Na-Ni-H Phase Formation at High Pressures and High Temperatures: Hydrido Complexes [NiH5]3- Versus the Perovskite NaNiH3

Hypervalent hydridosilicate in the Na-Si-H system

Hydrogenation reactions at gigapascal pressures can yield hydrogen-rich materials with properties relating to superconductivity, ion conductivity, and hydrogen storage. Here, we investigated the ternary Na-Si-H system by computational structure prediction and in situ synchrotron diffraction studies of reaction mixtures NaH-Si-H2 at 5-10 GPa. Structure prediction indicated the existence of various hypervalent hydridosilicate phases with compositions NamSiH(4+m) (m = 1-3) at comparatively low pressures, 0-20 GPa. These ternary Na-Si-H phases share, as a common structural feature, octahedral SiH6 2- complexes which are condensed into chains for m = 1 and occur as isolated species for m = 2, 3. In situ studies demonstrated the formation of the double salt Na3[SiH6]H (Na3SiH7, m = 3) containing both octahedral SiH6 2- moieties and hydridic H-. Upon formation at elevated temperatures (>500°C), Na3SiH7 attains a tetragonal structure (P4/mbm, Z = 2) which, during cooling, transforms to an orthorhombic polymorph (Pbam, Z = 4). Upon decompression, Pbam-Na3SiH7 was retained to approx. 4.5 GPa, below which a further transition into a yet unknown polymorph occurred. Na3SiH7 is a new representative of yet elusive hydridosilicate compounds. Its double salt nature and polymorphism are strongly reminiscent of fluorosilicates and germanates.

Formation and Polymorphism of Semiconducting K2SiH6 and Strategy for Metallization

K2SiH6, crystallizing in the cubic K2PtCl6 structure type (Fm3̅m), features unusual hypervalent SiH62- complexes. Here, the formation of K2SiH6 at high pressures is revisited by in situ synchrotron diffraction experiments, considering KSiH3 as a precursor. At the investigated pressures, 8 and 13 GPa, K2SiH6 adopts the trigonal (NH4)2SiF6 structure type (P3̅m1) upon formation. The trigonal polymorph is stable up to 725 °C at 13 GPa. At room temperature, the transition into an ambient pressure recoverable cubic form occurs below 6.7 GPa. Theory suggests the existence of an additional, hexagonal, variant in the pressure interval 3-5 GPa. According to density functional theory band structure calculations, K2SiH6 is a semiconductor with a band gap around 2 eV. Nonbonding H-dominated states are situated below and Si-H anti-bonding states are located above the Fermi level. Enthalpically feasible and dynamically stable metallic variants of K2SiH6 may be obtained when substituting Si partially by Al or P, thus inducing p- and n-type metallicity, respectively. Yet, electron-phonon coupling appears weak, and calculated superconducting transition temperatures are <1 K.

Hydrogen Absorption Reactions of Hydrogen Storage Alloy LaNi5 under High Pressure

Hydrogen can be stored in the interstitial sites of the lattices of intermetallic compounds. To date, intermetallic compound LaNi₅ or related LaNi₅-based alloys are known to be practical hydrogen storage materials owing to their higher volumetric hydrogen densities, making them a compact hydrogen storage method and allowing stable reversible hydrogen absorption and desorption reactions to take place at room temperature below 1.0 MPa. By contrast, gravimetric hydrogen density is required for key improvements (e.g., gravimetric hydrogen density of LaNi₅: 1.38 mass%). Although hydrogen storage materials have typically been evaluated for their hydrogen storage properties below 10 MPa, reactions between hydrogen and materials can be facilitated above 1 GPa because the chemical potential of hydrogen dramatically increases at a higher pressure. This indicates that high-pressure experiments above 1 GPa could clarify the latent hydrogen absorption reactions below 10 MPa and potentially explore new hydride phases. In this study, we investigated the hydrogen absorption reaction of LaNi₅ above 1 GPa at room temperature to understand their potential hydrogen storage capacities. The high-pressure experiments on LaNi₅ with and without an internal hydrogen source (BH₃NH₃) were performed using a multi-anvil-type high-pressure apparatus, and the reactions were observed using in situ synchrotron radiation X-ray diffraction with an energy dispersive method. The results showed that 2.07 mass% hydrogen was absorbed by LaNi₅ at 6 GPa. Considering the unit cell volume expansion, the estimated hydrogen storage capacity could be 1.5 times higher than that obtained from hydrogen absorption reaction below 1.0 MPa at 303 K. Thus, 33% of the available interstitial sites in LaNi₅ remained unoccupied by hydrogen atoms under conventional conditions. Although the hydrogen-absorbed LaNi₅H_x (x < 9) was maintained below 573 K at 10 GPa, LaNi₅H_x began decomposing into NiH, and the formation of a new phase was observed at 873 K and 10 GPa. The new phase was indexed to a hexagonal or trigonal unit cell with a ≈ 4.44 Å and c ≈ 8.44 Å. Further, the newly-formed phase was speculated to be a new hydride phase because the Bragg peak positions and unit cell parameters were inconsistent with those reported for the La-Ni intermetallic compounds and La-Ni hydride phases.

Na3FeH7 and Na3CoH6: Hydrogen-Rich First-Row Transition Metal Hydrides from High Pressure Synthesis

The formation of ternary hydrogen-rich hydrides involving the first-row transition metals TM = Fe and Co in high oxidation states is demonstrated from in situ synchrotron diffraction studies of reaction mixtures NaH–TM–H₂ at p ≈ 10 GPa. Na₃FeH₇ and Na₃CoH₆ feature pentagonal bipyramidal FeH₇^3– and octahedral CoH₆^3– 18-electron complexes, respectively. At high pressure, high temperature (300 < T ≤ 470 °C) conditions, metal atoms are arranged as in the face-centered cubic Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes undergo reorientational dynamics. Upon cooling, subtle changes in the diffraction patterns evidence reversible and rapid phase transitions associated with ordering of the complexes. During decompression, Na₃FeH₇ and Na₃CoH₆ transform to tetragonal and orthorhombic low pressure forms, respectively, which can be retained at ambient pressure. The discovery of Na₃FeH₇ and Na₃CoH₆ establishes a consecutive series of homoleptic hydrogen-rich complexes for first-row transition metals from Cr to Ni.

In situ synchrotron diffraction studies of reaction mixtures NaH−TM−H₂ (TM = Fe, Co) at p ≈ 10 GPa revealed the formation of ternary hydrides Na₃FeH₇ and Na₃CoH₆ featuring pentagonal bipyramidal Fe(IV)H₇³⁻ and octahedral Co(III)H₆³⁻ complexes, respectively. The discovery of Na₃FeH₇ and Na₃CoH₆ establishes a consecutive series of homoleptic hydrogen-rich complexes for first-row transition metals from Cr to Ni.