Investigation of the Defect Structure of Congruent and Fe-Doped LiNbO(3) Powders Synthesized by the Combustion Method

Fe-doped LiNbO(3) synthesized by the combustion method to seek new multiferroic materials exhibits room-temperature ferromagnetism, as reported in our previous work. In this work, the defect structure of congruent and Fe-doped LiNbO(3) (0.57–3.3 mol %) powders was investigated in detail by several methods. The molar ratio of [Li]/([Li]+[Nb]) was determined by the Curie temperature (T(c)) via DSC. Two peaks of T(c) were observed due to phase splitting, and the phase at lower T(c) disappears as the Fe doping concentration increases. The coexistence of two different oxidation states of Fe ions in LiNbO(3) was probed by XPS and UV-Vis spectroscopy. The Raman spectra exhibit displacements along the c axis of Li and Nb ions, and a deformation of the NbO(6) framework owing to Fe doping. Several doping models were applied in the Rietveld refinement of powder X-ray diffraction collected by synchrotron radiation. The fitting by the Nb vacancy model leads to an improbably distorted structure of congruent LiNbO(3). In Fe-doped LiNbO(3), we conjecture that Li and Nb vacancies coexist in the lattice structure; Fe(+2)/Fe(+3) ions are substituted for Li ions at the regular Li site and may push the anti-site Nb(Li) ion back to the regular Nb site.