Liquid hydrogen structure factor to 5 GPa and evidence of a crossover between two density evolutions

Abstract

The center-of-mass structure factor, $S(Q)$, of liquid hydrogen and liquid deuterium has been measured up to 5 GPa, mostly along the melting line from 50 to 296 K. Good quality data were achieved thanks to a novel synchrotron x-ray technique that can isolate the very weak x-ray scattering signal of the micrometric volume of hydrogen compressed in the diamond anvil cell. $S(Q)$ is dominated by a broad peak and hence, its wave-vector position, ${Q}_{m}$, is used to appreciate the structural changes in the system. An isotope effect in the position of ${Q}_{m}$ is observed that can be explained by a density effect. The shift of ${Q}_{m}$ towards higher $Q$ as density increases is followed over a threefold compression. Two simple liquid type evolutions are disclosed with a crossover between them around 37 ${\mathrm{nm}}^{\ensuremath{-}3}$/molecule. An interpretation is proposed based on the change in the zero-point motional renormalization of the interaction from anharmonic to harmonic.