Advancements in low-density crystalline silicon allotropes

While numerous crystalline Si allotropes have been predicted in recent years and, in several instances, synthesized under high pressure, the exploration of Si phases with a lower density than conventional diamond Si (d-Si) is still in its infancy. Theoretical calculations on the electronic properties of these expanded Si forms suggest that, unlike the most stable d-Si structure, many may possess direct or quasi-direct bandgaps and only exhibit slightly higher formation energies than d-Si. The few that have been synthesized already display exciting optical properties, making them promising candidates for optoelectronic and photovoltaic applications. Their unique open-framework, guest–host structures enable distinctive interactions between Si and interstitial guest/dopant atoms, offering exciting potentials in spintronics, energy storage, and bio/medical technologies. In this Perspective, we provide an introduction and overview of the latest theoretical and experimental advancements in low-density Si allotropes, emphasizing their potential in various electronic and energy-related applications. This work also highlights the critical challenges and future directions for the continued development of these Si allotropes for next-generation technological applications.