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Spectroscopic study of Zn-doped LiNbO3 Crystals (Substitution Mechanism)
Lecturer: Professor Chia Chih-Ta, Department of Physics, National Taiwan Normal University, Taiwan
Date: August 13, 2009
Time: 13:00 - 15:00
Room: TS126
Abstract
(P.-C. Tsai, M.-L. Sun, C.-C. Lee, C.-T. Chia,* H.-F. Lu, M.-L. Hu, J.-F. Lee)
In this talk, Raman, EXAFS, FTIR, Proton Exchange, Thermal Effect, and Coercive Field experiments were utilized to determine the defect structure of ZnO-doped LiNbO3 single crystals. The calculation of hybrid density functional theory OH─ absorption mode and IFEFFIT EXAFS analysis fitting were also included. Raman and FTIR measurements are clearly found the three stages of Zn substitution in LiNbO3 single crystal for doping concentration from 0.8 to 8.3 mol. %. The Extended X-ray Absorption Fine Structure (EXAFS) measurement at room temperature gives a decisive conclusion that the Zn atoms are sited on the Li atomic sites of LiNbO3 crystal for all doping concentration. The coercive filed measurement gives the substitution mechanism for doping below 7.5 mol. % Zn doping. An investigation of the OH¯ absorption spectra of Zn-doped LiNbO3 single crystals after proton exchange (PE) was carried out for determine the substitution mechanism for Zn-doping above 7.5 mol.%. Before PE treatment, the absorption bands are found centered at approximately 3485 cm─1 for Zn doping below 7.5 mol % concentrations, whereas two distinct bands at 3505 and 3530 cm─1 are clearly observed above 7.5 mol %. After PE treatment for 7.5 mol. % Zn doping sample, an absorption band at 3505 cm─1 is predominant for all the samples, and this is attributed to the high concentration of H＋ ions substituting Li atoms. For highly Zn-doped samples, the lineshape and intensity of the 3530 cm─1 mode remain the same during PE. From Coercive Field (CF) measurement, large numbers of ZnLi＋atoms of highly Zn-doped samples were moved leading to change OH¯ spectra nearby Nb vacancy structure. For lower doping samples, only fewer NbLi4+ atoms can move, so lower intensities of the OH¯ absorption areas was found. A theoretical investigation using the hybrid density functional B3LYP method with a simple cluster structure shows that the origins of the 3485 and 3530 cm─1 absorption modes correspond to the Li- and Nb-vacancy models. Based on the summary of our experiments, we propose the VNb5─ model for highly doping Zn-doped LiNbO3. This model is consistent with the calculation of hybrid density functional theory OH¯ absorption mode and EXAFS results. The Nb vacancies should be considered to be an essential factor in influencing the physical properties of Zn-doped LiNbO3 at levels above 7.5 mol % doping concentration.
More information: Krisztián Kordás