Structural and vibrational properties of the van derWaals compound (N$_2$)$_{11}$He up to 135 GPa
Résumé
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 ($P6_3/m$) with seven nonequivalent atoms: 6 N and 1 He. The N$_2$ molecule centered at (0,0,0) is orientationally disordered. A close relationship with the ε structure of pure solid N$_2$ is observed. A phase transition to a hexagonal superstructure occurs at 28 GPa, which is attributed to the ordering of these disordered 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 ε-N$_2$ are discussed.
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