▁this ▁co ols ▁down ▁it ▁is ▁stable ▁at ▁standard ▁conditions ▁for ▁months . ▁Arg on ▁atoms ▁occup y ▁oct ah ed ral ▁inter st it ial ▁sites . ▁The ▁cry st all ine ▁lattice ▁size ▁is ▁almost ▁un changed ▁at ▁room ▁temperature , ▁but ▁is ▁slightly ▁larger ▁than ▁pure ▁C 6 0 ▁below ▁ 2 6 5   K . ▁However ▁arg on ▁does ▁stop ▁the ▁bu ck y ball s ▁spin ning ▁below ▁ 2 5 0   K , ▁a ▁lower ▁temperature ▁than ▁in ▁pure ▁C 6 0 . ▁ ▁Sol id ▁C 7 0 ▁full er ene ▁will ▁also ▁abs orb ▁arg on ▁under ▁pressure ▁of ▁ 2 0 0   MP a ▁and ▁at ▁a ▁temperature ▁of ▁ 2 0 0   ° C . ▁C 7 0 · Ar ▁has ▁arg on ▁in ▁oct ah ed ral ▁sites ▁and ▁has ▁the ▁rock ▁salt ▁structure , ▁with ▁cub ic ▁cry st als ▁in ▁which ▁the ▁lattice ▁parameter ▁is ▁ 1 5 . 0 0 1   Å . ▁This ▁comp ares ▁to ▁the ▁pure ▁C 7 0 ▁lattice ▁parameter ▁of ▁ 1 4 . 9 6 4   Å , ▁so ▁the ▁arg on ▁forces ▁the ▁cry st als ▁to ▁expand ▁slightly . ▁The ▁C 7 0 ▁elli ps oid al ▁balls ▁rotate ▁freely ▁in ▁the ▁solid , ▁they ▁are ▁not ▁locked ▁into ▁position ▁by ▁extra ▁arg on ▁atoms ▁filling ▁the ▁holes . ▁Arg on ▁gradually ▁esc apes ▁over ▁a ▁couple ▁of ▁days ▁when ▁the ▁solid ▁is ▁stored ▁at