Electric relaxation and Mn.sup.3+/Mn.sup.4+ charge transfer in Fe-doped Bi.sub.12MnO.sub.20-BiMn.sub.2O.sub.5 structural self-composite

• Published in: Journal of materials science 2017-02, Vol.52 (4), p.2222
• Main Authors: Leonarska, A, Kadziolka-Gawel, M, Szeremeta, A. Z, Bujakiewicz-Koronska, R, Kalvane, A, Molak, A
• Format: Article
• Language: English
• Subjects: Activation energy; Analysis; Electric properties
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-016-0515-2

Fe-doped Bi.sub.12MnO.sub.20-BiMn.sub.2O.sub.5 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 Bi.sub.12MnO.sub.20 phase; and a = 7.545(4) Å, b = 8.538(1) Å, c = 5.758(3) Å-for the orthorhombic Pbam BiMn.sub.2O.sub.5 phase. The .sup.57Fe Mössbauer spectrum, recorded at room temperature, has shown pure electronic quadrupolar split. The major doublets reflected the occurrence of Fe.sup.3+ ions distributed in two sites, i.e., octahedral Fe.sup.4+O.sub.6 and square pyramidal Fe.sup.3+O.sub.5, with preferential occupation of the pyramidal sites, that was consistent with the Pbam phase symmetry. The third doublet resulted from the presence of iron Fe.sup.3+ in tetrahedral Fe.sup.3+O.sub.4 coordination and corresponded to a small admixture of the I23 phase. The DC resistivity [rho].sub.DC(T) dependence on temperature has shown thermally activated features, and the value of E.sub.a,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 E.sub.A,1 = 0.14-0.20 eV and characteristic time values of [tau].sub.01 = 10.sup.-10-10.sup.-12 s in 100-200 K range. It was attributed to the charge transfer between Mn.sup.4+/Mn.sup.3+ sites. The other relaxation occurred in the 170-220 K range, and it exhibited the following values: [tau].sub.02 = 10.sup.-11 s, and E.sub.A,2 = 0.27 eV. A disorder-related VRH polaron model was proposed for [rho].sub.DC(T) and for electric relaxation processes.