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PRINCIPAL PUBLICATION AND AUTHORS

Hypervalent hydridosilicates in the Na-Si-H system, K. Spektor (a,b), H. Kohlmann (a), D. Druzhbin (c), W.A. Crichton (c), S. Bhat (b), S.I. Simak (d), O. Yu Vekilova (e), U. Häussermann (e), Front. Chem. 11, 1251774 (2023); https:/doi.org/10.3389/fchem.2023.1251774 (a) Inorganic Chemistry, Faculty for Chemistry and Mineralogy, Leipzig University, Leipzig (Germany) (b) Deutsches Elektronen-Synchrotron DESY, Hamburg (Germany) (c) ESRF (d) Theoretical Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping (Sweden) (e) Department of Materials and Environmental Chemistry, Stockholm University, Stockholm (Sweden)

REFERENCES

[1] A. Drozdov et al., Nature 525, 73-76 (2015). [2] M. Somayazuluet et al., Phys. Rev. Lett. 122, 027001 (2019). [3] R. Lucrezi et al., npj Comput. Mater. 8, 119 (2022). [4] J. Nylén et al., J. Chem. Phys. 131, 104506 (2009). [5] T. Liang et al., J. Phys. Chem. Lett. 12, 7166-7172 (2021).

performed at beamline ID06-LVP. Angle-dispersive powder X-ray diffraction data were recorded using the Pilatus3X-900 kW CdTe high-resolution 2D detector. Figure 46 shows the evolution of diffraction patterns at P = ~9 GPa. Above 560°C, the formation of a new hydride phase of Na3SiH7 with a tetragonal structure (space group P4/mbm, Z = 2) became apparent, and remained present even when raising temperatures up to 850°C. Upon cooling to temperatures below 120°C, the tetragonal diffraction pattern showed a clear orthorhombic splitting, accompanied with a doubling of the unit cell. This pattern was assigned to an orthorhombic polymorph of Na3SiH7 with space group Pbam, Z = 4. Upon decompression, a further transition occurred below ~4.5 GPa into a not yet structurally characterised polymorph. Na3SiH7 represents a double salt, Na3[SiH6]H containing both octahedral SiH62- moieties and hydridic H (Figure 47). Hydridic H is octahedrally coordinated by six Na atoms, and HNa6 octahedra build up a corner-connected framework similar to the BO3 framework in perovskites ABO3. SiH62- octahedra

are located in the voids accommodating the (larger-sized) A constituent of perovskites. Thus, the Na3SiH7 structures may be considered as anti-perovskite arrangement (SiH62-) [HNa3]2+ and, as for perovskites, there is inherent structural flexibility from rotations and tilts of octahedra giving rise to polymorphism.

Na3SiH7 is a new representative of elusive hydridosilicate compounds from high-pressure synthesis featuring hypervalent all-hydrido SiH6 octahedral moieties. These structural units are unusual because typically hypervalency is associated with electronegative ligands such as F, whereas the polarity of the Si-H bond is very low. Electronic structure calculations suggest that hydridosilicates are semiconductors with band gaps around 2 2.5 eV and molecular dynamics simulations indicate superionicity of H- ions [5]. Further, hydridosilicates may adjoin to the predicted superconducting silicon polyhydrides BaSiH8 and SrSiH8, which possess dynamical stability pressures below 30 GPa [3].

Fig. 47: Crystal structures of high- temperature P4/mbm- and low-temperature Pbam-Na3SiH7, along with their observed pressure stability ranges. Below 4.5 GPa, Pbam-Na3SiH7 is superseded by another polymorph with a yet unknown structure.