Structures of the metallic and superconducting high pressure phases of solid CS2

Abstract

First principles structural prediction and molecular dynamics (MD) calculations have been performed to examine the structures responsible for the recently reported metallic and superconducting phases of highly compressed CS2. The low pressure experimental molecular crystal structure was found to be metastable and transformed into a disordered structure above 10 GPa. At 60 GPa, the predicted low energy structures show molecular CS2 is separated into C and S dominant regions. A crystalline structure with the P21/m symmetry was found to be most stable from 60 to 120 GPa. The structure is formed from alternate layers of hexagonal C rings and S 2D-square-nets linked by C-S bonds. A non-crystalline structure with similar features structure is also predicted by MD calculations. Electron-phonon coupling calculations show this crystalline phase is superconductive. Contrary to the suggestions made from the experiments, no magnetism was found in all predicted low enthalpy high pressure structures. Moreover, the theoretical results do not support the proposal on the existence of hypervalent 6-coordinated carbon at 120 GPa.

Creating new layered structures at high pressures: SiS2

“…SiS 2 is a member of an important family of group IV-VI AB 2 compounds made of light elements including well-known materials such as CO 2 1234567891011121314, SiO 2 1516171819202122, GeO 2 2324252627 and CS 2 28293031. Limited to relatively low pressures, the structural evolution of SiS 2 has also been accurately described32.…”

High-Pressure Structural Evolution of Disordered Polymeric CS$_2$

“…For providing a clue to the origin of the pressure evolution in the structure of disordered polymeric CS 2 , we decided to performed evolutionary search of crystal structures of stoichiometric CS 2 in the low-pressure range of 5-10 GPa, not covered in previous studies [20,21]. The benefit expected from this search is twofold.…”

“…In ref. [21] instead, fully unconstrained search was performed at 2, 60 and 100 GPa allowing for partial decomposition of the sample and spatial separation of the elements. Here we focus specifically on the lower pressures around 10 GPa where polymerization occurs in order to understand to which extent the experimental data could be explained by polymerization, without assuming disproportionation.…”

“…Subsequently, a constrained first-principles evolutionary search was conducted up to 200 GPa in order to identify the lowest-enthalpy structures of non-molecular CS 2 with C in 4-fold coordination by S [20], yet the structure of the experimentally obtained extended, disordered CS 2 remained unsolved. In another ab initio simulation study with constrained C:S=1:2 stoichiometry, the molecular crystal has been predicted to transform into non-molecular either amorphous or crystalline solids, above 10 GPa [21]. All these materials include C-C and S-S bonds along with C-S bonds, and also C and S are predicted to separate on the spatial scale of the simulation cell.…”

Pressure-induced Transformations of Dense Carbonyl Sulfide to Singly Bonded Amorphous Metallic Solid

“…The pathway to such an extended lattice (or alloy), however, is controlled by strong kinetics, giving rise to stable crystalline phases with complex structures as well as metastable amorphous solids as seen in silicate minerals 34 . Note that this pressure-induced ionization to an extended molecular alloy differs from chemical decomposition 35 , or phase separation, to heterogeneous mixtures of sulfur (or oxygen) and carbon phases via thermal diffusions typically at high temperatures. Inherently, CO 2 with its strong covalent CO bonds is more difficult to deform than OCS or CS 2 , which have more ionic CS bonds.…”