Hypervalent hydridosilicate in the Na-Si-H system.

Area of Science:

• High-pressure materials science
• Solid-state chemistry
• Computational materials discovery

Background:

• High-pressure hydrogenation is key to discovering novel hydrogen-rich materials with potential applications in superconductivity, ion conductivity, and hydrogen storage.
• Ternary systems involving alkali metals, silicon, and hydrogen are underexplored under extreme pressure conditions.

Purpose of the Study:

• To investigate the Na-Si-H ternary system under gigapascal pressures.
• To computationally predict and experimentally verify new hypervalent hydridosilicate phases.
• To characterize the structural properties and phase transitions of synthesized compounds.

Main Methods:

• Computational structure prediction using density functional theory.
• In situ synchrotron X-ray diffraction studies of NaH-Si-H₂ mixtures at 5-10 GPa.
• High-temperature synthesis and pressure-dependent structural analysis.

Main Results:

• Predicted existence of hypervalent hydridosilicate phases Na_mSiH_(4+m) (m = 1-3) at 0-20 GPa, featuring octahedral SiH₆^2- complexes.
• Experimental confirmation of the double salt Na₃SiH₇ (m = 3) formation, containing SiH₆^2- and H^- ions.
• Observed polymorphism of Na₃SiH₇, with tetragonal and orthorhombic (Pbam) structures at high temperatures and upon cooling, respectively. The Pbam phase is retained to ~4.5 GPa upon decompression.

Conclusions:

• Na₃SiH₇ represents a new class of elusive hydridosilicate compounds.
• The double salt nature and observed polymorphism are analogous to known fluorosilicates and germanates.
• This discovery expands the landscape of high-pressure hydrogen-rich materials and their structural diversity.