Abstract
Diatomic elemental solids are highly compressible due to the weak interactions between molecules. However, as the density increases the intra- and intermolecular distances become comparable, leading to a range of phenomena, such as structural transformation, molecular dissociation, amorphization, and metallisation. Here we report, following the crystallization of chlorine at 1.15(30) GPa into an ordered orthorhombic structure (oC8), the existence of a mixed-molecular structure (mC8, 130(10)–241(10) GPa) and the concomitant observation of a continuous band gap closure, indicative of a transformation into a metallic molecular form around 200(10) GPa. The onset of dissociation of chlorine is identified by the observation of the incommensurate structure (i-oF4) above 200(10) GPa, before finally adopting a monatomic form (oI2) above 256(10) GPa.
Highlights
The behaviour of the lighter halogens, fluorine (F2) and chlorine (Cl2), forms a link between the phenomena observed for bromine and iodine and that of hydrogen
We identify a continuous band gap closure, indicative of the onset of molecular metalicity in the halogens, and observe the continuous dissociation of Cl2 molecules through an incommensurate phase before adopting a purely atomic form at 258(10) GPa
The transition was confirmed by X-ray diffraction with peaks that were indexed to an orthorhombic space group Cmce [a = 5.8655(7), b = 4.1937 (2) and c = 7.9455(4) Å] with four chlorine molecules per unit cell
Summary
The behaviour of the lighter halogens, fluorine (F2) and chlorine (Cl2), forms a link between the phenomena observed for bromine and iodine and that of hydrogen. We identify a continuous band gap closure, indicative of the onset of molecular metalicity in the halogens, and observe the continuous dissociation of Cl2 molecules through an incommensurate phase before adopting a purely atomic form at 258(10) GPa. This study presents insights into the dissociation of elemental molecular systems at extreme conditions.
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