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

You-Yun Li; Hao-Long Chen; Guo-Ju Chen; Chia-Liang Kuo; Ping-Hung Hsieh; Weng-Sing Hwang

doi:10.3390/ma10040380

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

Materials (Basel). 2017 Apr 2;10(4):380. doi: 10.3390/ma10040380.

Authors

You-Yun Li¹, Hao-Long Chen², Guo-Ju Chen³, Chia-Liang Kuo⁴, Ping-Hung Hsieh⁵, Weng-Sing Hwang⁶

Affiliations

¹ Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan 70101, Taiwan. li.youyun@gmail.com.
² Department of Electronic Engineering, Kao Yuan University, No. 1821, Jhongshan Road, Lujhu District, Kaohsiung 82151, Taiwan. t11033@cc.kyu.edu.tw.
³ Department of Materials Science and Engineering, I-Shou University, No.1, Sec. 1, Syuecheng Road, Dashu District, Kaohsiung 84001, Taiwan. gjchen@isu.edu.tw.
⁴ Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan 70101, Taiwan. jdempire1981@gmail.com.
⁵ Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan 70101, Taiwan. hann0613@gmail.com.
⁶ Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan 70101, Taiwan. wshwang@mail.ncku.edu.tw.

Abstract

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 (T_c) via DSC. Two peaks of T_c were observed due to phase splitting [2], 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 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.

Keywords: Fe-doped lithium niobate; Rietveld refinement; combustion method; defect structure.