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. Mössbauer spectra confirm the presence of Sn(II) in a distorted coordination polyhedron. The study of the Sn 1−3 x Bi 2 x Se series has let us to identify a new phase Sn 4 Bi 2 Se 7 , with a NaCl-type structure, close to the theoretical cubic SnSe. A 119 Sn Mössbauer study shows that the octahedral coordination polyhedra are slightly distorted at low temperature. X-ray diffraction confirms that this distortion is a local effect.