Structure and temperature transformation of SnSe.: Stabilization of a new cubic phase Sn4Bi2Se7

We have studied the evolution of the orthorhombic structure of SnSe (space group Pnma) as a function of the temperature. The structure slightly changes below 723 K. From 723 K to 803 K some modifications take place and at temperatures higher than 813 K the structure becomes close to the TlI-type structure. The new space group is Cmcm. The transformation to NaCl structure can not be observed because it melts before. Mossbauer spectra confirm the presence of Sn(II) in a distorted coordination polyhedron. The study of the Sn1-3xBi2xSe series has let us to identify a new phase Sn4Bi2Se7, with a NaCl-type structure, close to the theoretical cubic SnSe. A Sn-119 Mossbauer study shows that the octahedral coordination polyhedra are slightly distorted at low temperature, X-ray diffraction confirms that this distortion is a local effect.