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 $a=5.326\mathrm{}30\left(4\right),$ $b=5.352\mathrm{}70\left(4\right),$ and $c=7.539\mathrm{}86\left(6\right)\mathrm{}\AA .$ 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) Å, and the Fe-O-Fe angles are 158.4(2)° and 158.1(1)°. At 15 K the crystal structure belongs to space group ${P2\mathrm{}}_1/n$ with $a=5.311\mathrm{}82\left(3\right),$ $b=5.347\mathrm{}75\left(4\right),$ $c=7.520\mathrm{}58\left(5\right)\mathrm{}\AA$ and $\beta =90.065\mathrm{}\left(1\right)\mathrm{}°,$ and contains two distinct Fe sites. The average Fe-O bond length is 1.872(6) Å about the one iron site, and 1.974(6) Å about the second site, with bond valence sums of 4.58 and 3.48, respectively. This provides quantitative evidence for charge disproportionation, ${2\mathrm{F}\mathrm{e}}^{4+}{\to \mathrm{Fe}}^{3+}{+\mathrm{F}\mathrm{e}}^{5+},$ 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 [δ,0,δ] with δ ∼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 $2.5{\mu }_B,$ 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).