(samarium fluoride) exhibits orthorhombic symmetry (space group Pnma; type) at room temperature and rhombohedral symmetry (space group type) at high temperature. In this paper we describe a study of the structural phase transition mechanism occurring in this compound performed by means of differential thermal analysis, x-ray diffraction and Raman scattering. The transition temperature was determined to be and the first-order character of the orthorhombic-rhombohedral phase transition was confirmed. The crystal structure of was refined in the two phases using a Rietveld powder diffraction method. The correlation between the coordination polyhedra appropriate to the two structure types has been established and a proposed transition mechanism is described in this paper. Moreover, a full vibrational investigation of both phases of is presented together with a group theory analysis.