Cation vacancy order in the K0.8+xFe1.6-ySe2 system: five-fold cell   expansion accommodates 20% tetrahedral vacancies
  Ordering of the tetrahedral site vacancies in two crystals of refined compositions K0.93(1)Fe1.52(1)Se2 and K0.862(3)Fe1.563(4)Se2 produces a fivefold expansion of the parent ThCr2Si2 unit cell in the ab plane which can accommodate 20% vacancies on a single site within the square FeSe layer. The iron charge state is maintained close to +2 by coupling of the level of alkali metal and iron vacancies, producing a potential doping mechanism which can operate at both average and local structure levels. 
J. Bacsa, A.Y. Ganin, Y. Takabayashi, K.E. Christensen, K. Prassides,   M.J. Rosseinsky, J.B. Claridge
cond-mat.supr-con