Structural and vibrational properties of the van der Waals compound (N2)11He up to 135 GPa

Abstract

The structural and vibrational properties of the van der Waals compound (N${}_{2}$)${}_{11}$He have been measured up to 175 GPa. The structure, as determined at 14.5 GPa using single-crystal x-ray diffraction, is hexagonal $(P{6}_{3}/m)$ with seven nonequivalent atoms: 6 N and 1 He. The ${\mathrm{N}}_{2}$ molecule centered at (0,0,0) is orientationally disordered. A close relationship with the $\ensuremath{\varepsilon}$ structure of pure solid ${\mathrm{N}}_{2}$ is observed. A phase transition to a hexagonal superstructure occurs at 28 GPa, which is attributed to the ordering of these disordered ${\mathrm{N}}_{2}$ molecules, driven by quadrupole-quadrupole interaction. A transition to an amorphous state then takes place at 135 GPa. Strong similarities in the structural evolution under pressure of the (N${}_{2}$)${}_{11}$He compound and of $\ensuremath{\varepsilon}$-N${}_{2}$ are discussed.