Dynamics of superionic Rb3H(SeO4)2 single crystals studied by H and Rb spin-lattice relaxation

The ¹H and ⁸⁷Rb spin-lattice relaxation and spin-spin relaxation times in superionic Rb₃H(SeO₄)₂ single crystals grown by the slow evaporation method were measured over the temperature range 160-450 K. The temperature dependencies of the ¹H T₁, T_1ρ, and T₂ are measured. In the ferroelastic phase, T₁ differs from T_1ρ, which is in turn different from T₂, although these three relaxation times converge to similar values near 410 K. This transition seems to occur at temperature which is about 40 K lower than the superionic transition temperature. The observation of liquid-like values of the ¹H T₁, T_1ρ, and T₂ in the high temperature is compatible with the phase being superionic, indicating that the destruction and reconstruction of hydrogen bonds does indeed occur at high temperature. In addition, the ⁸⁷Rb T₁ and T₂ values at high temperature were similar (on the order of milliseconds), a trend that was also observed for ¹H T₁ and T₂. This behavior is expected for most hopping-type ionic conductors, and could be attributed to interactions between the mobile ions and the neighboring group ions within the crystal. The motion giving rise to this liquid-like behavior is related to the superionic motion.     

MgH_2-Nb体系解氢能力与电子结构的第一原理计算

采用第一原理赝势平面波方法,构建了一个NbH0.6/MgH2相界模型,研究了Nb合金化对MgH2解氢能力与电子结构的影响.结果显示:NbH0.6/MgH2相界的结构稳定性比MgH2相差,表明Nb合金化利于提高MgH2相的解氢能力;Nb对MgH2相解氢能力增强的主要原因在于Nb-H间电子相互作用比Mg-H间强,有利于促进NbH0.6相形核,并且α-Mg在MgH2-Nb体系的NbH0.6/MgH2相界中形核比在MgH2相中容易.     

Preparation and Characterization of MgH<Subscript>2</Subscript> Mechanocomposites with Mg<Subscript>2</Subscript>NiH<Subscript>0.3</Subscript> + Mg<Subscript>2</Subscript>NiH<Subscript>4 – δ</Subscript> Two-Phase Mixture

The thermal behavior of MgH₂ mechanocomposites with Mg₂NiH_{4 – δ} + Mg₂NiH_0.3 two-phase mixture and phase equilibria in the system Mg–Ni–H involving MgH₂ were studied by powder X-ray diffraction and TG–DSC. The graphic representation of phase equilibria involving MgH₂ and Mg₂NiH_{4 – δ} and Mg₂NiH_x (0 < x < 0.3) solid solutions in the practically important field with high magnesium content area was revised. The possibility of MgH₂ mechanocomposites with Mg₂NiH_{4 – δ} and Mg₂NiHx for use as magnesium intercalating agents and alternative intercalating agents—organomagnesium compounds were considered.     

Transport, magnetic, internal friction, and Young's modulus in the Y-doped manganites La0.9−xYxTe0.1MnO3

The resistivity, magnetization, internal friction, and Young's modulus for the polycrystalline samples La_0.9−xY_xTe_0.1MnO₃ (x=0, 0.05, 0.10 and 0.15) have been investigated. All samples have rhombohedral crystallographic structure with the space group . The Curie temperature T_C of the studied samples decreases with increasing Y-doping level. For the samples with x=0,0.05 and 0.10, the temperature dependence of the resistivity ρ(T) exhibits two metal-insulator transitions (MIT) at T_p1 (which is close to its Curie temperature T_C) and T_p2 (which is below T_p1). When the doping level to 0.15, these two MIT temperatures are suppressed and an upturn at low temperatures below T* is observed from the ρ(T) curve. A change of Young's modulus E is observed in the vicinity of T_C accompanied by a broad peak of the internal friction Q⁻¹ for all studied samples. The values of the relative Young's modulus ΔE increase with increasing Y-doping level at the low temperatures. These results are discussed in terms of the local Jahn-Teller (JT) distortion by the substitution of smaller Y³⁺ ions for larger La³⁺ ions and the increased bending of the Mn-O-Mn bond with decreasing the average ionic radius of the A-site element 〈r_A〉 and the tolerance factor t, resulting in the narrowing of the bandwidth, the decrease of the mobility of e_g electrons and the weakening of double-exchange (DE) interaction.     

Role of O?H bonding in catalytic activity of Nb2O5 during the course of dehydrogenation of MgH2

The hydrogen desorption reaction of MgH₂, MgH₂ → Mg + H₂, is accelerated by the addition of metal oxide catalysts (e.g., Nb₂O₅). From our theoretical calculation of electronic structure, it was predicted that the catalytic activities of metal oxides are related closely to the O? H interaction operating at the interface between oxide catalyst and MgH₂. In this study, the O? H vibration on the Nb₂O₅‐catalyzed MgH₂ was investigated experimentally using FTIR spectroscopy. The broad absorption band due to the O? H stretching mode was observed in the region of 2,800–3,600 cm^?1 in the FTIR spectra of the specimens when hydrogen desorption reaction was in progress. The absorbance of the band decreased monotonously with decreasing hydrogen content in the specimen during the course of dehydrogenation of MgH₂. This experimental result was in agreement with our prediction for the existence of O? H interaction in the hydrogen desorption process. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A study of the phase transitions and proton dynamics of the superprotonic conductor Cs5H3(SO4)4·0.5H2O single crystal with H and Cs nuclear magnetic resonance

The phase transitions and proton dynamics of Cs₅H₃(SO₄)₄·0.5H₂O single crystals were studied by measuring the NMR line shape, the spin-lattice relaxation time, T₁, and the spin-spin relaxation time, T₂, of the ¹H and ¹³³Cs nuclei. The “acid” protons and the “water” protons in Cs₅H₃(SO₄)₄·0.5H₂O were distinguished. The loss of water protons was observed above T_C1, whereas the content of water protons was found to recover above T_C2. Therefore, the water protons play a special role in the stability of the superprotonic phase at high temperatures. The mechanism of fast proton conduction was found to consist of hydrogen-bond proton transfer involving the breakage of the weak part of the hydrogen bond and the formation of a new hydrogen bond. Thus, these structural phase transitions probably involve significant reorientation of the SO₄ tetrahedra and dynamical disorder of the hydrogen bonds between them.     

Renewed Insight into the Promoting Mechanism of Magnesium Hydride on Ammonia Borane

Our previous study found that mechanically milling with magnesium hydride (MgH₂) could dramatically improve the dehydrogenation property of ammonia borane (AB). Meanwhile, it appears that the MgH₂ additive maintains its phase stability in the milling and subsequent heating process. In an effort to further the mechanistic understanding of the AB/MgH₂ system, we reinvestigated the property and structure evolution in the hydrogen release process of the AB/0.5MgH₂ sample. Property examination using volumetric method and synchronous thermal analyses showed that the AB/0.5 MgH₂ sample releases ～13.8 wt % hydrogen after being heated at 300 °C. This hydrogen amount is in excess of that available from AB, indicative of the participation of a faction of MgH₂ in the dehydrogenation process of AB. Structural and chemical state analyses using Fourier transformation infrared spectroscopy and solid‐state ¹¹B nuclear magnetic resonance techniques further showed that part of MgH₂ participates in the dehydrogenation process of AB from the first step, resulting in the formation of Mg? B? N? H intermediate species. The incorporation of Mg in AB is believed to be a crucial event leading to dehydrogenation property improvements, particularly for the release of the last equivalent of H₂ in AB at relatively moderate temperature. These findings have provided renewed insight into the promoting mechanism of MgH₂ on the hydrogen release from AB.     

Heat capacity of pure and doped V2O3 single crystals

Heat capacities have been measured for single crystals of V₂O₃, either pure or doped with 1 and 1.4 mole% Cr₂O₃ and Al₂O₃ over the temperature range 100–700°K. V₂O₃ undergoes a fairly sharp transition at low temperatures (～170°K) but fails to exhibit any thermal anomaly above 300°K. The thermal behavior of (M_xV_1?x)₂O₃, M = Cr, Al, is manifested by two transitions: one at low temperatures, 170–180°K for x = 0.01 and 180–190°K for x = 0.014, and the other at high temperatures. For x = 0.01, the high-temperature (HT) anomaly extended over the range 325–345°K (Cr-doped V₂O₃) and 345–365°K (Al-doped V₂O₃), respectively. The corresponding ranges for x = 0.014 were found to be 260–280°K and 270–290°K, respectively. Further, the HT anomaly was characterized by a large hysteresis (～50°K). The values of lattice heat capacity of pure and doped V₂O₃ were, however, found to be almost the same and could be empirically represented by the Debye (D)?Einstein (E) function $\text{D}\text{(}\text{580}\text{T}\text{) + 4}\text{E}\text{(}\text{θ}\text{T}\text{)}$ with θ values 430°K (T = 100–230°K) and 465°K (T > 230°K), respectively. Further, the enthalpy change ΔH associated with the HT anomaly in doped V₂O₃ (80 ≤ ΔH ≤ 510 J/mole) was 5–10 times smaller than the ΔH corresponding to the lower-temperature transition. The results cited here appear incompatible with the Mott transition model that has been invoked to explain the HT anomaly.     

Dynamics of [Zn(D2O)6] in [Zn(D2O)6][SiF6] crystal as studied by 1D, 2D spectra and spin-lattice relaxation time of H NMR

The dynamics of [Zn(D₂O)₆]²⁺ in [Zn(D₂O)₆][SiF₆] was investigated by ²H NMR one-dimensional spectra, two-dimensional exchange spectra and spin-lattice relaxation time (T₁). The lineshapes of those spectra and T₁ were dominated by the 180° flip of the water molecules and the reorientation of [Zn(D₂O)₆]²⁺ about the C₃ axis. The variation of lineshape of the one-dimensional spectrum below room temperature can be explained by only the 180° flip of the water molecules. The spectrum at room temperature showed a typical shape due to the rapid 180° flip of water molecules. The change in lineshape of the one-dimensional ²H NMR spectrum is caused by the three-site jump of [Zn(D₂O)₆]²⁺ about its C₃ axis above 333 K. Information of the reorientation of [Zn(D₂O)₆]²⁺ below 333 K could not be obtained from the one-dimensional spectrum and T₁. In this temperature range, the two-dimensional exchange spectrum was effective for analysis of molecular motion. The effects of multiple motions, the 180° flip of the water molecules and the reorientation of [Zn(D₂O)₆]²⁺ about the C₃ axis, on the lineshape of the two-dimensional exchange spectrum were studied using spectral simulation.     

Organomagnesium clusters: Structure, stability, and bonding in archetypal models

We have studied the molecular structure and the nature of the chemical bond in the monomers and tetramers of the Grignard reagent CH₃MgCl as well as MgX₂ (X = H, Cl, and CH₃) at the BP86/TZ2P level of theory. For the tetramers, we discuss the stability of three possible molecular structures of C_2h, D_2h, and T_d symmetry. The most stable structure for (MgCl₂)₄ is D_2h, the one for (MgH₂)₄ is C_2h, and that of (CH₃MgCl)₄ is T_d. The latter is 38 kcal/mol more stable with chlorines in bridge positions and methyl groups coordinated to a Mg vertex than vice versa. We find through a quantitative energy decomposition analysis (EDA) that the tetramerization energy is predominantly composed of electrostatic attraction ΔV_elstat (60% of all bonding terms ΔV_elstat + ?E_oi) although the orbital interaction ?E_oi also provides an important contribution (40%).     

Chelation of manganese(II) with quinaldic acids. An NMR relaxation study

Manganese(II) enhanced spin-lattice relaxation rates (1/T_1p) of ¹H and ¹³C nuclei in quinoline-2-carboxylic acid, 8-methoxyquinoline-2-carboxylic acid, 8-hydroxyquinoline-2-carboxylic acid, 8-aminoquinoline-2-carboxylic acid, and 6-(hydroxymethyl)pyridine-2-carboxylic acid were measured in aqueous solution at various temperatures. Relative metal-nucleus distances were calculated from the r^?6 dependence of 1/T_1p. The results indicate that the Mn²⁺ ion in the 8-methoxyquinaldic acid chelate is coordinated to the carboxyl oxygen atom and the nitrogen atom but not t the methoxyl oxygen atom.     

Ti掺杂的MgH2和Mg2NiH4的放氢性能

以TiF₃和Ti(OBu-n)₄为催化剂, 研究了Ti离子掺杂对MgH₂和Mg₂NiH₄放氢性能的影响. 结果表明, 未掺杂的MgH₂起始放氢温度为420 ℃, 掺杂TiF₃和Ti(OBu-n)₄后分别降低到360和410 ℃; Mg₂NiH₄在掺杂TiF₃后放氢温度由230 ℃降低到220 ℃, 而掺杂Ti(OBu-n)₄后没有变化. 可见无论对MgH₂或Mg₂NiH₄, 在降低放氢温度方面TiF₃都明显优于Ti(OBu-n)₄. 另外, 研究还发现, TiF₃掺杂对MgH₂放氢动力学有显著的提高, 但对Mg₂NiH₄没有明显的提高. 结合XRD和FTIR的测试分析, 我们认为: 催化作用很大程度上取决于氢化物自身的晶体结构和催化剂的电子结构; 降低氢化物放氢温度和提高动力学性能的原因是催化剂与氢化物之间的相互作用削弱了氢化物中Mg—H或Ni—H键, 使得活泼的H…H原子对容易形成, 从而有利于H₂的释出.     

Density Functional Investigations of Electronic Structure and Dehydrogenation Reactions of Al‐ and Si‐Substituted Magnesium Hydride

The effect on the hydrogen storage attributes of magnesium hydride (MgH₂) of the substitution of Mg by varying fractions of Al and Si is investigated by an ab initio plane‐wave pseuodopotential method based on density functional theory. Three supercells, namely, 2×2×2, 3×1×1 and 5×1×1 are used for generating configurations with varying amounts (fractions x=0.0625, 0.1, and 0.167) of impurities. The analyses of band structure and density of states (DOS) show that, when a Mg atom is replaced by Al, the band gap vanishes as the extra electron occupies the conduction band minimum. In the case of Si‐substitution, additional states are generated within the band gap of pure MgH₂—significantly reducing the gap in the process. The reduced band gaps cause the Mg? H bond to become more susceptible to dissociation. For all the fractions, the calculated reaction energies for the stepwise removal of H₂ molecules from Al‐ and Si‐substituted MgH₂ are much lower than for H₂ removal from pure MgH₂. The reduced stability is also reflected in the comparatively smaller heats of formation (ΔH_f) of the substituted MgH₂ systems. Si causes greater destabilization of MgH₂ than Al for each x. For fractions x=0.167 of Al, x=0.1, 0.167 of Si (FCC) and x=0.0625, 0.1 of Si (diamond), ΔH_f is much less than that of MgH₂ substituted by a fraction x=0.2 of Ti (Y. Song, Z. X. Guo, R. Yang, Mat. Sc. & Eng. A 2004 , 365, 73). Hence, we suggest the use of Al or Si instead of Ti as an agent for decreasing the dehydrogenation reaction and energy, consequently, the dehydrogenation temperature of MgH₂, thereby improving its potential as a hydrogen storage material.     

Improvement in Hydrogen Desorption from β‐ and γ‐MgH2 upon Transition‐Metal Doping

A thorough study of the structural, electronic, and hydrogen‐desorption properties of β‐ and γ‐MgH₂ phases substituted by selected transition metals (TMs) is performed through first‐principles calculations based on density functional theory (DFT). The TMs considered herein include Sc, V, Fe, Co, Ni, Cu, Y, Zr, and Nb, which substitute for Mg at a doping concentration of 3.125 % in both the hydrides. This insertion of TMs causes a variation in the cell volumes of β‐ and γ‐MgH₂. The majority of the TM dopants decrease the lattice constants, with Ni resulting in the largest reduction. From the formation‐energy calculations, it is predicted that except for Cu and Ni, the mixing of all the selected TM dopants with the MgH₂ phases is exothermic. The selected TMs also influence the stability of both β‐ and γ‐MgH₂ and cause destabilization by weakening the Mg?H bonds. Our results show that doping with certain TMs can facilitate desorption of hydrogen from β‐ and γ‐MgH₂ at much lower temperatures than from their pure forms. The hydrogen adsorption strengths are also studied by density‐of‐states analysis.     

First-Principles Study of Pd Single-Atom Catalysis to Hydrogen Desorption Reactions on MgH2(110) Surface

MgH₂ is a promising and popular hydrogen storage material. In this work, the hydrogen desorption reactions of a single Pd atom adsorbed MgH₂(110) surface are investigated by using first-principles density functional theory calculations. We find that a single Pd atom adsorbed on the MgH₂(110) surface can signi cantly lower the energy barrier of the hydrogen desorption reactions from 1.802 eV for pure MgH₂(110) surface to 1.154 eV for Pd adsorbed MgH₂(110) surface, indicating a strong Pd single-atom catalytic effect on the hydrogen desorption reactions. Furthermore, the Pd single-atom catalysis significantly reduces the hydrogen desorption temperature from 573 K to 367 K, which makes the hydrogen desorption reactions occur more easily and quickly on the MgH₂(110) surface. We also discuss the microscopic process of the hydrogen desorption reactions through the reverse process of hydrogen spillover mechanism on the MgH₂(110) surface. This study shows that Pd/MgH₂ thin films can be used as good hydrogen storage materials in future experiments.     

Influence of the preparation history of α-Fe2O3 on its reactivity for hydrogen reduction

TG experiments on the hydrogen reduction of α-Fe₂O₃ were carried out to elucidate the influence of the preparation history of the oxide on its reactivity. α-Fe₂O₃ samples were prepared by the thermal decomposition of seven iron salts in a stream of oxygen, air or nitrogen at temperatures of 500–1200°C for 1 h. Thirteen metal ions such as Cu²⁺, Ni²⁺, etc. were used as doping agents. The reactivity of the oxide was indicated by the initial reduction temperature (T_i. α-Fe₂O₃ prepared at lower temperatures showed lower T_i values and the reduction proceeded stepwise (Fe₂O₃ → Fe₃O₄ → Fe). T_i values increased with the rise in the preparation temperature of the oxide. The oxides prepared at higher temperatures showed that two reduction steps (Fe₂O₃ → Fe₃O₄ → Fe) proceed simultaneously. the preparation in oxygen gave higher T_i than that in air or nitrogen. The doping by metal ions, except Ti⁴⁺, lowered the T_i of α-Fe₂O₃. The Cu²⁺ ion showed the lowest T_i, while Ti⁴⁺ showed the highest T_i and the inhibition effect.The reduction process was expressed by two equations; Avrami—Erofeev's equation for α-Fe₂O₃ → Fe₃O₄ and Mampel's equation for Fe₃O₄ → Fe.     

Spin-lattice relaxations study of water mobility in natural natrolite

The mobility of water molecules in natural natrolite (Na₂Al₂Si₃O₁₀?2H₂O) is investigated by the ¹H NMR method. The spin-lattice relaxation times in the laboratory and rotating frames (T₁ and T_1ρ) are measured as a function of the temperature for a polycrystalline sample. From experimental T₁ data it follows that at T > 286 K the diffusion of water molecules along channels parallel to the c axis is observed. From experimental T_1ρ data it follows that at T > 250 K the diffusion of water molecules in transversal channels of natrolite is also observed. At a low temperature (T < 250 K) the dipolar interaction with paramagnetic impurities (presumably Fe³⁺ ions) becomes significant as a relaxation mechanism of ¹H nuclei.     

Raman scattering study of CaB6 and YbB6

Phonon spectra of CaB₆ and RB₆ (R=Yb, Ce, and Pr) have been investigated by Raman scattering. We found clear spectral difference between divalent cation hexaboride and trivalent one. E_g mode shows the doublet spectra for only the divalent crystals of CaB₆ and YbB₆. The doublet spectra are understood by the two-dimensional charge distribution on B₆ without lattice distortion. In addition, the scattered intensities of the phonons change at around the ferromagnetic Curie temperature for YbB₆ and at T?600 K for CaB₆. These are the characteristic temperatures due to the change of the electronic system.     

An Analytical Bond Order Potential for Mg−H Systems

Magnesium-based materials provide some of the highest capacities for solid-state hydrogen storage. However, efforts to improve their performance rely on a comprehensive understanding of thermodynamic and kinetic limitations at various stages of (de)hydrogenation. Part of the complexity arises from the fact that unlike interstitial metal hydrides that retain the same crystal structures of the underlying metals, MgH₂ and other magnesium-based hydrides typically undergo dehydrogenation reactions that are coupled to a structural phase transformation. As a first step towards enabling molecular dynamics studies of thermodynamics, kinetics, and (de)hydrogenation mechanisms of Mg-based solid-state hydrogen storage materials with changing crystal structures, we have developed an analytical bond order potential for Mg−H systems. We demonstrate that our potential accurately reproduces property trends of a variety of elemental and compound configurations with different coordinations, including small clusters and bulk lattices. More importantly, we show that our potential captures the relevant (de)hydrogenation chemical reactions 2H (gas)→H₂ (gas) and 2H (gas)+Mg (hcp)→MgH₂ (rutile) within molecular dynamics simulations. This verifies that our potential correctly prescribes the lowest Gibbs free energies to the equilibrium H₂ and MgH₂ phases as compared to other configurations. It also indicates that our molecular dynamics methods can directly reveal atomic processes of (de)hydrogenation of the Mg−H systems.     

Molecular reorientation of liquid benzene. Anisotropic Raman scattering of the CH and CD stretching modes in the deuterated molecules

The anisotropic Raman spectra of the CH and CD stretching modes in seven deuterated benzenes of D_6h, D_3h, D_2h and C_2h symmetry are reported. The reorientational linewidths are interpreted within the model of anisotropic rotational diffusion. The data are consistent with NMR relaxation studies. The study covers the temperature range between T/T_c = 0.49 and T/T_c = 0.97.