▁mole cules ▁are ▁similar ▁in ▁size , ▁so ▁that ▁a ▁greater ▁range ▁of ▁mis ci b ility ▁occurs ▁compared ▁to ▁other ▁gas ▁mi xt ures . ▁Sol id ▁arg on ▁can ▁dissol ve ▁up ▁to ▁ 5 % ▁o xygen ▁without ▁changing ▁structure . ▁Below ▁ 5 0 % ▁o xygen ▁a ▁hex ag onal ▁close ▁pack ed ▁phase ▁exists . ▁This ▁is ▁stable ▁from ▁about ▁ 3 G Pa ▁to ▁ 8 . 5   G Pa . ▁Typ ical ▁formula ▁is ▁Ar O . ▁With ▁more ▁o xygen ▁between ▁ 5 . 5 ▁and ▁ 7   G Pa , ▁a ▁cub ic ▁P m 3 n ▁structure ▁exists , ▁but ▁under ▁higher ▁pressure ▁it ▁changes ▁to ▁a ▁I - 4 2 d ▁space ▁group ▁form . ▁With ▁more ▁than ▁ 8 . 5   G Pa ▁these ▁allo ys ▁separate ▁to ▁solid ▁arg on ▁and ▁ ε - ox y gen . ▁The ▁cub ic ▁structure ▁has ▁a ▁unit ▁cell ▁edge ▁of ▁ 5 . 7 8 2 8   Å ▁at ▁ 6 . 9   G Pa . ▁The ▁representative ▁formula ▁is ▁Ar ( O 2 ) 3 . ▁ ▁Ar He 2 ▁Using ▁density - function al ▁theory ▁Ar He 2 ▁is ▁predicted ▁to ▁exist ▁with ▁the ▁M g C u 2 ▁L aves ▁phase ▁structure ▁at ▁high ▁press ures ▁below ▁ 1 3 . 8   G Pa . ▁A bove ▁ 1 3 . 8   G Pa ▁it ▁transform s ▁to ▁Al B 2