First Tin Pnictide Halides Sn₂₄P_19.3I₈ and Sn₂₄As_19.3I₈: Synthesis and the Clathrate-I Type of the Crystal Structure

Sn24P19.3I8 (I) and Sn24As19.3I8 (II) have been prepared by a standard ampule synthesis. I crystallizes in a cubic space group Pm3̄n, a = 10.9540(10) Å, z = 1. The crystal structure of I is built of the 3D net composed of tin and phosphorus atoms, while iodine atoms occupy large polyhedral holes of two different types, pentagonal dodecahedral and tetrakaidodecahedral. An arrangement of such polyhedra follows that of the clathrate-I type. The 3D net has vacancies at one of the phosphorus atoms positions. The vacancies cause the split of the tin atomic position into two, having different coordination, which is reflected in the 119Sn Mössbauer spectrum. The vacancy concentration correlates well with the occupancy factors of the split tin atomic positions, and in accordance with the Zintl−Klemm formalism for valence compounds, I is a narrow-gap semiconductor. Powder diffraction data shows that II belongs to the same clathrate family, but has an 8 times larger face-centered cubic unit cell.