Crystal structure and phase transition mechanisms in CsFe2F6

For the first time, structural phase transitions induced by the temperature were found in A xMxIIM(1−x)IIIF3 fluorides with the defect pyrochlore structure (Fd View the MathML source3¯m , Z =8). The room temperature structure of CsFe2F6 was determined using X-ray powder diffraction technique. This phase was found to be ordered with the Pnma space group. The study of the temperature stability of orthorhombic structure by differential scanning calorimeter between 100 K and 700 K has shown a succession of phase transitions. The Pnma (Z =4)→Imma (Z =4)→I 41/amd (Z =4)→Fd View the MathML source3¯m (Z=8) structural sequence was proposed to occur within a rather narrow temperature range 500–560 K. The mechanism of structural transition has been mainly associated with the rotation of (MF6) octahedra and small displacements of some Fe atoms. This assumption is in good agreement with the low experimental entropy value, which is characteristic for displacive transformations.