Abstract :
Mössbauer studies of 57Fe-doped CaMn7O12 have been carried out over two paramagnetic ranges of temperature. The observed hyperfine parameters of 57Fe spectra were discussed assuming that Fe3+ cations are mainly substituted for manganese cations in six-coordinated oxygen polyhedra. In the first temperature range 90 K < T < 380 K , the values of quadrupole splitting ( Δ 1 ≫ Δ 2 ) are evidence for two types of structural distortion of (MnO6) polyhedra due to the ordering of Mn3+ and Mn4+ cations in (9d) and (3b) sites of a trigonal structure with a charge ordering. In the second temperature range 380 K < T < 460 K , the observed temperature evolution of the line shape in 57Fe spectra confirms two phenomena: (i) the structural phase transition of the trigonal phase to a high-temperature cubic structure, with the coexistence of both phases between 380 and 450 K; (ii) the existence of only non-distorted (MnO6) octahedra due to the fast electronic exchange between Mn3+ and Mn4+ cations.
