Search

Silicon (Si) is one of the most essential elements, as it is indispensable for modern electronic technology. The standard Si structure at ambient conditions is the cubic diamond structure, and it has an indirect band gap, which prevents it from being considered as a next-generation platform for semiconductor technologies. Therefore, the search for new allotropes of silicon has attracted great attention. Herein, first principles swarm-intelligence structure searches coupled with density-functional theory were performed to explore the stable high-pressure phases of silicon-rich lithium containing compounds, LiSix ( x = 4 – 8 ). The LiSi4 stoichiometry was predicted to be stable, and it was found to assume one of the following space groups, P4/mnc, Cmmm, and C2/m within the pressure range of 0 to 50 GPa. By removing the Li atoms from these compounds, three silicon allotropes were obtained that were metastable at ambient pressures. Our work illustrates how novel silicon allotropes can be predicted using the CALYPSO method.

High pressure is well known to be a useful tool to explore novel crystal structures of materials with unique chemical and physical properties that are not possible to access at ambient conditions. We here report some interesting crystal structures of CsSi6 using a global structure search at a pressure range of 0–30 GPa. Remarkably, we identify a new cubic Im-3m phase of CsSi6 at ambient conditions, and that phase undergoes to an orthorhombic Cmcm phase at ∼8.2 GPa, then transforms to a monoclinic C2/m phase at ∼20.8 GPa. It is found that Si atoms have 5-fold coordination in the high-pressure C2/m phase. These newly predicted structures are all dynamically stable, and their electronic properties are well presented in this work. The present results clearly show some interesting crystal structures of CsSi6 under high pressure, providing insights for exploring other high-pressure behaviors of alkali metal silicides.

Transition metal nitrides have been long thought to hold a promise for hard materials. In this work, we systematically explored the crystal structures of transition metal nitrides via a model system, namely Ti–N system, to investigate their physical related properties employing a global structure searches within ab initio electronic structure framework. As a result of the advanced crystal structure searches, several stoichiometries, e.g. TiN5, TiN2, Ti3N4, TiN, Ti2N, and Ti3N, were found to become stable at high pressures. A sequence of stable Ti–N compounds identified all exhibit metallic behaviors with the evidence that several bands crossing the Fermi level. The present results do show these predicted compounds are hard materials as expected. The findings of new hard materials here put forward further understanding of the crystal structures and electronic properties of Ti–N compounds at high pressures.