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

Abstract

Abstract 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 Sn1−3x Bi2x Se series has let us to identify a new phase Sn4Bi2Se7, with a NaCl-type structure, close to the theoretical cubic SnSe. A 119Sn 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.