Structural studies of charge disproportionation and magnetic order in CaFeO3

The crystal and magnetic structures of CaFeO3 have been determined at 300 and 15 K using synchrotron x-ray and neutron powder-diffraction techniques. At 300 K, CaFeO3 adopts: the GdFeO3 struture, space group P-bnm with unit-cell dimensions a = 5.326 30(4), b = 5.352 70(4), and c = 7.539 86(6) Angstrom. This structure is distorted from the ideal perovskite structure by tilting of the FeO6 octahedra about [110] and [001]. The average Fe-O distance is 1.922(2) Angstrom, and the Fe-O-Fe angles are 158.4(2)degrees and 158.1(1)degrees. At 15 K the crystal structure belongs to space group P2(1)/n with a = 5.311 82(3), b = 5.347 75(4), c = 7.520 58(5) Angstrom and beta = 90.065(1)degrees, and contains two distinct Fe sites. The average Fe-O bond length is 1.872(6) Angstrom about the one iron site, and 1.974(6) Angstrom about the second site. with bond valence sums of 4.58 and 3.48, respectively. This provides quantitative evidence for charge disproportionation. 2Fe(4+) --> Fe3+ + Fe5+, 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 [delta,0,delta] with delta similar to 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. Sec. Jpn. 67, 1529 (1998)], with Fe moments of 3.5 and 2.5 mu(B), respectively. However, a comparable lit 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).