The ammonia-hydrogen system under pressure

Binary mixtures of hydrogen and ammonia were compressed in diamond anvil cells to 15 GPa at room temperature over a range of compositions. The phase behavior was characterized using optical microscopy, Raman spectroscopy, and synchrotron X-ray diffraction. Below 1.2 GPa we observed two-phase coexistence between liquid ammonia and fluid hydrogen phases with limited solubility of hydrogen within the ammonia-rich phase. Complete immiscibility was observed subsequent to the freezing of ammonia phase III at 1.2 GPa, although hydrogen may become metastably trapped within the disordered face-centered-cubic lattice upon rapid solidification. For all compositions studied, the phase III to phase IV transition of ammonia occurred at ~3.8 GPa and hydrogen solidified at ~5.5 GPa, transition pressures equivalent to those observed for the pure components. A P-x phase diagram for the NH(3)-H(2) system is proposed on the basis of these observations with implications for planetary ices, molecular compound formation, and possible hydrogen storage materials.     

Rb3ReH10, ein neues Hydrid des siebenwertigen Rheniums – Hochdrucksynthese und Kristallstruktur

Rb₃ReH₁₀, a New Rhenium(VII) Hydride – High Pressure Synthesis and Crystal Structure The ternary hydride Rb₃ReH₁₀ can be synthesised by reacting rubidium hydride with rhenium in a hydrogen atmosphere under a pressure of above 4000 bar in a temperature range between 700 and 870 K. X-ray investigations on powder samples and elastic neutron diffraction experiments on the deuterated compound led to the crystal structure. According to the formula [ReD₉]DRb₃ the atomic arrangement of the room temperature modification corresponds to that of the perowskite structure type. The coordination polyhedron of the hydrogen atoms that surround each rhenium atom can be described crystallographically as a statistical occupation of two 24-fold positions with hydrogen. In the orthorhombic low-temperature modification the deuterium ligands are arranged in ordered positions. They form monocapped square antiprisms. Magnetic susceptibility measurements revealed Rb₃ReD₁₀ to be diamagnetic.     

Structural changes of filled ice Ic hydrogen hydrate under low temperatures and high pressures from 5 to 50 GPa

Low-temperature and high-pressure experiments were performed on the filled ice Ic structure of hydrogen hydrate at previously unexplored conditions of 5-50 GPa and 30-300 K using diamond anvil cells and a helium-refrigeration cryostat. In situ x-ray diffractometry revealed that the cubic filled ice Ic structure transformed to tetragonal at low temperatures and high pressures; the axis ratio of the tetragonal phase changed depending on the pressure and temperature. These results were consistent with theoretical predictions performed via first principle calculations. The tetragonal phase was determined to be stable above 20 GPa at 300 K, above 15 GPa at 200 K, and above 10 GPa at 100 K. Further changes in the lattice parameters were observed from about 45-50 GPa throughout the temperature region examined, which suggests the transformation to another high-pressure phase above 50 GPa. In our previous x-ray study that was performed up to 80 GPa at room temperature, a similar transformation was observed above 50 GPa. In this study, the observed change in the lattice parameters corresponds to the beginning of that transformation. The reasons for the transformation to the tetragonal structure are briefly discussed: the tetragonal structure might be induced due to changes in the vibrational or rotational modes of the hydrogen molecules under low temperature and high pressure.     

Room-temperature structures of solid hydrogen at high pressures

By employing first-principles metadynamics simulations, we explore the 300 K structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa, we find the ambient-pressure disordered hexagonal close-packed (hcp) phase transited into an insulating partially ordered hcp phase (po-hcp), a mixture of ordered graphene-like H(2) layers and the other layers of weakly coupled, disordered H(2) molecules. Within this phase, hydrogen remains in paired states with creation of shorter intra-molecular bonds, which are responsible for the very high experimental Raman peak above 4000 cm(-1). At 275 GPa, our simulations predicted a transformation from po-hcp into the ordered molecular metallic Cmca phase (4 molecules∕cell) that was previously proposed to be stable only above 400 GPa. Gibbs free energy calculations at 300 K confirmed the energetic stabilities of the po-hcp and metallic Cmca phases over all known structures at 220-242 GPa and >242 GPa, respectively. Our simulations highlighted the major role played by temperature in tuning the phase stabilities and provided theoretical support for claimed metallization of solid hydrogen below 300 GPa at 300 K.     

Gaseous products of dimethylamine borane oxidation in chemically catalyzed deposition of nickel-rhenium-boron coatings

The gaseous products of the oxidation of hydride hydrogen of the dimethylamine borane (DMAB) (CH₃)₂NH · BH₃ reducer used for depositing Ni-Re-B coatings and hydrolysis of DMAB on them were studied by mass spectrometry of the isotope composition of the gas. The oxidation level of hydride hydrogen of the DMAB reducer was found to depend on the catalytic activity of the Ni-Re-B alloy, on which heterogeneous hydrolysis takes place. For Ni-Re-B alloys with low rhenium concentrations (0?C13 at %), the heterogeneous hydrolysis of DMAB proceeds with hydride hydrogen oxidation to the atomic state, as it does in the deposition of the Ni-B alloy. In contrast, at high rhenium concentrations (40?C46 at %), the oxidation proceeds to the proton H⁺, leading to an antibatic dependence of the hydrolysis and alloy reduction rates. An analysis of the partial rates of the process and isotope composition of the evolved gas revealed two different mechanisms of the chemical-catalytic reduction of Ni-Re-B alloys at concentrations of potassium perrhenate of 0?C4 and 4?C16 mM in solution.     

First principles simulation of a superionic phase of hydrogen fluoride (HF) at high pressures and temperatures

We have conducted ab initio molecular dynamics simulations of hydrogen fluoride (HF) at pressures of 5-66 GPa along the 900 K isotherm. We predict a superionic phase at 33 GPa, where the fluorine atoms are fixed in a bcc lattice while the hydrogen atoms diffuse rapidly with a diffusion constant between 2 x 10(-5) and 5 x 10(-5)cm(2)s. We find that a transformation from asymmetric to symmetric hydrogen bonding occurs in HF at 66 GPa and 900 K. With superionic HF we have discovered a model system where symmetric hydrogen bonding occurs at experimentally achievable conditions. Given previous results on superionic H(2)O [Goldman et al., Phys. Rev. Lett. 94, 217801 (2005)] and NH(3) [Cavazzoni et al., Science 283, 44 (1999)], we conclude that high P, T superionic phases of electronegative element hydrides could be common.     

Dissociation of methane under high pressure

Methane is an extremely important energy source with a great abundance in nature and plays a significant role in planetary physics, being one of the major constituents of giant planets Uranus and Neptune. The stable crystal forms of methane under extreme conditions are of great fundamental interest. Using the ab initio evolutionary algorithm for crystal structure prediction, we found three novel insulating molecular structures with P2(1)2(1)2(1), Pnma, and Cmcm space groups. Remarkably, under high pressure, methane becomes unstable and dissociates into ethane (C(2)H(6)) at 95 GPa, butane (C(4)H(10)) at 158 GPa, and further, carbon (diamond) and hydrogen above 287 GPa at zero temperature. We have computed the pressure-temperature phase diagram, which sheds light into the seemingly conflicting observations of the unusually low formation pressure of diamond at high temperature and the failure of experimental observation of dissociation at room temperature. Our results support the idea of diamond formation in the interiors of giant planets such as Neptune.     

Mechanistic aspects of the solid-state transformation of ammonium cyanate to urea at high pressure

The chemical transformation of ammonium cyanate into urea has been of interest to many generations of scientists since its discovery by Friedrich W?hler in 1828. Although widely studied both experimentally and theoretically, several mechanistic aspects of this reaction remain to be understood. In this paper, we apply computational methods to investigate the behavior of ammonium cyanate in the solid state under high pressure, employing a theoretical approach based on the self-consistent-charges density-functional tight-binding method (SCC-DFTB). The ammonium cyanate crystal structure was relaxed under external pressure ranging from 0 to 700 GPa, leading to the identification of five structural phases. Significantly, the phase at highest pressure (above 535 GPa) corresponds to the formation of urea molecules. At ca. 25 GPa, there is a phase transition of ammonium cyanate (from tetragonal P4/nmm to monoclinic P21/m) involving a rearrangement of the ammonium cyanate molecules. This transformation is critical for the subsequent transformation to urea. The crystalline phase of urea obtained above 535 GPa also has P21/m symmetry (Z = 2). This polymorph of urea has never been reported previously. Comparisons to the known (tetragonal) polymorph of urea found experimentally at ambient pressure suggests that the new polymorph is more stable above ca. 8 GPa. Our computational studies show that the transformation of ammonium cyanate into urea is strongly exothermic (enthalpy change -170 kJ mol-1 per formula unit between 530 and 535 GPa). The proposed mechanism for this transformation involves the transfer of two hydrogen atoms of the ammonium cation toward nitrogen atoms of neighboring cyanate anions, and the remaining NH2 group creates a C-NH2 bond with the cyanate unit.     

Electroless deposition and properties of Co–Re–B alloys

The data on the mechanism of electroless (catalytic) deposition of Co–Re–B coatings are obtained by determining the donor capacitance of dimethylamine borane (DMAB) (CH₃)₂HN · BH₃ reductant and the oxidation level of its hydride hydrogen. From the results of the study of isotopic composition of evolved hydrogen, it is concluded that the oxidation level of DMAB hydride hydrogen depends on the catalytic activity of the alloy. The alloys containing up to 46 at % rhenium were produced by the electroless deposition.     

Al 掺杂对Mg2Ni 合金的电子结构及贮氢性能的影响

采用基于密度泛函理论(DFT)的平面波赝势(PW-PP)方法, 计算Mg_2-xAl_xNiH₄ (x=0, 0.125, 0.25)合金的晶胞体积、电子态密度、键序、电荷布居、生成焓, 分析原子间成键和结构的稳定性, 研究Al 部分替代Mg 对Mg₂Ni 合金及其氢化物的结构和储氢性能的影响. 结果表明: 随着Al 含量的增加, Mg₂Ni 合金晶胞体积减小, 不利于氢原子进入合金中, 导致合金的储氢容量降低. 在Mg_2-xAl_xNiH₄ (x=0, 0.125, 0.25)中, Mg-H和Al-H相互作用远小于Ni-H的相互作用, 随着Al 含量的增加, 氢化物生成焓减少以及Ni-H的相互作用减弱, 氢化物的结构稳定性降低, Al 部分替代Mg能有效改善Mg₂Ni 合金释氢动力学性能.     

Gravimetric determination of rhenium as 2,4,6-triphenylpyrylium perrhenate

A gravimetric method for determination of rhenium is based on formation of sparingly soluble 2,4,6-triphenylpyrylium perrhenate. The conditional solubility product in O.1N sulphuric acid medium was found to be (3.72 +/- 0.08) x 10(-9). The interference of foreign anions (simple or complex) is eliminated by preliminary extraction of perrhenate with acetone from strongly alkaline medium. The method is applied to the determination of rhenium in perrhenates and rhenium alloys.     

The Origin of the Catalytic Activity of a Metal Hydride in CO2 Reduction

Atomic hydrogen on the surface of a metal with high hydrogen solubility is of particular interest for the hydrogenation of carbon dioxide. In a mixture of hydrogen and carbon dioxide, methane was markedly formed on the metal hydride ZrCoH_x in the course of the hydrogen desorption and not on the pristine intermetallic. The surface analysis was performed by means of time‐of‐flight secondary ion mass spectroscopy and near‐ambient pressure X‐ray photoelectron spectroscopy, for the in situ analysis. The aim was to elucidate the origin of the catalytic activity of the metal hydride. Since at the initial stage the dissociation of impinging hydrogen molecules is hindered by a high activation barrier of the oxidised surface, the atomic hydrogen flux from the metal hydride is crucial for the reduction of carbon dioxide and surface oxides at interfacial sites.     

Hydrogen desorption exceeding ten weight percent from the new quaternary hydride Li3BN2H8

Mobile applications of hydrogen power have long demanded new solid hydride materials with large hydrogen storage capacities. We report synthesis of a new quaternary hydride having the approximate composition Li(3)BN(2)H(8) with 11.9 wt % theoretical hydrogen capacity. It forms by reacting LiNH(2) and LiBH(4) powders in a 2:1 molar ratio either by ball milling or by heating the mixed powders above 95 degrees C. This new quaternary hydride melts at approximately 190 degrees C and releases > or =10 wt % hydrogen above approximately 250 degrees C. A small amount of ammonia (2-3 mol % of the generated gas) is released simultaneously. Preliminary calorimetric measurements suggest that hydrogen release is exothermic and, hence, not easily reversible.     

Structural refinement of the high-pressure phase of aluminum trihydroxide: in-situ high-pressure angle dispersive synchrotron X-ray diffraction and theoretical studies

In-situ high-pressure synchrotron angle-dispersive X-ray diffraction for gibbsite (aluminum trihydroxide) was performed at room temperature up to 20 GPa. A pressure-induced phase transition was observed at 2.6 GPa. The new high-pressure phase can be recovered at ambient pressure. Rietveld refinement shows that the new phase of Al(OH)(3) has an orthorhombic structure, spacegroup Pbca, and the lattice parameters at ambient condition are a = 868.57(5) pm, b = 505.21(4) pm, c = 949.54(6) pm, V = 416.67(6) x 10(6) pm(3) with Z = 8. The compressibility of gibbsite and the high-pressure polymorph was analyzed, and their bulk moduli were estimated as 49.8 +/- 1.8 and 81.0 +/- 5.2 GPa, respectively. First-principles calculations of the high-pressure phase were performed to determine the hydrogen positions and to confirm the structural stability of the new phase.     

Hydride donor abilities and bond dissociation free energies of transition metal formyl complexes

The hydride complex [Pt(dmpe)2H]+ (dmpe = 1,2-bis(dimethylphosphino)ethane) reversibly transfers H- to the rhenium carbonyl complex [CpRe(PMe3)(NO)(CO)]+, giving the formyl CpRe(PMe3)(NO)(CHO). From the equilibrium constant for the hydride transfer (16.2), the DeltaGdegrees for the reaction was determined (-1.6 kcal/mol), as was the hydride-donating ability of the formyl (44.1 kcal/mol). The hydride-donating ability, DeltaGdegrees(H-), is defined as the energy required to release the hydride ion into solution by the formyl complex [i.e. M(CHO) right arrow M(CO)+ + H-]. Subsequently, the hydride-donating ability of a series of formyl complexes was determined, ranging from 44 to 55 kcal/mol. With use of this information, two rhenium carbonyl complexes, [CpRe(NO)(CO)2]+ and [Cp*Re(NO)(CO)2]+, were hydrogenated to formyls, employing [Pt(dmpp)2]2+ and Proton-Sponge. Finally, the E(1/2)(I/0) values for five rhenium carbonyl complexes were measured by cyclic voltammetry. Combined with the known DeltaGdegrees(H-) values for the complexes, the hydrogen atom donating abilities could be determined. These values were all found to be approximately 50 kcal/mol.     

Spinel-to-CaFe2O4-type structural transformation in LiMn2O4 under high pressure

A new form of LiMn2O4 is reported. The structure is the CaFe2O4-type and 6% denser than the spinel. The structure transformation was achieved by heating at 6 GPa. Analysis of the neutron diffraction pattern confirmed an average of the structure; the unit cell was orthorhombic at a = 8.8336(5) angstroms, b = 2.83387(18) angstroms, and c = 10.6535(7) angstroms (Pnma). Electron diffraction patterns indicated an order of superstructure 3a x b x c, which might be initiated by Li vacancies. The exact composition is estimated at Li(0.92)Mn2O4 from the structure analysis and quantity of intercalated Li. The polycrystalline CaFe2O4-type compound showed semiconducting-like characters over the studied range above 5 K. The activation energy was reduced to approximately 0.27 eV from approximately 0.40 eV at the spinel form, suggesting a possible enhancement of hopping mobility. Magnetic and specific-heat data indicated a magnetically glassy transition at approximately 10 K. As the CaFe2O4-type transition was observed for the mineral MgAl2O4, hence the new form of the lithium manganese oxide would provide valuable opportunities to study not only the magnetism of strongly correlated electrons but also the thermodynamics of the phase transition in the mantle.     

Experimental study of the dehydration reactions gypsum-bassanite and bassanite-anhydrite at high pressure: indication of anomalous behavior of H(2)O at high pressure in the temperature range of 50-300 degrees C

The system CaSO(4)-H(2)O, characterized by the three dehydration reactions gypsum-anhydrite, gypsum-bassanite, and bassanite-anhydrite, was reexamined by in situ differential pressure analysis in the temperature range of 60-350 degrees C up to 3.5 GPa pressure. The investigation revealed a fine structure in the dehydration boundaries of gypsum-bassanite and bassanite-anhydrite, each characterized by three inflections at 0.9-1.0, 1.9-2.0, and 2.6-28 GPa. In addition, the phase transition of anhydrite high pressure anhydrite (monazite structure) was established for the first time at high P-T conditions intersecting the bassanite-anhydrite dehydration boundary at 2.15 GPa250 degrees C. Furthermore, the triple point gypsum-bassanite-anhydrite was redetermined with 235 MPa80.5 degrees C. The evaluation of the gypsum-bassanite dehydration boundary with respect to the volume and entropy change of the reaction, DeltaV(react) and DeltaS(react), by means of the Clausius-Clapeyron relation yields for the entropy parameter an unusually large increase over the range of the noted inflections. This is interpreted as anomalous entropy behavior of H(2)O related presumably to a dramatic increase in fluctuations of the hydrogen network of the liquid leading possibly into a new structural state. The effect is strongly related to the three noted pressure levels of 0.9-1.0, 1.9-2.0, and 2.6-28 GPa. In a synopsis of data including also a previous high pressure study in the temperature range between 0 and 80 degrees C, a tentative P-T diagram of H(2)O is proposed.     

Electrochemical behavior of aqueous acid perrhenate-containing solutions on noble metals: critical review and new experimental evidence

The actual state of the art in the reduction of perrhenate ions on noble metals is reviewed and discussed. Also, with the aim of contributing to better knowledge of this process, results of several experiments are presented. For the first time, spectroscopic evidence on the nature of the deposited rhenium layer on Pt and Rh and the detection of an intermediate in the reduction pathway toward metallic rhenium is provided. The role of the substrate in the electroreduction of perrhenate ions in aqueous acid media is emphasized, because it is directly associated with the formation of different H-containing species as reducing agents. Thus, those metals capable of adsorbing H atoms are able to reduce ReO(4)(-) to ReO(2) by H(ad) at potentials more positive than that of the hydrogen evolution reaction. Moreover, H(ad) reacts with the ReO(2) layer previously deposited, resulting in the formation of Re(III)-soluble species, which subsequently undergo disproportionation to Re and ReO(2). For metals that are not capable of adsorbing H, i.e., Au, molecular hydrogen is the reducing agent, leading to the formation of metallic Re. In addition, ReO(4)(-) is chemically reduced to metallic Re by hydride.     

Diverse roles of hydrogen in rhenium carbonyl chemistry: hydrides, dihydrogen complexes, and a formyl derivative

Rhenium carbonyl hydride chemistry dates back to the 1959 synthesis of HRe(CO)? by Hieber and Braun. The binuclear H?Re?(CO)? was subsequently synthesized as a stable compound with a central Re?(μ-H)? unit analogous to the B?(μ-H)? unit in diborane. The complete series of HRe(CO)(n) (n = 5, 4, 3) and H?Re?(CO)(n) (n = 9, 8, 7, 6) derivatives have now been investigated by density functional theory. In contrast to the corresponding manganese derivatives, all of the triplet rhenium structures are found to lie at relatively high energies compared with the corresponding singlet structures consistent with the higher ligand field splitting of rhenium relative to manganese. The lowest energy HRe(CO)? structure is the expected octahedral structure. Low-energy structures for HRe(CO)(n) (n = 4, 3) are singlet structures derived from the octahedral HRe(CO)? structure by removal of one or two carbonyl groups. For H?Re?(CO)? a structure HRe?(CO)?(μ-H), with one terminal and one bridging hydrogen atom, lies within 3 kcal/mol of the structure Re?(CO)?(η2-H?), similar to that of Re?(CO)??. For H?Re?(CO)(n) (n = 8, 7, 6) the only low-energy structures are doubly bridged singlet Re?(μ-H)?(CO)(n) structures. Higher energy dihydrogen complex structures are also found.     

H2O and D2 mixtures under pressure: spectroscopy and proton exchange kinetics

We have investigated the pressure-induced spectral changes and the proton exchange reactions of D(2)-H(2)O mixtures to 64 GPa using micro-Raman spectroscopy. The results show the profound difference in the rotational and vibrational Raman spectra of hydrogen isotopes from those of the pure samples, showing the vibrational modes at higher frequencies and continuing to increase with pressure without apparent turnover. This indicates the repulsive nature of D(2)-H(2)O interaction without hydrogen bonds between the two and, thus, interstitial fillings of D(2) molecules into the bcc-like ice lattice. The spectral analysis using the Morse potential yields a hydrogen bond distance of 0.734 ? at 6 GPa--slightly shorter than that in pure--attributed to the repulsive interaction. The pressure-dependent spectral changes suggest that the proton-ordering transition in the ice lattice occurs over a large pressure range between 28 and 50 GPa, which is substantially lower than that of pure ice (40-80 GPa). This again indicates the presence of high internal pressure arising from the repulsive interaction. The Raman spectra show evidences that the proton exchange occurs in various phases including in solid D(2) and H(2)O mixtures. Based on the time-dependent spectral changes, we obtained the proton exchange rates of k ~ 0.085 h(-1) at 0.2 GPa in fluid D(2) and water mixtures, k ~ 0.03 h(-1) and 0.003 h(-1) at 2 GPa and 4 GPa, respectively, in fluid D(2)-ice mixtures, and k ~ 10(-3) h(-1) at 8 GPa in solid D(2) and ice mixtures.