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.

Structural diversity and hydrogen storage properties in the system K-Si-H

KSiH₃ exhibits 4.1 wt% experimental hydrogen storage capacity and shows reversibility under moderate conditions, which provides fresh impetus to the search for other complex hydrides in the K-Si-H system. Here, we reproduce the stable Fm3̄m phase of K₂SiH₆ and uncover two denser phases, space groups P3̄m1 and P6₃mc at ambient pressure, by means of first-principles structure searches. We note that P3̄m1-K₂SiH₆ has a high hydrogen content of 5.4 wt% and a volumetric density of 88.3 g L^-1. Further calculations suggest a favorable dehydrogenation temperature T_des of -20.1/55.8 °C with decomposition into KSi + K + H₂. The higher hydrogen density and appropriate dehydrogenation temperature indicate that K₂SiH₆ is a promising hydrogen storage material, and our results provide helpful and clear guidance for further experimental studies. We found three further potential hydrogen storage materials stable at high pressure: K₂SiH₈, KSiH₇ and KSiH₈. These results suggest the need for further investigations into hydrogen storage materials among such ternary hydrides at high pressure.

Tracking the Progression of Anion Reorientational Behavior between α-Phase and β-Phase Alkali-Metal Silanides, MSiH3, by Quasielastic Neutron Scattering

Quasielastic neutron scattering (QENS) measurements over a wide range of energy resolutions were used to probe the reorientational behavior of the pyramidal SiH₃ ^- anions in the monoalkali silanides (MSiH₃, where M = K, Rb, and Cs) within the low-temperature ordered β-phases, and for CsSiH₃, the high-temperature disordered α-phase and intervening hysteretic transition region. Maximum jump frequencies of the β-phase anions near the β-α transitions range from around 10⁹ s^-1 for β-KSiH₃ to 10¹⁰ s^-1 and higher for β-RbSiH₃ and β-CsSiH₃. The β-phase anions undergo uniaxial 3-fold rotational jumps around the anion quasi-C ₃ symmetry axis. CsSiH₃ was the focus of further studies to map out the evolving anion dynamical behavior at temperatures above the β-phase region. As in α-KSiH₃ and α-RbSiH₃, the highly mobile anions (with reorientational jump frequencies approaching and exceeding 10¹² s^-1) in the disordered α-CsSiH₃ are all adequately modeled by H jumps between 24 different locations distributed radially around the anion center of gravity, although even higher anion reorientational disorder cannot be ruled out. QENS data for CsSiH₃ in the transition region between the α- and β-phases corroborated the presence of dynamically distinct intermediate (i-) phase. The SiH₃ ^- anions within i-phase appear to undergo uniaxial small-angular-jump reorientations that are more akin to the lower-dimensional β-phase anion motions rather than to the multidimensional α-phase anion motions. Moreover, they possess orientational mobilities that are an order-of-magnitude lower than those for α-phase anions but also an order-of-magnitude higher than those for β-phase anions. Combined QENS and neutron powder diffraction results strongly suggest that this i-phase is associated chiefly with the more short-range-ordered, nanocrystalline portions (invisible to diffraction) that appear to dominate the CsSiH₃.

Hydrogenation properties of KSi and NaSi Zintl phases

The recently reported KSi-KSiH(3) system can store 4.3 wt% of hydrogen reversibly with slow kinetics of several hours for complete absorption at 373 K and complete desorption at 473 K. From the kinetics measured at different temperatures, the Arrhenius plots give activation energies (E(a)) of 56.0 ± 5.7 kJ mol(-1) and 121 ± 17 kJ mol(-1) for the absorption and desorption processes, respectively. Ball-milling with 10 wt% of carbon strongly improves the kinetics of the system, i.e. specifically the initial rate of absorption becomes about one order of magnitude faster than that of pristine KSi. However, this fast absorption causes a disproportionation into KH and K(8)Si(46), instead of forming the KSiH(3) hydride from a slow absorption. This disproportionation, due to the formation of stable KH, leads to a total loss of reversibility. In a similar situation, when the pristine Zintl NaSi phase absorbs hydrogen, it likewise disproportionates into NaH and Na(8)Si(46), indicating a very poorly reversible reaction.

Polyanionic Hydrides from Polar Intermetallics AeE2 (Ae = Ca, Sr, Ba; E = Al, Ga, In)

The hydrogenation behavior of the polar intermetallic systems AeE2 (Ae = Ca, Sr, Ba; E = Al, Ga, In) has been investigated systematically and afforded the new hydrides SrGa2H2 and BaGa2H2. The structure of these hydrides was characterized by X-ray powder diffraction and neutron diffraction of the corresponding deuterides. Both compounds are isostructural to previously discovered SrAl2H2 (space group P3m1, Z = 1, SrGa2H2/D2: a = 4.4010(4)/4.3932(8) A, c = 4.7109(4)/4.699(1) A; BaGa2H2/D2: a = 4.5334(6)/4.5286(5) A, c = 4.9069(9)/4.8991(9) A). The three hydrides SrAl2H2, SrGa2H2, and BaGa2H2 decompose at around 300 degrees C at atmospheric pressure. First-principles electronic structure calculations reveal that H is unambiguously part of a two-dimensional polyanion [E2H2]2- in which each E atom is tetrahedrally coordinated by three additional E atoms and H. The compounds AeE2H2 are classified as polyanionic hydrides. The peculiar feature of polyanionic hydrides is the incorporation of H in a polymeric anion where it acts as a terminating ligand. Polyanionic hydrides provide unprecedented arrangements with both E-E and E-H bonds. The hydrogenation of AeE2 to AeE2H2 takes place at low reaction temperatures (around 200 degrees C), which suggests that the polyanion of the polar intermetallics ([E2]2-) is employed as precursor.