According to textbook definitions 1 , there exists no physical observable able to distinguish a liquid from a gas beyond the critical point, and hence only a single fluid phase is defined. There are, however, some thermophysical quantities, having maxima that define a line emanating from the critical point, named 'the Widom line' 2 in the case of the constant-pressure specific heat. We determined the velocity of nanometric acoustic waves in supercritical fluid argon at high pressures by inelastic X-ray scattering and molecular dynamics simulations. Our study reveals a sharp transition on crossing the Widom line demonstrating how the supercritical region is actually divided into two regions that, although not connected by a first-order singularity, can be identified by different dynamical regimes: gas-like and liquid-like, reminiscent of the subcritical domains. These findings will pave the way to a deeper understanding of hot dense fluids, which are of paramount importance in fundamental and applied sciences. Throughout the past century great effort was devoted to the investigation of the physics of fluid systems: all of their thermodynamical properties in the phase diagram below the critical point are nowadays well known 3. On the other hand, experimental studies in the supercritical region have been limited so far, owing to technical difficulties. The fluid pressure-temperature (P-T) phase diagram includes a subcritical region with two different phases (liquid and gas, separated by the liquid-vapour coexistence line) and a single-phase supercritical region. Structural and dynamical investigations, aiming to extend the study of the fluid phase diagram well beyond the critical point play a crucial role in many fundamental and applied research fields, such as condensedmatter physics, Earth and planetary science, nanotechnology and waste management 4-8. From an experimental point of view, the challenge is to close the gap between studies on fluid and solid phases using diamond anvil cell (DAC) techniques 9-12 and studies on hot dense fluids by shock waves 13,14. As this gap typically overlaps with the supercritical fluid region, it is crucial to track the evolution of transport properties of fluids beyond the critical point. In the specific case of acoustic waves, most of the liquids show the so-called positive dispersion. This is an increase of the speed of sound as a function of wavelength from the continuum limit (λ → ∞)-in which the acoustic waves propagate adiabatically-to the short-wavelength limit, that is, on approaching the interparticle distances 15-17. The ultimate origin of this effect can be traced back to the presence of one (or more)

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, 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 A) 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.

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The incorporation of carbon dioxide or argon stabilizes the structure of the microporous silica polymorph silicalite well beyond the stability range of tetrahedrally coordinated SiO(2) and, in fact, beyond even the metastability range of low-pressure silica polymorphs such as quartz and cristobalite at room temperature. The bulk modulus of silicalite strongly increases as a result of the incorporation of CO(2) or Ar and is equivalent to that of quartz. The insertion of these species deactivates the normal compression and pressure-induced amorphization mechanisms in this material, impeding the softening of low-energy vibrations, amorphization, and the eventual increase in silicon coordination up to at least 25 GPa.