properties of the hydrides R₃Fe_29−xV_xH_y (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed in this work. The lattice constants , and c and the unit cell volume of R₃Fe_29−xV_xH_y decrease with increasing rare-earth atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Hydrogenation results in regular anisotropic expansions along the a-, b-, and c-axes in this series of hydrides. Abnormal crystallographic and magnetic properties of Ce₃Fe_27.5V_1.5H_6.5, like Ce₃Fe_27.5V_1.5, suggest that the Ce ion is non-triply ionized. Hydrogenation leads to the increase in both Curie temperature for all the compounds and in the saturation magnetization at 4.2 K and RT for R₃Fe_29−xV_x with R=Y, Ce, Nd, Sm, Gd, and Dy, except for Tb. Hydrogenation also leads to a decrease in the anisotropy field at 4.2 K and RT for R₃Fe_29−xV_x with R=Y, Ce, Nd, Gd, Tb, and Dy, except for Sm. The Ce₃Fe_27.5V_1.5 and Gd₃Fe_28.4V_0.6 show the larger storage of hydrogen with y=6.5 and 6.9 in these hydrides.     

Mg-TM-H (TM=Sc,Ti, V,Y, Zr,Nb)晶体形成能力和电子性能

基于密度泛函理论的第一性原理的Vienna Ab initio Simulation Package (VASP)软件系统研究了Mg₇TMH₁₆ (TM=Sc, Ti, V, Y, Zr, Nb)的晶体结构和电子性能. 分析了结合能和形成焓,计算得到的结合能显示Mg₇TiH₁₆和Mg₇NbH₁₆的稳定性最强,形成焓的变化表明Mg₇TMH₁₆具有比MgH₂更低的分解温度,电子态密度显示Mg₇TMH₁₆表现出明显的金属特性. 关键词： 第一原理 7TMH₁₆ (TM=Sc,Ti,V,Y,Zr,Nb)')" href="#">Mg₇TMH₁₆ (TM=Sc,Ti,V,Y,Zr,Nb) 结合能 形成焓     

FeAl(B2) 合金La, Ac, Sc 和 Y 元素微合金化的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法, 研究了稀土元素(La, Ac, Sc 和 Y) 微合金化对FeAl (B2) 有序金属间化合物合金晶体结构、 弹性和电子性能的影响. 计算结果表明: 稀土元素Y 易于取代Fe位, 而Sc, La和Ac易于取代Al位, 其中Ac元素的加入使晶格点阵发生最大的变形. 弹性性能的计算表明La, Ac, Sc 和 Y 元素的加入可以改善FeAl (B2) 的塑性, 其中Fe₇Al₈Sc具有最好的塑性和硬度. 稀土元素对合金性能的影响, 主要是由于稀土原子的加入改变了Fe和Al电子之间的杂化作用. 计算结果与已有的试验结果和理论结果相一致.     

First-principles investigation of V2CSe2 MXene as a potential anode material for non-lithium metal ion batteries

The electrochemical performances of the V₂CSe₂ MXene as anode materials for Na, K, Mg, Ca, and Al-ion batteries have been systematically investigated by the first-principles method. The adsorption energies of metal atoms show that Na, K, and Ca atoms can effectively adsorb on the V₂CSe₂, except for Mg and Al atoms. The large diffusion constants for Na, K, and Ca atoms calculated by the diffusion energy barriers (0.098 eV for Na, 0.066 eV for Ca, and 0.24 eV for Ca) indicate the high mobility on the V₂CSe₂ surface. Significantly, the maximum theoretical capacities of V₂CSe₂ reach up to 394.12 mA h/g for Na and Ca ions. Furthermore, the low average open-circuit voltage (OCV) (0.150 V for Na, 0.175 V for K, and 0.072 V for Ca) indicates the V₂CSe₂ is a suitable anode material. These results provide fundamental guidance for the V₂CSe₂ monolayer as anode materials of non-lithium metal-ion batteries.     

Two-dimensional MTe2 (M = Co,Fe, Mn,Sc, Ti) transition metal tellurides as sodium ion battery anode materials: Density functional theory calculations

We performed density functional theory calculations to probe sodium adsorption and diffusion properties on two-dimensional (2D) MTe₂ (M = Co, Fe, Mn, Sc, Ti) first-row transition metal tellurides, and gauge their potentials as anode materials for sodium-ion batteries (NIBs). In this work, we found that all considered MTe₂ possess strong sodium adsorption properties and excellent diffusion kinetics. Moreover, sodium atoms prefer to bind on sites that are farther apart rather than on nearby sites, implying that (1) the sodium clustering is not favored and (2) the large adsorption energies are essentially due to the sodium-MTe₂ interaction. We further adopted ab initio random structure search to compute probable stable sodium adsorption configurations, to obtain more accurate capacities and open circuit voltages. The calculated capacities and open circuit voltage are reasonable, and are suitable for anode applications. Our results show that in general, 2D MTe₂ sheets have suitable sodium adsorption energies and diffusion barriers, and could be applied as sodium ion battery anode materials.     

Three-body interaction between an atom and a monolayer film

An atom (B) situated near a plane of atoms (A) experiences an interaction potential energy V = (V₂ + V₃ + … The leading term is the sum over the monlayer of 2-body interactions. We compute here the 3-body correction V₃. Numerical results are presented for the case of noble gases, H and CH₄. For H or He near a Xe layer, V₃ reduces the well depth by ~ 10%.     

Effect of metal catalyst on the mechanism of hydrogen spillover in three-dimensional covalent-organic frameworks

Hydrogen spillover mechanism of metal-supported covalent-organic frameworks COF-105 is investigated by means of the density functional theory, and the effects of metal catalysts M_4(Pt_4, Pd_4, and Ni_4) on the whole spillover process are systematically analyzed. These three metal catalysts exhibit several similar phenomena:(i) they prefer to deposit on the tetra(_4-dihydroxyborylphenyl) silane(TBPS) cluster with surface-contacted configuration;(ii) only the H atoms at the bridge site can migrate to 2,3,6,7,10,11-hexahydroxy triphenylene(HHTP) and TBPS surfaces, and the migration process is an endothermic reaction and not stable;(iii) the introduction of M_4 catalyst can greatly reduce the diffusion energy barrier of H atoms, which makes it easier for the H atoms to diffuse on the substrate surface. Differently, all of the H2 molecules spontaneously dissociate into H atoms onto Pt_4 and Pd_4clusters. However, the adsorbed H2 molecules on Ni_4 cluster show two types of adsorption states: one activated state with stretched H–H bond length of 0.88 ?A via the Kubas interaction and five dissociated states with separated hydrogen atoms. Among all the M_4 catalysts, the orders of the binding energy of M_4 deposited on the substrate and average chemisorption energy per H2 molecule are Pt_4Ni_4Pd_4. On the contrary, the orders of the migration and diffusion barriers of H atoms are Pt_4Ni_4Pd_4, which indicates that Pt_4 is the most promising catalyst for the hydrogen spillover with the lowest migration and diffusion energy barriers. However, the migration of H atoms from Pt_4 toward the substrate is still endothermic. Thus direct migration of H atom from metal catalyst toward the substrate is thermodynamically unfavorable.     

Excitation functions for 44Sc, 46Sc, and 47Sc radionuclides produced in the interaction of 45Sc with deuterons and 6He

Experimental excitation functions are presented for ⁴⁵Sc(d, p)⁴⁶Sc, ⁴⁵Sc(d, t)⁴⁴Sc, ⁴⁵Sc(⁶He, ⁵He*)⁴⁶Sc and ⁴⁵Sc(6He, α)⁴⁷Sc reactions at projectile energies near the Coulomb barrier. The obtained excitation functions for reactions ⁴⁵Sc(d, p)⁴⁶Sc and ⁴⁵Sc(⁶He, ⁵He*)⁴⁶Sc have similar behavior and have a maxima near the Coulomb barriers of these reactions. The compilation of the available experimental data, obtained at deuteron- and ⁶He-energies near the Coulomb barrier, showed that the values of the cross sections at the maxima of the excitation functions obtained in (d, p) reactions and the reactions for one-neutron pickup from the ⁶He projectiles have a different Z-dependence.     

A novel double-pulse laser plasma spectroscopic technique for H analysis in metal samples utilizing transversely excited atmospheric-pressure CO2 laser-induced metastable He atoms

The analysis of hydrogen in a metal sample (zircaloy-4), which is usually difficult to perform using conventional laser-induced breakdown spectroscopy (LIBS) techniques, has been achieved using a double-pulse technique under He gas at atmospheric pressure. In this technique, a transversely excited atmospheric-pressure (TEA) CO₂ laser (1.5 J, 200 ns) was focused onto the metal surface to induce a strong He gas plasma whilst simultaneously focussing a Nd-doped yttrium aluminum garnet (Nd:YAG) laser (120 mJ, 8 ns), synchronized with the TEA CO2 laser, onto the metal to ablate atoms into the resulting He gas plasma. The emission spectrum obtained shows a narrow H linewidth with low background intensity and long lifetime emission, thereby indicating that excitation takes place via metastable He atoms. The H emission from H₂O can be suppressed by a careful pretreatment involving heating the sample in a vacuum chamber.     

First principle study of structural,elastic and electronic properties of APt3 (A=Mg,Sc, Y and Zr)

We report results obtained from first principle calculations on APt₃ compounds with A=Mg, Sc, Y and Zr. Our results of the lattice parameter a are in good agreement with experimental data, with deviations less than 0.8%. Single crystal elastic constants are calculated, then polycrystalline elastic moduli (bulk, shear and Young moduli, Poisson ration, anisotropy factor) are presented. Based on Debye model, Debye temperature ?_D is calculated from the sound velocities V_l, V_t and V_m. Band structure results show that the studied compounds are electrical conductors, the conduction mechanism is assured by Pt-d electrons. Different hybridisation states are observed between Pt-d and A-d orbitals. The study of the charge density distribution and the population analysis shows the coexistence of ionic, covalent and metallic bonds.     

Electronic structure and stability of complex hydrides Mg<Subscript>2</Subscript><Emphasis Type="Italic">M</Emphasis>H<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">M</Emphasis> = Fe,Co)

This paper reports on the results of theoretical investigations carried out for the hydrides Mg₂FeH₆ and Mg₂CoH₅ and the mixed hydride Mg₂(FeH₆)_0.5(CoH₅)_0.5 in terms of the full-potential linearized augmented plane wave (FLAPW) method. It has been shown that the partial substitution of the Co atoms for the Fe atoms leads to a slight increase in the stability of the hydride, but, at the same time, makes it impossible to increase the stability of the alloy. The high stability of the hydrides under investigation has been explained by the strong bonding between atoms of the transition metal and hydrogen.     

H,O,N,He原子掺杂Al原子的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法,对Al中分别加入H,O,N和He原子后的晶体状态进行了研究.通过晶体结构和形成能分析比较了杂质原子占据不同位置的难易程度及对晶体稳定性的影响,并从态密度、电荷密度和电荷布居的角度,分析了其电子结构.结果表明:H、O和N原子占据金属Al的四面体间隙最稳定,而He原子主要占据金属Al的八面体间隙. O和N原子与Al原子具有强烈的共价作用,H原子与Al原子存在共价作用但相对较弱,而He原子与Al原子的相互作用以范德华力为主.进一步揭示了四种原子在金属Al中不同行为的电子机制.