OH^- absorption of Zn-doped LiNbO₃ single crystals after proton exchange

An investigation of the OH^- absorption spectra of Zn-doped LiNbO₃ single crystals with doping concentrations from 0.0 to 8.3 mol % after proton exchange (PE) is carried out. Before PE treatment, the absorption bands are found centered at approximately 3485 cm^-1 for the samples with Zn-doping concentrations below 7.5 mol %, whereas two distinct bands at 3505 and 3530 cm^-1 are clearly observed for the samples with Zn-doping concentrations above 7.5 mol %. After PE treatment, 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. However, for the highly Zn-doped samples, the lineshape and intensity of the 3530 cm^-1 mode remain the same during PE. 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, respectively.