Structure and electron density distribution of CaFeO3 in the ''charge disproportionate'' state

Mossbauer spectroscopy on CaFeO3 suggests a ''charge disproportion'' reaction of Fe4+ into Fe3+ and Fe5+ below 290 K. Crystal structure was investigated at 298 and 113 K (in ''charge disproportionate'' state) by using a mono-chromated synchrotron radiation at KEK-PF in Tsukuba. The crystal structure is similar to GdFeO3 with Pbnm at 293 K. At 113 K, two kinds of nonequivalent Fe cation (which should be assigned to Fe3+ and Fe5+) form NaCl-type arrangement on Fe sub-lattice; the space group is monoclinic P2(1)/n which is one of the sub-groups of Pbnm. On the difference Fourier map, a residual peak is observed at the center of Fe-O bond; this suggests the strong covalent mixing between e(g)(Fe) levels and 2p(O) ones. The Fe-O-Fe bond angle in CaFeO3 is determined to be about 160 degrees, which significantly differs from the 180 degrees angle bonding in SrFeO3. It indicates that the e(g) bandwidth in CaFeO3 is narrowed and that eventually the itinerant nature is suppressed. Therefore, the charge disproportion tends to occur in CaFeO3 with the decrease in temperature.