Theoretical Structure Prediction and High-Pressure Synthesis of a Layered Sodium Silicide Zintl Phase: Na₂Si₃

The discovery of silicon-rich compounds with tailored electronic and thermal properties remains an important objective in materials chemistry. Here, we report the high-pressure/high-temperature (HP-HT) synthesis and theoretical characterization of a new sodium silicide, Na₂Si₃, stabilized near 5 GPa. Na₂Si₃ is recoverable to 1 bar. Evolutionary crystal structure prediction combined with in situ synchrotron X-ray diffraction and density functional theory calculations shows that Na₂Si₃ adopts a tetragonal structure (space group P-42₁m, Z = 2) composed of layered silicon slabs separated by sodium atoms. The bonding can be rationalized within the Zintl-Klemm framework with a formal charge distribution (Na⁺)₂(Si⁰)(Si^-)₂. Thermodynamic calculations indicate that Na₂Si₃ becomes stable at ∼5 GPa while remaining metastable at ambient pressure. Electronic structure calculations predict semiconducting behavior with an indirect band gap of ∼1.14 eV that increases under compression. The pentagonal topology of the silicon layers and the relatively low predicted thermal conductivity suggest potential interest for thermoelectric material design.