K(Th_0.75Sr_0.25)₂Se₆: Structural Change Resulting from the Disorder of Differently Charged Cations

Single crystals of K(Th_0.75Sr_0.25)₂Se₆ were obtained by a standard solid-state chemistry route at 1173 K. This compound does not belong to the AAn₂Q₆ family (A = K, Rb, Cs, or Tl; An = Th, U, or Np; Q = S, Se, or Te) that possesses infinite Q-Q-Q chains and where a charge distribution of A⁺, 2 × An⁴⁺, 2 × Q^2-, 2 × (Q₂^2.5-) has been proposed and hence a charge of -1.25 on Q of the "dichalcogenide". Rather in K(Th_0.75Sr_0.25)₂Se₆, where the Th and Sr cations randomly occupy the same site, incorporation of these differently charged cations breaks the infinite Se-Se-Se chains into a structure that has typical Se₂^2- pairs.