Abstract
We report a complete characterization of the crystal structure between 400 and 80 K for ( = Rm, Yb, and Lu) compounds using high resolution x-ray synchrotron powder diffraction. The three samples have a hexagonal structure (space group ) characterized by a sequence of double layers of mixed valence iron and oxygen atoms forming two-dimensional triangular layers separated by a single -O layer along the axis. This structure is stable down to 80 K for and though a sudden expansion in the axis is observed at around 300 K coupled to a variation in the electrical properties. However, exhibits two structural transitions upon cooling. The splitting of some reflections and the occurrence of superstructure peaks below 320 K reveal a structural phase transition. The unit cell is monoclinic (space group ), and there are four nonequivalent Fe sites with a maximum charge disproportionation of 0.5 . The hexagonal to monoclinic transition is characterized by a sudden expansion of the axis on cooling, and it seems to be driven by the condensation of modes. At lower temperatures (∼170 K) additional splitting of several peaks indicate that the unit cell is no longer monoclinic but triclinic (space group ). This transition is characterized by a contraction of the monoclinic plane, while the axis remains almost unchanged. There are six nonequivalent Fe sites in the triclinic cell, and the charge disproportionation magnitude is little affected.
2 More- Received 26 June 2014
- Revised 15 September 2014
DOI:https://doi.org/10.1103/PhysRevB.90.094119
©2014 American Physical Society