Scientists at Washington State University and University Saskatchewan have recently found experimental and theoretical evidence for simple molecular carbon disulfide transformations under progressively higher pressures. An insulating black polymer with three-fold carbon atoms occurs at 9 GPa, then transforms to a semiconducting polymer above 30 GPa, and finally transforms to a metallic solid above 50 GPa . The final metallic phase is a highly disordered three-dimensional network structure with four-fold carbon atoms at the carbon-sulfur distance of ~1.70Å, in plausible structures similar to those of extended phases of its chemical analog carbon dioxide. Carbon dioxide is an important terrestrial volatile often considered to exist in the deep interior of the Earth and may have strong implication on the deep carbon cycles .
The finding of highly disordered extended carbon disulfide showing high metallic conductivity, similar to those of elemental metals, is quite surprising, because the most “metallic” organic polymers exhibit barely metallic conductivity. In this regard, the origin of high metallic conductivity in highly disordered, extended carbon disulfide phase is quite puzzling. Its occurrence deserves further experimental and theoretical studies to determine the exact nature of structural disorder and chemical bonding in this simple organic polymer of highly compressed carbon disulfide.
In the recent article , the researchers also present two plausible structures, based on first-principles calculations, for the carbon disulfide’s metallic phase: α-chalcopyrite and tridymite, both of which exhibit metallic ground states and disordered diffraction features similar to that measured. They also present the phase and chemical transformation diagram for carbon disulfide, showing a large stability field of the metallic phase to 100 GPa and 800 K.
This work has been supported by the NSF-DMR (Grant No. 0854618) and DTRA (HDTRA1-09-1-0041).
Figure: Microphotographs of carbon disulfide under high pressure showing its transformation from (a) transparent fluid to (b,c) molecular solid (Cmca) at ~1 GPa, to (d,e,g) black polymer above 10 GPa ((-S-(C=S)-)p or CS3), and eventually to (f,h) a highly reflecting extended solid above 48 GPa (CS4) at ambient temperature. The right-most image (h) illustrates the metallic reflectivity of CS2 samples above 55 GPa similar to those of Pt probes in a four-probe configuration for resistance measurements.