Structures Of Hydrides Research Articles

Solving crystal structures of metal and chemical hydrides

Abstract The methods of structural characterization of metal and chemical hydrides are reviewed. The existing difficulties and problems are outlined and possible solutions presented. It is shown that powder diffraction, and especially the Direct Space Method, is essential component of hydride research. Crystal structures containing as many as 55 independent atoms (including hydrogen) have been fully characterized using powder diffraction. This is of great importance, because rapid collection of powder data thanks to modern synchrotron and neutron time-of-flight sources opens the possibility for fast in-situ studies, mapping of phase transitions induced by the temperature, pressure, hydrogen content, and chemical reactions. The progress in structural characterization of hydrides goes hand in hand with the progress in the powder diffraction methodology.

Structures of aluminium-based light weight hydrides

Abstract Light weight hydrides based on aluminium are among the most promising materials for hydrogen storage applications since the hydrogen content is up to 11 wt%. The crystal structures of many promising aluminium-based compounds have been determined and refined during the last years. This review gives an overview of structures of tetra-alanates (with AlH4 tetrahedra), hexa-alanates (with AlH6 octahedra) and different modifications of AlH3. The structure determinations are based on powder diffraction with X-rays, synchrotron X-rays and neutrons. In most structural work neutron diffraction has been used in order to determine accurate hydrogen/deuterium positions. In addition, density functional theory has been used both to predict new structures among these compounds and in combination with experimental work.

Structural investigations of two new ternary magnesium–niobium hydrides, Mg 6.5NbH ∼14 and MgNb 2H ∼4

Two new magnesium–niobium hydrides, Mg 6.5NbH ∼14 with a face centred cubic unit cell a = 9.548(1) Å (space group F m 3 ¯ m (No. 225), Z = 4, V = 870.49 Å 3) and MgNb 2H ∼4 with a C-centred monoclinic unit cell a = 5.685 (4) Å, b = 3.2914(6) Å, c = 7.924(2) Å and β = 103.82(3)° (space group C2/ m (No. 12), Z = 2, V = 143.98 Å 3), were synthesized by reacting MgH 2 and Nb in a high-pressure anvil cell at 8 GPa and 600 °C. The structures of the new hydrides were refined from synchrotron radiation data. Mg 6.5NbH ∼14 has a structure related to Mg 7TiH ∼16 ( a = 9.532(2) Å space group F m 3 ¯ m (No. 225)) (D. Kyoi, T. Sato, E. Rönnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Compd. 372 (2004) 213). Where the metal atom structure is of the Ca 7Ge type. Mg 6.5NbH ∼14, however, has some vacancies on one of the magnesium sites similar to the recently discovered Mg 6VH ∼14 (D. Kyoi, T. Sato, E. Rönnebro, Y. Tsuji, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Compd. 375 (2004) 253). The metal atoms in MgNb 2H ∼4 have the AuTe 2 type structure.

Nature of the chemical bond in complex hydrides, NaAlH 4, LiAlH 4, LiBH 4 and LiNH 2

The most stable crystal structures of complex hydrides, MXH n (NaAlH 4, LiAlH 4, LiBH 4 and LiNH 2) were simulated by the plane-wave pseudopotential method. The local chemical bonds between constituent ions were simulated using the DV-X α molecular orbital method. As a result, it was found that the covalent interaction is operating between X and H ions to form a XH n ion in MXH n . In addition, the ionic interaction is operating between M and XH n ions through the charge transfer from M to XH n ions. On the basis of this understanding of the nature of the chemical bond between ions, a phase stability diagram of complex hydrides was proposed using two parameters. One is the bond energy of XH diatomic molecules and the other is electronegativity difference, Δ Φ X M , between M and X ions. The calculated stability change by doping into NaAlH 4 could by explained qualitatively following this diagram. This diagram will provide us a clue to the modification of hydrides to lower the hydrogen decomposition temperature.

Novel intermetallic hydrides

The paper focuses on structural chemistry of novel intermetallic hydrides with unusual structural properties. In such “anisotropic” hydrides, a huge expansion proceeds in a sole crystallographic direction and leads to a dramatic differentiation of the properties of the hydrides along the direction of the expansion and normal to it. The behaviour of the “anisotropic” hydrides is dominated by the metal–hydrogen and hydrogen–hydrogen interactions in contrast to the “conventional” intermetallic hydrides where the metal–metal interactions are the most important ones. In sharp contrast to the known crystal structures of intermetallic hydrides, in “anisotropic” hydrides deuterium atoms do not fill initially existing interstices but, instead, attract rare earth atoms into their surrounding and form new D- occupied sites. This paper will summarise our recent research on the “anisotropic” hydrides with a particular focus on two groups of materials: (a) RENiIn-based deuterides (RE = rare earth metal) containing the shortest known separation of hydrogen atoms in the structures of metal hydrides and (b) RENi 3–(CeNi 3) and RE 2Ni 7–(La 2Ni 7)-based deuterides which develop unusually large (59–63%) expansion of the constituent RENi 2 layers.

The Additive Nature of Energy Penalties in 10‐Vertex nido‐(Car)boranes

AbstractA structural increment system, i.e. quantitative rules that govern the relative stabilities of 10‐vertex nido‐boranes and‐carboranes, has been determined. Density functional theory computations at the B3LYP/6‐311+G(d,p)//B3LYP/6‐31G(d) level with ZPE corrections were carried out for 81 different boron hydride and carborane structures from [B10H12]2– to C3B7H11 to determine their relative stabilities. A set of eleven disfavored geometrical features that destabilize a cluster structure relative to a hypothetical ideal situation were identified and weighted by so‐called energy penalties. The latter show good additive behavior and allow us to reproduce the DFT computed relative energies mostly with an accuracy of 6.0 kcal mol–1. Some unknown 10‐vertex nido‐carboranes that are thermodynamically more stable than their known isomers are also identified. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Structural Increment System for 11-Vertexnido-Boranes and Carboranes

An increment system forming a set of quantitative rules that govern the relative stabilities of 11-vertex nido-boranes and carboranes is presented. Density functional theory computations at the B3LYP/6-311+G//B3LYP/6-31G level with ZPE corrections were carried out for 61 different boron hydride and carborane structures from [B(11)H(14)](-) to C(4)B(7)H(11) to determine their relative stabilities. Disfavored structural features that destabilize a cluster structure relative to a hypothetical ideal situation were identified and weighted by so-called energy penalties. The latter show additive behavior and allow us to reproduce (within 5 kcal mol(-)(1)) the DFT computed relative energies. Energy penalties for four structural features, i.e., adjacent carbon atoms, CC, a hydrogen atom bridging between a carbon and a boron atom, CH-B, an endo-terminal hydrogen atom at an open face carbon atom, CH(2) and an endo-H between two carbon atoms, C(BH(2))C for the 11-vertex nido-cluster are quite similar to those reported for the 6-vertex nido-cluster, thus showing a behavior independent of the cluster size. Hydrogen structural features, however, vary strongly with the cluster size. Two unknown 11-vertex nido-carboranes were identified which are thermodynamically more stable than known positional isomers.

Neutron Scattering as a Powerful Tool in Solid State Chemistry. Examples from the Investigations in Crystal Structures and Properties of Metal Hydrogen and Metal Chalcogen Compounds

AbstractNeutron scattering experiments play a key role in the investigation in crystal structures and properties of solid compounds which is demonstrated with examples from the compound series AxMyXz (A ≙ alkali metal, M ≙ transition metal and X ≙ H, S or Se). Measurements on powdered samples provide as well the hydrogen positions within the crystal structures of ternary hydrides by using the analogous deuterium compounds as the lithium positions in the compounds with A ≙ Li. Ferromagnetic and antiferromagnetic arrangements are investigated by neutron diffraction which lead to the determination of the magnetic moments of transition metal atoms providing information about chemical bonding. Finally, it is demonstrated that inelastic neutron scattering is an independent method used in a complementary way to infrared or Raman spectroscopy, and, due to the interactions of neutrons with the magnetic states of transition metal atoms, to susceptibility or magnetisation.

Influence of hydrogen on the stability of iron phases under pressure

The influence of hydrogen presence on the stability of iron phases (bcc, hcp, dhcp, fcc, simple cubic) in a wide pressure interval at 0 K has been studied by the first‐principles projector augmented‐wave (PAW) method. Hydrogen is shown to occupy different interstitial lattice positions depending on the type of structure and pressure. An introduction of hydrogen impurities (∼6 at. %) leads to a stabilization of the close‐packed iron structures, shifting the calculated pressure of the bcc‐hcp transition from ∼9 GPa for pure iron to 7 GPa for Fe (6 at. % H). This tendency is further enhanced in the iron hydride structures. The iron hydrides in the close‐packed structures (hcp, dhcp, fcc) are essentially degenerate in energy and found to be most stable in the whole pressure range.

Oscillating dependence between magnetic and chemical orderings in the frustrated Laves hydridesRMn2Hx(R=Lu,Tm,Er)

We have studied crystal and magnetic structures of the hexagonal Laves hydrides $R{\mathrm{Mn}}_{2}{\mathrm{H}}_{x}$ $(R=\mathrm{L}\mathrm{u},\mathrm{T}\mathrm{m},\mathrm{E}\mathrm{r};2lxl4)$ by neutron powder diffraction. Interstitial hydrogen expands the metal lattice and stabilizes magnetic moments on the Mn sites. We found very unusual interplay between the chemical (hydrogen) and the magnetic orderings. While the hydrogen sublattice becomes gradually more ordered as the hydrogen density increases, the correlation length in the magnetic sublattice shows an oscillating dependence with hydrogen content. For some hydrogen concentrations less chemically ordered samples show better ordered spin structures.

Unusual effects on hydrogenation: anomalous expansion and volume contraction

The present paper considers two unique volume effects associated with the formation of intermetallic hydrides, anomalously high linear and volume expansion (CeNi3→CeNi3D2.8) and unusual volume contraction (HoNiSn→HoNiSnD0.67). The crystal structures of both deuterides were solved on the basis of powder neutron diffraction data. The hexagonal CeNi3 anisotropically expands on hydrogenation along 00z (30.7%). In orthorhombic CeNi3D2.8 (space group Pmcn (No. 62); a=4.8748(3); b=8.5590(5); c=21.590(2) Å) this expansion proceeds within the CeNi2 slabs only (63.1%), with a ‘shrinking’ of the CeNi5 parts (−2.8%). All D atoms are located inside the CeNi2 part and on the border of CeNi2 and CeNi5. In sharp contrast to the known crystal structures of intermetallic hydrides, in CeNi3D2.8 deuterium atoms do not fill initially existing interstices but, instead, attract cerium atoms into their surrounding and form new D-occupied sites, Ce3Ni and Ce3Ni3. The hydrogenation of HoNiSn causes a transition from TiNiSi to the ZrNiAl type of structure. Transformation of the metallic sublattice into the ZrNiAl type results in a ‘shrinking’ of the Ho3Ni tetrahedra filled by D. Their occupation in HoNiSnD0.67 (space group P6̄2m (No. 189); a=7.24197(10); c=3.93514(7) Å) proceeds with a formal ‘negative’ volume expansion effect and leads to the formation of strong Ho–H bonds.

Effects of Protide Structures on Hysteresis in Ti-Cr-V Protium Absorption Alloys

Ti–Cr–V alloys have a higher protium (hydrogen atom) desorption capacity, but a larger hysteresis in the absorption and desorption processes than AB2 and AB5 type protium absorption alloys. It is desirable that the alloys have a smaller hysteresis for the use of hydrogen storage tanks. This paper aims to clarify the effects of protide (hydride) structures on the hysteresis in Ti–Cr–V protium absorption alloys. It was found that the β phases observed after desorbing at the desorption plateau region had the BCC structure for the alloys (Ti:Cr=2:3) containing less than 70 at%V, but the BCT structure for the alloys (Ti:Cr=2:3) containing more than 70 at%V . A major reason for the large hysteresis in the Ti–Cr–V alloys appears to be the a-axial contraction in the absorbing process and the a-axial expansion in the desorbing process as opposed to the c-axial changes.

An interrelation of RH x coordination and H ordering in the structures of intermetallic hydrides

The hydrogen sublattice in the structures of intermetallic hydrides is considered as built from RH x polyhedra formed by H around R atoms. This approach allowed to describe successfully an interrelation between different structures formed on the basis of isostructural intermetallics and to rationalise the occurring order–disorder transitions. A detailed analysis has been performed for the RT 2H x hydrides formed by MgCu 2-type intermetallic compounds.

Phase transformations, crystal and magnetic structures of high-pressure hydrides of d-metals

This paper will briefly discuss the high-pressure-hydrogen techniques used at the Institute of Solid State Physics, Russian Academy of Sciences, the T– P diagrams of the studied binary metal–hydrogen systems and the crystal and magnetic structures of high-pressure hydrides formed in those systems. A compilation of the available experimental data and a list of relevant publications are provided for reference purposes.

High-pressure synthesis and crystal structures of new ternary caesium magnesium hydrides, CsMgH 3, Cs 4Mg 3H 10 and Cs 2MgH 4

The title compounds were synthesized by heating stoichiometric mixtures of the binary hydrides (deuterides) to 820 K in a multi-anvil press at 30 kbar pressure. X-ray and neutron powder diffraction on the deuterides at ambient conditions indicates rhombohedral symmetry for CsMgH 3 (space group R-3m, a=6.2521(6), c=22.382(3) Å, BaRuO 3 type structure), orthorhombic symmetry for Cs 4Mg 3H 10 ( Cmca, a=6.2569(9), b=14.743(2), c=13.791(2) Å, Cs 4Mg 3F 10 type structure), and tetragonal symmetry for Cs 2MgH 4 ( I4/mmm, a=4.3264(5), c=14.760(2) Å, K 2NiF 4 type structure). The compounds contain octahedral [MgH 6] 4− anions and have considerably better volume efficiencies for hydrogen storage than those containing tetrahedral [MgH 4] 2− anions that are synthesised under hydrogen gas pressure.

Thermoanalytic and spectroscopic investigations of hydride structures on the surface of amorphous silicon dioxide

The surface of amorphous silicon dioxide is hydrogenated and the hydride structures obtained are investigated by means of thermoanalytic and infrared spectroscopic measurements. Different hydride groups are found by IR spectroscopy. The results are confirmed by thermal measurements. The experimentally determined bond parameters of the surface silane centers document the differences in comparison with the Si–H bonds of other silanes. Model structures with comparable characteristics of the silicon–hydrogen bond lead to conclusions concerning the surface texture of the adsorbent.

Synthesis and Structures of Germanium(II) Fluorides and Hydrides

Treatment of [{HC(CMeNAr)2}GeCl] (Ar = 2,6-iPr2C6H3 (1), 2,6-Me2C6H3 (2)) with Me3SnF in dichloromethane at room temperature afforded the corresponding fluoride [{HC(CMeNAr)2}GeF] (Ar = 2,6-iPr2C6H3 (3), 2,6-Me2C6H3 (4)), while with NaBH4 in THF under reflux for 12 h gave the hydride [{HC(CMeNAr)2}GeH(BH3)] (Ar = 2,6-iPr2C6H3 (5), 2,6-Me2C6H3 (6)). Reaction of 3 with Me3SiN3 in toluene provided [{HC(CMeNAr)2}Ge(F)NSiMe3] (Ar = 2,6-iPr2C6H3 (7)). The BH3 in 5 can be removed with Me3P to afford [{HC(CMeNAr)2}GeH] (Ar = 2,6-iPr2C6H3 (8)). Treatment of 5 with tBuLi in diethyl ether led to [{HC(C(CH2)NAr)CMeNAr}Ge(H)BH3]Li(Et2O)3 (Ar = 2,6-iPr2C6H3 (9)), in which a hydrogen of one of the Me groups was eliminated, and this consequently resulted in the formation of a methylene group. Compounds 3−6 are the first examples of structurally characterized germanium(II) fluorides and hydrides. Single-crystal X-ray structural analyses indicate that compounds 3, 5, and 9 are monomeric and the germanium center resides in ...

In Situ Electrochemical ATR-FTIR Spectroscopic Investigation of Hydrogen-Terminated Si(110) Surface in Dilute NH 4 F Solution

In situ electrochemical attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy has been employed to study the dependence of hydride structures on a Si(110) surface in dilute solution. In situ ATR-FTIR results show that the Si(110) surface remains H-terminated in dilute solution and the hydride on Si(110) has a strong interaction with water molecules under cathodic potential control. In addition, is formed even at cathodic potentials. Infrared roughness factors, defined as the ratio of monohydride at steps or dihydride over the monohydride on the (110) plane, suggest that monohydride-terminated (111) facets are the dominant defects on the Si(110) surface in dilute solution at cathodic electrode potential. At anodic potential, vertical dihydrides, besides (111) facets, increased on the Si(110) surface leading to a roughened morphology. © 2001 The Electrochemical Society. All rights reserved.

In Situ Electrochemical ATR-FTIR Spectroscopic Investigation of Hydrogen-Terminated Si(111) Surface in Diluted NH 4 F Solution

In situ real-time and electrochemical attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy has been employed to study the dependence of hydride structures of an Si(111) surface in diluted solution on etching time and applied potential. The in situ real-time ATR-FTIR result shows that the Si(111) surface remains H terminated in diluted solution, and the hydride on Si(111) has a strong interaction with water molecules under cathodic potential control. Potential dependence of the in situ electrochemical ATR-FTIR spectra reveals that Si(111) surface is free of oxide at the open circuit potential (OCP) and is formed only at potentials positive of the OCP. Infrared roughness factors, defined as the ratio of monohydride at steps or dihydride over the monohydride on the (111) plane, suggest that monohydride chain steps are the dominating defects on the Si(111) surface in at cathodic electrode potential. Horizontal (D′) and vertical (D) dihydrides are predominant on the Si(111) surface leading to roughened morphology via oxidation at anodic potential. © 2001 The Electrochemical Society. All rights reserved.

Empirical and ab initio energy/architectural patterns for 73 nido-6-carborane isomers, from B(6)H(9)(-) to C(4)B(2)H(6).

Qualitative rules governing carbon and bridge-hydrogen placement permit the prediction of the most stable isomeric structures for the various carboranes. Seventy-three isomeric boron hydride and carborane structures, from B(6)H(9)(-) to C(4)B(2)H(6), were computed at the ab initio MP2(fc)/6-31G level to determine their relative stabilities quantitatively. Specific architectural features, recognized to be unfavorable, were assigned "energy penalty" values that allow the projection of comprehensive thermodynamic stability values via a simple additivity procedure. These values match the ab initio results with surprising precision. Our study includes Siebert's nido-2,3,5-C(3)B(3)H(7) and Wrackmeyer's nido-2,4-C(2)B(4)H(8) nido-6<V> carboranes, which contain "unusual" CH-B-bridge hydrogens.