Crystallization and phase transitions of C₆H₆:C₆F₆ complex under extreme conditions using laser-driven shock

The C6H6:C6F6 cocrystal is one of the simplest organic cocrystals with a molecule having a C–F bond and without any hydrogen bonding. It has a crystal structure very different from its constituents, C6H6 and C6F6, and its higher melting point indicates its increased stability relative to these two materials. So far, no studies are available on the phase transitions of this interesting adduct under dynamic compression. In this study, we present the findings of phase transitions of an equimolar mixture of C6H6:C6F6 observed under rapid shock compression at pressures of up to 4.15 GPa using time-resolved Raman spectroscopy. The compression is driven by a 2 J Nd:YAG laser with an 8 ns pulse length. Four prominent modes at 370 cm−1 (ν10F mode), 443 cm−1 (ν6F mode), 560 cm−1 (ν1F mode), and 991 cm−1 (ν1H mode) exhibit a blue shift with scaling factors of 2.41, 2.26, 2.39, and 2.67 cm−1/GPa, respectively. The liquid → solid-I phase transition is observed at around 0.49 GPa shock pressure. The second phase transition from solid-I → solid-VI is observed between 1.32 and 2.60 GPa, and no signature of the solid-V phase is observed unlike in the case of static compression[Wang et al., J. Phys. Chem. C 120, 29510 (2016)]. Another phase transition solid-VI → solid-VII is observed between 3.9 and 4.15 GPa. The shock velocities in the sample at two laser intensities, 1.47 GW/cm2 (300 mJ) and 2.46 GW/cm2 (500 mJ), are calculated by measuring the intensity ratio of Raman modes emerging from the shocked region to that of the whole sample and are 3.13 and 4.05 km/s, respectively. To compare with the experimental results, 1D radiation hydrodynamics simulations are also performed. The experimental and simulated shock velocities are in good agreement. The mode Grüneisen parameter for the ν1H, ν1F, ν6F, ν10F, and ν10' F modes are γi = 0.011(2), 0.022(2), 0.011(1), 0.024(3), and 0.379(14), respectively.