types ▁often ▁occur ▁within ▁the ▁same ▁spec imen . ▁Their ▁structure ▁is ▁based ▁on ▁the ▁Ni As ▁unit ▁cell . ▁As ▁such , ▁Fe ▁occup ies ▁an ▁oct ah ed ral ▁site ▁and ▁the ▁sul f ide ▁cent ers ▁occup y ▁trig onal ▁pr ism atic ▁sites . ▁ ▁Material s ▁with ▁the ▁Ni As ▁structure ▁often ▁are ▁non - sto ichi ometric ▁because ▁they ▁lack ▁up ▁to ▁ 1 / 8 th ▁fraction ▁of ▁the ▁metal ▁ ions , ▁creating ▁vac an cies . ▁One ▁of ▁such ▁structures ▁is ▁p yr r hot ite - 4 C ▁( Fe 7 S 8 ). ▁Here ▁" 4 " ▁indicates ▁that ▁iron ▁vac an cies ▁define ▁a ▁super l attice ▁that ▁is ▁ 4 ▁times ▁larger ▁than ▁the ▁unit ▁cell ▁in ▁the ▁" C " ▁direction . ▁The ▁C ▁direction ▁is ▁convention ally ▁chosen ▁parallel ▁to ▁the ▁main ▁symmetry ▁axis ▁of ▁the ▁cry stal ; ▁this ▁direction ▁usually ▁corresponds ▁to ▁the ▁largest ▁lattice ▁spacing . ▁Other ▁poly types ▁include : ▁p yr r hot ite - 5 C ▁( Fe 9 S 1 0 ), ▁ 6 C ▁( Fe 1 1 S 1 2 ), ▁▁ 7 C ▁( Fe 9 S 1 0 ) ▁and ▁ 1 1 C ▁( Fe 1 0 S 1 1 ). ▁Every ▁poly type ▁can ▁have ▁mon oc lin ic ▁( M ) ▁or ▁hex ag onal ▁( H ) ▁symmetry , ▁and ▁therefore ▁some ▁sources ▁label ▁them , ▁for ▁example , ▁not ▁as ▁ 6