  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.
