Order-disorder phase transition in type-I clathrate Cs₈Sn ₄₄□₂

The clathrate compound α- Cs₈Sn₄₄D□₂ has been synthesized from its elements under inert gas conditions and has been characterized by single-crystal and powder X-ray diffraction. At room temperature, it crystallizes with cubic symmetry [a = 24.256(3) Å, space group Ia3d, Z = 8] and adopts a 2 x 2 x 2 superstructure of the type-I clathrate and a high ordering of the vacancies (□) in the Sn framework. Single crystals of α-Cs₈Sn ₄₄D□₂ reversibly transform at 90°C to the high-temperature β form with primitive symmetry [a = 12.135(1) Å, space group Pm3n, Z = 1] and a lower ordering of the defects. Differential thermal analysis corroborates the reversible character of the phase transition, which occurs with an enthalpy change of approximately 0.38 J g^-1. An atom-migration mechanism describing the order-disorder transition involving spiro-connected six-membered rings only (scsr mechanism) is proposed.