Sodium silicide

Sodium silicide (NaSi, Na₄Si₄) is a binary inorganic compound consisting of sodium and silicon. It is a solid black or grey crystalline material.^[1] It can be synthesized by melting sodium or a sodium-potassium alloy with finely powdered silica gel. Temperature control of the reaction determine the final product's reactivity; heating above 400 °C creates an air-stable and less reducing product, while the product produced at room temperature is a pyrophoric and highly moisture-sensitive powder.^[2]

Sodium silicide's empirical formula (NaSi) is deceptive, as it is a Zintl phase: instead of containing discrete Si⁻ anions, it contains Si4−4 tetrahedra, which leads to the more descriptive formula Na₄Si₄. These silicon tetrahedra condense into various crosslinked silicon polymers at high pressure.^[3]

Sodium silicide reacts readily with water yielding gaseous hydrogen and aqueous sodium silicate in an exothermic reaction (~175 kJ·mol⁻¹):^[4]

2 NaSi + 5 H₂O → 5 H₂ + Na₂Si₂O₅

This is used in hydrogen technologies to generate hydrogen as a fuel. It is also used as high energy dense storage for hydrogen under low pressure.^[5]