Crystal structure and phase transition mechanisms in CsFe{sub 2}F{sub 6}

For the first time, structural phase transitions induced by the temperature were found in A{sub x}M{sub x}{sup II}M{sub (1−x)}{sup III}F{sub 3} fluorides with the defect pyrochlore structure (Fd3{sup ¯}m, Z=8). The room temperature structure of CsFe{sub 2}F{sub 6} 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)→I4{sub 1}/amd (Z=4)→Fd3{sup ¯}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 (MF{sub 6}) 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. - Graphical abstract: Mechanism of phase transition between the HT cubic form of CsFe{sub 2}F{sub 6} at 573 K (left) and the room temperature orthorhombic form at 298 K (right). The grey rectangles are clusters of five FeF{sub 6} octahedra. Highlights: ► Structural transition found for the first time in CsFe{sub 2}F{sub 6} with defect pyrochlore type. ► Fe{sup II} and Fe{sup III} atoms are ordered in room temperature Pnma form of CsFe{sub 2}F{sub 6}. ► Pnma(Z=4)→Imma(Z=4)→I4{sub 1}/amd(Z=4)→Fd-3m(Z=8) transition sequence is proposed. ► Structural transition due to rotation of MF{sub 6} groups+small displacements of Fe atoms. ► The low value of the entropy is in agreement with a displacive-type transition.