  The compression of arsenolite (cubic As2O3) has been studied from a joint
experimental and theoretical point of view. Experimental X-ray diffraction and
Raman scattering measurements of this molecular solid at high pressures with
different pressure-transmitting media have been interpreted with the help of ab
initio calculations. Our results confirm arsenolite as one of the softest
minerals in absence of hydrogen bonding and provide evidence for helium
trapping above 3 GPa between adamantane-type As4O6 cages, thus leading to a new
compound with stoichiometry He2As4O6. Helium trapping alters all properties of
arsenolite. In particular, pressure-induced amorphization, which occurs in pure
arsenolite above 15 GPa, is impeded when He is trapped between the As4O6 cages;
thus resulting in a mechanical stability of He2As4O6 beyond 30 GPa. Our work
paves the way for the modification of the properties of other molecular solids
by compression depending on their ability to trap relatively small atomic or
molecular species and form new compounds. Furthermore, our work suggests that
compression of molecular solids with noble gases as helium could result in
unexpected results compared to other pressure-transmitting media.
