Fig. 3.

X-ray diffraction measurements. a High-pressure X-ray diffraction patterns (λ = 0.3344 Å) of chlorine with increasing pressure. At 251 GPa, chlorine adopts an incommensurate structure based on a face-centred orthorhombic parent phase whose reflections are marked with the upper set of tick marks. Satellite reflections are shown with the lower set of tick marks. At 266 GPa chlorine has transformed to an atomic phase crystallising in a body-centred orthorhombic unit cell; b volume per Cl atom as a function of pressure. A third Birch-Murnaghan equation of state⁵³ was used to describe the pressure evolution (V₀ = 29.1222 Å, K₀ = 8.3(11) GPa, K’ = 5.6(4)), denoted by the solid green curve, to the molecular oC8 and mC8 phases. The 1-D phase diagram atop of the figure provides the phase behaviour of chlorine as observed with X-ray diffraction. Inset—crystal structures in the b–c plane of oC8, i-oF4 and oI2 of chlorine, differences in the opacity represent atoms belonging to different layers in the structures. Top panel—oC8 (Cmce), where distinct differences in intermolecular and intramolecular distances are observed. Middle Panel—i-oF4, a modulated structure as a precursor to complete molecular dissociation. Bottom panel—oI2 (Immm), the simple close-packed structure of atomic chlorine