Electric relaxation and Mn3+/Mn4+ charge transfer in Fe-doped Bi12MnO20–BiMn2O5 structural self-composite

Abstract

Fe-doped Bi12MnO20–BiMn2O5 ceramics was sintered at 1130 K for 6 h in ambient air. Two centro-symmetric phases formed thermodynamically stable self-composite material that was deduced from X-ray pattern analysis. The lattice parameters were a = 10.147(8) Å—for the cubic I23 Bi12MnO20 phase; and a = 7.545(4) Å, b = 8.538(1) Å, c = 5.758(3) Å—for the orthorhombic Pbam BiMn2O5 phase. The 57Fe Mössbauer spectrum, recorded at room temperature, has shown pure electronic quadrupolar split. The major doublets reflected the occurrence of Fe3+ ions distributed in two sites, i.e., octahedral Fe4+O6 and square pyramidal Fe3+O5, with preferential occupation of the pyramidal sites, that was consistent with the Pbam phase symmetry. The third doublet resulted from the presence of iron Fe3+ in tetrahedral Fe3+O4 coordination and corresponded to a small admixture of the I23 phase. The DC resistivity ρDC(T) dependence on temperature has shown thermally activated features, and the value of Ea,DC varied in the range of 0.22–0.37 eV. The electric impedance was measured in the f = 20 Hz–1 MHz and 100–690 K range. Two electrical relaxations were determined using the electric modulus formalism M″(T). Low-temperature relaxation has shown the temperature-dependent activation energy EA,1 = 0.14–0.20 eV and characteristic time values of τ01 = 10−10–10−12 s in 100–200 K range. It was attributed to the charge transfer between Mn4+/Mn3+ sites. The other relaxation occurred in the 170–220 K range, and it exhibited the following values: τ02 = 10−11 s, and EA,2 = 0.27 eV. A disorder-related VRH polaron model was proposed for ρDC(T) and for electric relaxation processes.