title: Sn(20.5) square(3.5)As(22)I(8): a largely disordered cationic clathrate with a new type of superstructure and abnormally low thermal conductivity.
abstract: Sn(20.5)As(22)I(8), a new cationic clathrate, has been prepared by using an ampoule technique. According to the X-ray powder diffraction data, it crystallizes in the face-centered cubic space group F23 or Fm(-)3 with a unit-cell parameter of a=22.1837(4) A. Single-crystal X-ray data allowed solution of the crystal structure in the subcell with a unit-cell parameter of a(0)=11.092(1) A and the space group Pm(-)3n (R=5.7 %). Sn(20.5)As(22)I(8) (or Sn(20.5) square(3.5)As(22)I(8), accounting for the vacancies in the framework) possesses the clathrate-I type crystal structure, with iodine atoms occupying the cages of the cationic framework composed of tin and arsenic atoms. The crystal structure is strongly disordered. The main features are a random distribution of vacancies, and shifts of the tin and arsenic atoms away from their ideal positions. The coordination of the tin atoms has been confirmed by using (119)Sn Mössbauer spectroscopy. Electron diffraction and high-resolution electron microscopy (HREM) analyses have confirmed the presence of the superstructure ordering, which results in a doubling of the unit-cell parameter and a change of the space group from Pm(-)3n to either F23 or Fm(-)3. Analysis of the crystal structure has led to the construction of four ordering models for the superstructure, which have been corroborated by HREM, and has also led to the identification of disordered regions originating from overlap of the different types of ordered domains. Sn(20.5)As(22)I(8) is a diamagnetic semiconductor with an estimated band gap of 0.45 eV; it displays abnormally low thermal conductivity, with the room temperature value being just 0.5 W m(-1) K(-1).
authors: Inorganic Synthesis Laboratory, Department of Chemistry, Moscow State University, 119992 Moscow, Russia.
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