Triggering dynamics of the high-pressure benzene amorphization

Success in designing and tailoring solid-state reactions depends
on the knowledge of the mechanisms regulating the reactivity at
the microscopic level. In spite of several attempts to rationalize
the reactivity of crystals [1-5], the question of the existence of a
critical distance for a reaction to occur remains unsolved. In
this framework, the role of lattice phonons, which continuously
tune the relative distance and orientation of the molecules,
is still not fully understood. Here, we show that at the onset
of the transformation of crystalline benzene to an amorphous
hydrogenated carbon the intermolecular C-C distance is always
the same (about 2.6 Angstrom) once collective motions are taken into
account, and it is independent of the pressure and temperature
conditions. This conclusion is supported by first-principles
molecular-dynamics simulations. This is a clear demonstration
of the role of lattice phonons in driving the reactivity in
the crystalline phase by fine-tuning of the nearest-neighbour
distances. The knowledge of the critical C-C distance can be
crucial in planning solid-state reactions at moderate pressure.