STRUCTURE CHANGE DUE TO THE CHARGE DISPROPORTIONATION IN CaFeO₃ WITH Fe⁴⁺ S. Morimoto, T. Yamanaka* and M. Tanaka**, Faculty of Engineering Science, Osaka Univ., Toyonaka 560, Japan., *Faculty of Science, Osaka Univ., Toyonaka 560, Japan., **Photon Factory, National Laboratory for High Energy Physics, Tsukuba 305, Japan.

Fe⁴⁺ in CaFeO₃ shows "charge disproportionation" into Fe³⁺ and Fe⁵⁺ at low temperature and the structure analysis is of great interest.

The crystal structures of CaFeO₃ have been investigated at 293 K and 113 K which is low temperature enough to exhibit the charge disproportionation of Fe⁴⁺. The cube-shaped crystalline samples with edge size of 0.06 mm were synthesized under 3 GPa and at around 1273 K. Data collections of diffraction intensity were carried out in the Photon Factory with the incident beam of ca. 0.7 A in wavelength.

CaFeO₃ was revealed to have a same structure as GdFeO₃ with Pbnm at 293 K [Z = 4, a = 5.3480(3), b* = a, c = 7.5730(5) A]. At 113 K, Fe³⁺ and Fe⁵⁺ are suggested to arrange NaCl type sub-lattice of Fe. Thereby the space group at 113 K is resulted to be monoclinic P2₁/n [Z = 4, a = 5.3409(3), b* = a, c = 7.5585(13) A, [[beta]]* = 90] which is one of sub-groups of Pbnm. The smaller bond angle (ca. 160 deg.) of Fe-O-Fe in CaFeO₃ than that in SrFeO₃ should suppress a itinerant nature of e_g(Fe) electrons and causes the disproportionation. The FeO₆ octahedron with negligibly small Jahn-Teller distortion can be regarded as an ideal O_h symmetry at both temperatures. This is explained by delocalization of e_g(Fe) electrons at 293 K and charge disproportionation at 113 K.

Residual charge densities were revealed by difference Fourier synthesis at both temperatures. A residual peak with 1.2 e/A³ as a maxima is confirmed clearly at the center of Fe-O bond. This suggests that strong covalency due to overlapping of e_g(Fe) orbitals and 2p(O) ones.

^*coincidence within experimental error