The AFeO2 (A=K, Rb and Cs) family: A comparative study of structures and structural phase transitions

Structures and phase transitions for the isostructural series of compounds KFeO2, RbFeO2 and CsFeO2 have been systematically studied by synchrotron X-ray high resolution powder diffraction experiments and in case of CsFeO2 also by single crystal diffractometry. At room temperature, all of the three compounds crystallize in the orthorhombic (Pbca) KGaO2 type of structure consisting of a three dimensional network of corner-sharing [FeO4/2]− tetrahedra, which at elevated temperatures shows a reversible phase transformation to a cubic structure (space group Fd3¯m). For KFeO2, RbFeO2 and CsFeO2 this phase transformation takes place at 1003 K, 737 K and 350 K respectively, as confirmed by differential scanning calorimetry and X-ray diffraction. Upon heating through the transitions the major structural changes are driven by the onset or enhancement of librational motion of the FeO4 tetrahedra. Due to this phenomenon the Fe–O–Fe bonds appear to step-wise getting straight, seemingly approaching 180° within the time and space averaged structure.

Graphical abstractReciprocal layer hk0 for the orthorhombic room temperature phase, with pseudo cubic symmetry due to twinning, and the cubic high temperature phase of CsFeO2. Upon heating the major structural changes are driven by the enhancement of librational motion of the [FeO4/2]− corner-sharing tetrahedra, a phenomenon related to the high/low phase transition of cristobalite.Figure optionsDownload full-size imageDownload as PowerPoint slide