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

Abstract

The application of pressure, internal or external, transforms molecular solids into non-molecular extended network solids with diverse crystal structures and electronic properties. These transformations can be understood in terms of pressure-induced electron delocalization; however, the governing mechanisms are complex because of strong lattice strains, phase metastability and path dependent phase behaviors. Here, we present the pressure-induced transformations of linear OCS (R3m, Phase I) to bent OCS (Cm, Phase II) at 9 GPa; an amorphous, one-dimensional (1D) polymer at 20 GPa (Phase III); and an extended 3D network above ~35 GPa (Phase IV) that metallizes at ~105 GPa. These results underscore the significance of long-range dipole interactions in dense OCS, leading to an extended molecular alloy that can be considered a chemical intermediate of its two end members, CO2 and CS2.

Phase diagram of carbonyl sulfide: An analogy to carbon dioxide and carbon disulfide

“…2,6) Despite an extensive amount of studies on CO 2 and CS 2 , few studies have been done on OCS at high pressures. 18,32) This is a serious shortfall, considering the unique role of OCS as a non-centro-symmetric triatomic molecule, and underscores the significance of the present study on dense OCS to 50 GPa and 4 K as described below.…”

“…However, they follow quite different mechanisms. For example, it occurs rather abruptly in OCS (or CS 2 ) 18,26,31) to an intermediary three-fold 1D polymeric configuration at 20 GPa (or 10 GPa), which then convert slowly to a four-fold 3D network polymer over a large pressure region of 25-70 GPa (30-70 GPa). In contrast, the Cmca phase of CO 2 develops strong lattice strength and disorders over a large pressure range of 20-40 GPa before it transforms to a non-molecular amorphous solid (a-carbonia or a-CO 2 ) with three-and four-fold carbons at ambient temperature.…”

“…Importantly, upon increasing pressure, the bending ν 2 and its overtone 2ν 2 modes split into doublets at 10 GPa, suggesting a phase transition. A previous study 18) has attributed this transition to a structural change from linear OCS-I (R3m) to bent OCS-II (Cm), which is caused by a simple rotation of molecule from the body diagonal to the face diagonal. Therefore, it is likely that the observed splitting is due to the factor group splitting associated with the molecular bending and is also responsible to its intensity increase observed at 10 GPa.…”

“…Recently, we have reported a series of phase=chemical transformations in OCS to 130 GPa from linear OCS (R3m, phase I) to bent OCS (Cm, phase II) at 9 GPa; an amorphous, one-dimensional (1D) polymer at 20 GPa (phase III); and an extended 3D network above ∼35 GPa (phase IV) that metallizes at ∼105 GPa. 18) In this study, we have extended the study of OCS to low temperatures to 4 K, using confocal micro-Raman spectroscopy and four-probe electric resistance measurements, and complete the phase diagram of OCS to 50 GPa and 4 K. The phase diagram of OCS underscores the significance of long-range dipole interactions in the intermediate pressure regime, leading to an extended molecular alloy that can be considered a chemical intermediate of its two end members, CO 2 and CS 2 . In this paper, following the description of experimental methods (Sect.…”