Structure and Magnetic Properties of a Nanosized Iron-Doped Bismuth Titanate Pyrochlore

Abstract

The nanosized (50–70 nm) pyrochlore Bi1.5Fe0.5Ti2O7−δ was prepared by a coprecipitation technique. Characterization of Bi1.5Fe0.5Ti2O7−δ was carried out by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Raman spectroscopy, Mössbauer spectroscopy, and magnetic susceptibility measurements. The study of Fe doping in Bi2Ti2O7 was performed by means of density functional theory (DFT) calculations. The nanosized Bi1.5Fe0.5Ti2O7−δ sample crystallizes in the structural type of pyrochlore (Fd3̅m). The distribution of Fe atoms over the sites of Bi and Ti was studied from DFT simulations and then confirmed by the XRD analysis and Mössbauer method. The local distribution, electronic structure, and magnetic behavior of nanosized Bi1.5Fe0.5Ti2O7−δ are determined by the local microstructure of the metastable nanosized sample. Based on the examination of the Mössbauer spectrum of the Bi1.5Fe0.5Ti2O7−δ nanopowder, the following states of oxidation were revealed for iron atoms: Fe4+ in the titanium sites with a fraction of ∼5.7% and two states of Fe3+ (in the Bi and Ti sites) with different geometries of the oxygen surrounding. The ratio of Fe3+ distributed over the sites correlates well with the distribution in the ceramic sample. The presence of Fe4+ was found only in the nanosized Bi1.5Fe0.5Ti2O7−δ. The experimental effective magnetic moment of Fe atoms in the nanosized Bi1.5Fe0.5Ti2O7−δ appeared noticeably lower than that in the ceramic sample. The temperature dependence of μeff within the temperature range of 50–300 K is adequately described by the model of coexistence of Fe3+ and Fe4+ and the existence of clusters.

Synthesis and Oxygen Mobility of Bismuth Cerates and Titanates with Pyrochlore Structure

“…While compared with other Bi titanate-based pyrochlores, the oxygen tracer diffusion coefficient values of Y-doped Bi titanate are slightly lower than those for Sc-doped Bi titanate and significantly lower than those for Mg- and Zn-doped Bi titanates ( Figure 8 , curves 4–6) previously studied by authors [ 18 , 24 ]. This is probably due to the difference in cation size and charge as well as A-site stoichiometry (the samples from works [ 20 , 26 ] are A-site deficient) and, hence, oxygen vacancy content and space in the lattice for oxygen migration.…”

“…The isotope exchange was carried out in He + 1% C 18 O 2 + 1% Ar flow (25 mL min −1 ) while heating from 50 to 800 • C with a constant ramp of 5 • C min −1 . 18 O atomic fraction (α) and C 16 O 18 O atomic fraction (f 16-18 ) responses were analyzed in order to estimate isotope exchange kinetic parameters.…”

“…The Arrhenius plots of the oxygen tracer diffusion coefficient are given in Figure 8. Note: * means that this is a tracer diffusion coefficient related to the isotope tracer 18 O. It is related to the oxygen self-diffusion coefficient via a correlation factor f corresponding to the counterflows of isotopes 16 O and 18 O within the sample bulk (D* = f × DO, f ≈ 0.5, .…”

“…Oxides and solid solutions with the pyrochlore structure A 2 B 2 O 7 (or A 2 B 2 O 6 O’, where A and B are rare earth or transition elements) attracted a lot of attention as materials for many applications such as oxygen [ 1 , 2 , 3 , 4 ] and hydrogen [ 5 , 6 ] separation membranes, solid oxide fuel cell/electrolyzer electrolytes [ 1 , 7 , 8 , 9 ] and electrodes [ 8 , 10 , 11 ], catalysts for various transformation processes [ 12 , 13 ], pigments [ 14 , 15 ], etc. [ 16 , 17 , 18 ]. The prospects of using pyrochlores in various electrochemical devices are provided by their high ionic or mixed ionic-electronic conductivity, depending on their composition and synthesis conditions [ 1 , 8 , 9 , 10 , 11 , 19 , 20 ].…”