Ge40.0Te5.3I8: synthesis, crystal structure, and properties of a new clathrate-I compound

Abstract

A new clathrate phase with the composition Ge40.0Te 5.3I8 has been prepared and its structure deter- mined from single crystal X-ray data. It crystallizes in the cubic space group Pm3n with a unit cell parameter a ¼ 10.815(1) � and Z ¼ 1( R ¼ 0.031 for 272 indepen- dent reflections and 19 variables). Germanium and tellur- ium form a cationic clathrate framework hosting the guest iodine anions in cages of two different shapes. The catio- nic framework features a joint occupation on two of the three independent sites by germanium and tellurium, lead- ing to the formation of Ge- -Ge and Ge- -Te bonds and vacancies on the third position. Electron diffraction and high-resolution electron microscopy reveal that the bulk material is not uniform; it contains domains showing a tendency for segregation of tellurium and germanium within the 4-fold positions of the space group P23, which is a subgroup of Pm3n .G e 40.0Te 5.3I8 is a diamagnetic semiconductor with a band gap of about 0.8 eV and its composition agrees with the Zintl formalism for the charge-balanced compounds.