Structural study of the ferroelectric instability in Sn

The structural change occurring in Sn2P2Se6 between the paraelectric and the ferroelectric phases is investigated by means of X-ray diffraction. Details of the structure in both phases are obtained and the role of the Sn2+ lone pair is discussed in the light of structural data. In agreement with the 2/m to m symmetry lowering, polar displacements are found within the m plane away from a particular crystallographic direction but antiparallel displacements occur also. These results are discussed in the frame of the phenomenological theory which predicts a particular temperature dependence of the dielectric polarization in this crystal