Structural studies of charge disproportionation and magnetic order inCaFeO3

The crystal and magnetic structures of ${\mathrm{CaFeO}}_{3}$ have been determined at 300 and 15 K using synchrotron x-ray and neutron powder-diffraction techniques. At 300 K, ${\mathrm{CaFeO}}_{3}$ adopts the ${\mathrm{GdFeO}}_{3}$ struture, space group Pbnm with unit-cell dimensions $a=5.32630(4),$ $b=5.35270(4),$ and $c=7.53986(6)\AA{}.$ This structure is distorted from the ideal perovskite structure by tilting of the ${\mathrm{FeO}}_{6}$ octahedra about [110] and [001]. The average Fe-O distance is 1.922(2) \AA{}, and the Fe-O-Fe angles are 158.4(2)\ifmmode^\circ\else\textdegree\fi{} and 158.1(1)\ifmmode^\circ\else\textdegree\fi{}. At 15 K the crystal structure belongs to space group ${P2}_{1}/n$ with $a=5.31182(3),$ $b=5.34775(4),$ $c=7.52058(5)\AA{}$ and $\ensuremath{\beta}=90.065(1)\ifmmode^\circ\else\textdegree\fi{},$ and contains two distinct Fe sites. The average Fe-O bond length is 1.872(6) \AA{} about the one iron site, and 1.974(6) \AA{} about the second site, with bond valence sums of 4.58 and 3.48, respectively. This provides quantitative evidence for charge disproportionation, ${2\mathrm{F}\mathrm{e}}^{4+}{\ensuremath{\rightarrow}\mathrm{Fe}}^{3+}{+\mathrm{F}\mathrm{e}}^{5+},$ at low temperature. The temperature evolution of the lattice parameters indicates a second- (or higher-) order phase transition from the orthorhombic charge-delocalized state to the monoclinic charge-disproportionated state, beginning just below room temperature. The magnetic structure at 15 K is incommensurate, having a modulation vector [\ensuremath{\delta},0,\ensuremath{\delta}] with \ensuremath{\delta} \ensuremath{\sim}0.322, corresponding to one of the 〈111〉 directions in the pseudocubic cell. A reasonable fit to the magnetic intensities is obtained with the recently proposed screw spiral structure [S. Kawasaki et al., J. Phys. Soc. Jpn. 67, 1529 (1998)], with Fe moments of 3.5 and $2.5{\ensuremath{\mu}}_{B},$ respectively. However, a comparable fit is given by a sinusoidal amplitude-modulated model in which the Fe moments are directed along [010], which leaves open the possibility that the true magnetic structure may be intermediate between the spiral and sinusoidal models (a fan structure).