Defect‐Controlled Electronic Properties in AZn2Sb2 Zintl Phases

Experimentally, AZn₂Sb₂ samples (A=Ca, Sr, Eu, Yb) are found to have large charge carrier concentrations that increase with increasing electronegativity of A. Using density functional theory (DFT) calculations, we show that this trend can be explained by stable cation vacancies and the corresponding finite phase width in A_1?xZn₂Sb₂ compounds.     

Low-Barrier Hydrogen Bonds in Negative Thermal Expansion Material H3[Co(CN)6]

The covalent nature of the low-barrier N−H−N hydrogen bonds in the negative thermal expansion material H₃[Co(CN)₆] has been established by using a combination of X-ray and neutron diffraction electron density analysis and theoretical calculations. This finding explains why negative thermal expansion can occur in a material not commonly considered to be built from rigid linkers. The pertinent hydrogen atom is located symmetrically between two nitrogen atoms in a double-well potential with hydrogen above the barrier for proton transfer, thus forming a low-barrier hydrogen bond. Hydrogen is covalently bonded to the two nitrogen atoms, which is the first experimentally confirmed covalent hydrogen bond in a network structure. Source function calculations established that the present N−H−N hydrogen bond follows the trends observed for negatively charge-assisted hydrogen bonds and low-barrier hydrogen bonds previously established for O−H−O hydrogen bonds. The bonding between the cobalt and cyanide ligands was found to be a typical donor–acceptor bond involving a high-field ligand and a transition metal in a low-spin configuration.     

The Hydrogenation of the Zintl Phase NdGa Studied by in situ Neutron Diffraction

The hydrogenation of the Zintl phase NdGa was studied by in situ neutron powder diffraction. We find a compositional range of 0.1 < x < 0.8 in NdGaH_1+x. Hydrogen atoms are located in two different positions, in HNd₄ tetrahedra, and close to the polyanionic chains. For the latter, the Ga–H distance in NdGaH_1.66 is quite long (ca. 200 pm) with a trigonal bipyramidal Nd₃Ga₂ surrounding of hydrogen atoms. Hydrogen poor NdGaH_<1 phases as known for similar systems were not observed. The changing hydrogen content shows no measureable effect on the unit cell volume, but on lattice parameter ratios. Superstructures occur for 0.53 < x < 0.66 and 0.73 < x < 0.8, leading to a doubling or tripling of the lattice parameter a. They are probably caused by partial hydrogen ordering. The threefold superstructure contains a ¹_∞[(Ga–H–Ga–H–Ga)^6–] moiety with hydrogen bridging two gallium chains.     

Tailoring Thermodynamics and Kinetics for Hydrogen Storage in Complex Hydrides towards Applications

Solid‐state hydrogen storage using various materials is expected to provide the ultimate solution for safe and efficient on‐board storage. Complex hydrides have attracted increasing attention over the past two decades due to their high gravimetric and volumetric hydrogen densities. In this account, we review studies from our lab on tailoring the thermodynamics and kinetics for hydrogen storage in complex hydrides, including metal alanates, borohydrides and amides. By changing the material composition and structure, developing feasible preparation methods, doping high‐performance catalysts, optimizing multifunctional additives, creating nanostructures and understanding the interaction mechanisms with hydrogen, the operating temperatures for hydrogen storage in metal amides, alanates and borohydrides are remarkably reduced. This temperature reduction is associated with enhanced reaction kinetics and improved reversibility. The examples discussed in this review are expected to provide new inspiration for the development of complex hydrides with high hydrogen capacity and appropriate thermodynamics and kinetics for hydrogen storage.

     

A Composite of Complex and Chemical Hydrides Yields the First Al‐Based Amidoborane with Improved Hydrogen Storage Properties

The first Al‐based amidoborane Na[Al(NH₂BH₃)₄] was obtained through a mechanochemical treatment of the NaAlH₄–4 AB (AB=NH₃BH₃) composite releasing 4.5 wt % of pure hydrogen. The same amidoborane was also produced upon heating the composite at 70 °C. The crystal structure of Na[Al(NH₂BH₃)₄], elucidated from synchrotron X‐ray powder diffraction and confirmed by DFT calculations, contains the previously unknown tetrahedral ion [Al(NH₂BH₃)₄]^?, with every NH₂BH₃^? ligand coordinated to aluminum through nitrogen atoms. Combination of complex and chemical hydrides in the same compound was possible due to both the lower stability of the Al?H bonds compared to the B?H ones in borohydride, and due to the strong Lewis acidity of Al³⁺. According to the thermogravimetric analysis–differential scanning calorimetry–mass spectrometry (TGA–DSC–MS) studies, Na[Al(NH₂BH₃)₄] releases in two steps 9 wt % of pure hydrogen. As a result of this decomposition, which was also supported by volumetric studies, the formation of NaBH₄ and amorphous product(s) of the surmised composition AlN₄B₃H_(0–3.6) were observed. Furthermore, volumetric experiments have also shown that the final residue can reversibly absorb about 27 % of the released hydrogen at 250 °C and p(H₂)=150 bar. Hydrogen re‐absorption does not regenerate neither Na[Al(NH₂BH₃)₄] nor starting materials, NaAlH₄ and AB, but rather occurs within amorphous product(s). Detailed studies of the latter one(s) can open an avenue for a new family of reversible hydrogen storage materials. Finally, the NaAlH₄–4 AB composite might become a starting point towards a new series of aluminum‐based tetraamidoboranes with improved hydrogen storage properties such as hydrogen storage density, hydrogen purity, and reversibility.     

LaSr3NiRuO4H4: A 4d Transition‐Metal Oxide–Hydride Containing Metal Hydride Sheets

The synthesis of the first 4d transition metal oxide–hydride, LaSr₃NiRuO₄H₄, is prepared via topochemical anion exchange. Neutron diffraction data show that the hydride ions occupy the equatorial anion sites in the host lattice and as a result the Ru and Ni cations are located in a plane containing only hydride ligands, a unique structural feature with obvious parallels to the CuO₂ sheets present in the superconducting cuprates. DFT calculations confirm the presence of S= Ni⁺ and S=0, Ru²⁺ centers, but neutron diffraction and μSR data show no evidence for long‐range magnetic order between the Ni centers down to 1.8 K. The observed weak inter‐cation magnetic coupling can be attributed to poor overlap between Ni 3d and H 1s in the super‐exchange pathways.     

Structural and dynamic properties of the polyanionic hydrides SrAlGeH and BaAlGeH

The quaternary aluminium hydrides SrAlGeH and BaAlGeH were synthesized from either hydrogenating the intermetallic AlB₂-type precursors SrAlGe and BaAlGe or reacting SrH₂ with a mixture of Al and Ge in the presence of pressurized hydrogen. Their structures were characterized by X-ray and neutron powder diffraction of the corresponding deuterides. The compounds crystallize with the trigonal SrAlSiH structure type (space group P3m1, Z = 1, a = 4.2435(2) and 4.3450(2) Å, c = 4.9710(3) and 5.2130(4) Å for SrAlGeH and BaAlGeH, respectively) and feature a two-dimensional polyanion [AlGeH]²⁻ which represents a corrugated hexagon layer built from three-bonded Al and Ge atoms. H is terminally attached to Al. Polyanions [AlGeH]²⁻ are electron precise and, according to electronic structure calculations, the quaternary hydrides display band gaps with sizes between 0.7 and 0.8 eV. Infrared and inelastic neutron scattering spectroscopy show Al–H stretching and bending mode frequencies at around 1250 and 870 cm⁻¹, respectively. SrAlGeH and BaAlGeH are thermally stable up to at least 500 °C. When exposed to air the hydrides decompose rapidly to amorphous, orange colored materials.     

Atomic Imaging of Subsurface Interstitial Hydrogen and Insights into Surface Reactivity of Palladium Hydrides

Resolving interstitial hydrogen atoms at the surfaces and interfaces is crucial for understanding the mechanical and physicochemical properties of metal hydrides. Although palladium (Pd) hydrides hold important applications in hydrogen storage and electrocatalysis, the atomic position of interstitial hydrogen at Pd hydride near surfaces still remains undetermined. We report the first direct imaging of subsurface hydrogen atoms absorbed in Pd nanoparticles by using differentiated and integrated differential phase contrast within an aberration-corrected scanning transmission electron microscope. In contrast to the well-established octahedral interstitial sites for hydrogen in the bulk, subsurface hydrogen atoms are directly identified to occupy the tetrahedral interstices. DFT calculations show that the amount and the occupation type of subsurface hydrogen atoms play an indispensable role in fine-tuning the electronic structure and associated chemical reactivity of the Pd surface.     

Oxidative Addition Reactions of Element–Hydrogen Bonds with Different Polarities to a Gallium(I) Compound

The gallium(I) derivative [Ga({N(dipp)CMe}₂CH)] ( 1 ; dipp=2,6‐diisopropylphenyl) undergoes facile oxidative addition reactions with various element–hydrogen bonds including N? H, P? H, O? H, Sn? H, and H? H bonds. This was demonstrated by its reaction with triphenyltin hydride, ethanol, water, diethylamine, diphenylphosphane, and dihydrogen. All products were characterized by means of single‐crystal X‐ray structure determination, NMR spectroscopy, IR spectroscopy, and mass spectrometry.     

Cationic Channels with Partial Anion Occupation in the Zintl Phases Ba2Mg12Ge7.33 and Ba6Mg17.4Li2.6Ge12O0.64

In the system Ba/(Mg, Li)/Ge, two new Zintl phases with the composition Ba₂Mg₁₂Ge_7.33 (P6₃/m, Z = 1, a = 1121.7(5) pm, c = 440.2(2) pm) and Ba₆Mg_17.4Li_2.6Ge₁₂O_0.64 (P6₃/m, Z = 1, a = 1537.8(8) pm, c = 454.6(2) pm) are found and structurally characterized. Their structures are described with respect to the Zintl‐Klemm concept, structure directing rules, and chemical twinning. These new compounds contain as a specific structural feature cationic channels with partial anion occupation which allows to adjust the electron count. In Ba₂Mg₁₂Ge_7.33, the channels are formed by Mg²⁺ cations and are partially filled with germanium dumb‐bells, while the channels in Ba₆Mg_17.4Li_2.6Ge₁₂O_0.64 are formed by Li⁺ and Mg²⁺ cations and host O^2— anions. The electronic structure of both compounds has been investigated using Extended‐Hückel calculations with special emphasis on the states of the cationic channels and their interstitial heteroatoms. The potentiality of using the electron localization function (ELF) to find missing atoms in structures has been tested and verified for both compounds.     

Novel Arachno‐type X56– Zintl Anions in Sr3Sn5, Ba3Sn5, and Ba3Pb5 and Charge Influence on Zintl Clusters

Two new binary Zintl phases, Sr₃Sn₅ and Ba₃Sn₅ were synthesized and structurally characterized. The revised structure of Ba₃Pb₅ is also reported. All three compounds are isotypic and crystallize with a modified Pu₃Pd₅ structure type. The anionic substructure is composed of X₅^6– square pyramidal clusters (X = Sn, Pb), which are described as arachno clusters according to the Wade‐Mingos electron counting rules. The electronic structure of the pyramidal Zintl anions and the influence of the number of skeletal electrons of these clusters are investigated using the electron localization function (ELF). The structural relationship between Ba₃Sn₅ and the Zintl phases Ba₃Si₄ and Ba₃Ge₄ are analyzed. Additionally, two new Zintl phases Ba₃Ge_2.82Sn_2.18 and Ba₃Ge_3.94Sn_0.06, have been synthezised and their structures are reported, which directly show that the exchange of tin against germanium leads to a change from the M₃X₅ to the M₃X₄ structure type. This effect is traced back to the maximal charge acquisition property of the Zintl anions of heavier and lighter tetralides.     

High‐Pressure Raman and Calorimetry Studies of Vanadium(III) Alkyl Hydrides for Kubas‐Type Hydrogen Storage

Reversible hydrogen storage under ambient conditions has been identified as a major bottleneck in enabling a future hydrogen economy. Herein, we report an amorphous vanadium(III) alkyl hydride gel that binds hydrogen through the Kubas interaction. The material possesses a gravimetric adsorption capacity of 5.42 wt % H₂ at 120 bar and 298 K reversibly at saturation with no loss of capacity after ten cycles. This corresponds to a volumetric capacity of 75.4 kgH₂ m^?3. Raman experiments at 100 bar confirm that Kubas binding is involved in the adsorption mechanism. The material possesses an enthalpy of H₂ adsorption of +0.52 kJ mol^?1 H₂, as measured directly by calorimetry, and this is practical for use in a vehicles without a complex heat management system.     

Morphology-Dependent Stability of Complex Metal Hydrides and Their Intermediates Using First-Principles Calculations

Complex light metal hydrides are promising candidates for efficient, compact solid-state hydrogen storage. (De)hydrogenation of these materials often proceeds via multiple reaction intermediates, the energetics of which determine reversibility and kinetics. At the solid-state reaction front, molecular-level chemistry eventually drives the formation of bulk product phases. Therefore, a better understanding of realistic (de)hydrogenation behavior requires considering possible reaction products along all stages of morphological evolution, from molecular to bulk crystalline. Here, we use first-principles calculations to explore the interplay between intermediate morphology and reaction pathways. Employing representative complex metal hydride systems, we investigate the relative energetics of three distinct morphological stages that can be expressed by intermediates during solid-state reactions: i) dispersed molecules; ii) clustered molecular chains; and iii) condensed-phase crystals. Our results verify that the effective reaction energy landscape strongly depends on the morphological features and associated chemical environment, offering a possible explanation for observed discrepancies between X-ray diffraction and nuclear magnetic resonance measurements. Our theoretical understanding also provides physical and chemical insight into phase nucleation kinetics upon (de)hydrogenation of complex metal hydrides.     

Creating Iron– and Rhenium–Bismuth Bonds by Reactions with Organometallic Hydrides

While addition of [Cp₂ReH] to [Bi(OtBu)₃] leads to an equilibrium containing [Cp₂Re‐Bi(OtBu)₂], [{Cp₂Re}₂Bi(OtBu)], tBuOH and [CpRe(μ‐η⁵,η¹‐C₅H₄)Bi–ReCp₂], in the presence of water [{(Cp₂Re)₂Bi}₂O] ( 1 ) is formed selectively. Also [FpH] [Fp = (η⁵‐C₅H₅)(CO)₂Fe] can be employed as a precursor to form heterometallic bismuth compounds. Synthesis of [FpBi{OCH(CF₃)₂}₂]₂ ( 5 ) can be achieved by reaction of [FpH] with [Bi{OCH(CF₃)₂}₃(thf)]₂ and carboxylates [FpBi(O₂CR)₂]₂ are generated upon treatment of [FpH] with [Bi(O₂CR)₃] (R = CH₃, tBu). While the compounds [Fp‐Bi(O₂CR)₂]₂ can also be obtained from reactions with Fp‐Fp, they are formed far more readily using [FpH] as the precursor. They typically crystallize as dimers, like the alkoxide 5 . A monomeric compound of the type [Fp‐BiX₂] ( 6 ) could be isolated for X = thd (tetramethylheptanedionate), that is, after the reaction of [FpH] with [Bi(thd)₃]. Altogether, the results demonstrate the potential of [FpH] as a precursor for [Fp‐BiX₂] compounds, which are formed in reactions with bismuth alkoxides, carboxylates and diketonates.