Infinite family of bc8 -like metastable phases in silicon

Abstract

We show that new silicon crystalline phases, observed in the experiment with the laser-induced microexplosions inside silicon crystals (Rapp et al. // Nat. Commun. 6, 7555 (2015)), are all superstructures of a disordered high-symmetry phase with $Ia\bar{3}d$ cubic space group, as well as known for many years phases $bc8$ (Si-III) and $r8$ (Si-XII). The physics of this phenomenon is rather nontrivial: The $bc8$-like superstructures appear as regularly ordered patterns of switchable atomic strings, preserving everywhere the energetically favorable tetrahedral coordination of silicon atoms. The variety of superstructures arises because each string can be switched between two states independently of the others. An infinite family of different phases can be obtained this way and a number of them are considered here in detail. In addition to the known $bc8$, $bt8$, and $r8$ crystals, 128 tetrahedral structures with 16 (6 phases), 24 (22 phases), and 32 (100 phases) atoms per primitive cell are generated and studied, most of them are new ones. For the coarse-grain description of the structures with two possible states of switchable strings, the black/white (switched/nonswitched) Shubnikov symmetry groups has been used. An ab initio relaxation of the atomic positions and lattice parameters shows that all the considered phases are metastable and have higher density and energy relative to the $bc8$ phase at the ambient pressure. A possible scenario for appearance of those phases from the high-temperature amorphous phase is discussed.