Investigation of the Defect Structure of Congruent and Fe-Doped LiNbO₃ Powders Synthesized by the Combustion Method.

Fe-doped LiNbO₃ synthesized by the combustion method to seek new multiferroic materials exhibits room-temperature ferromagnetism, as reported in our previous work [1]. In this work, the defect structure of congruent and Fe-doped LiNbO₃ (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 (Tc) via DSC. Two peaks of Tc were observed due to phase splitting [2], and the phase at lower Tc disappears as the Fe doping concentration increases. The coexistence of two different oxidation states of Fe ions in LiNbO₃ 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 NbO₆ 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 LiNbO₃. In Fe-doped LiNbO₃, 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 NbLi ion back to the regular Nb site.