Double-Atom Substitution for Suppressing the THz Absorption of LiNbO 3

Lithium niobate (LiNbO3, LN) is a cornerstone inorganic nonlinear optical crystal for high-field terahertz (THz) generation. However, its application in broadband and high-frequency regimes is severely hindered by a low laser damage threshold and strong intrinsic phonon absorption above 4.5 THz. In this work, we propose a material-design strategy based on double-atom substitution to address these limitations. By synergistically replacing two Nb5+ cations with one tetravalent Ge4+ and one hexavalent Te6+, we synthesized Li2GeTeO6 (LGT), which possesses a structural framework closely related to LN but with the space group symmetry reduced from R3c to R3. A previous study has reported that LGT achieves a laser damage threshold of 1725 MW/cm2, approximately 17 times higher than that of conventional LN (Wang, D.; Zhang, Y.; Liu, Q.; Zhang, B.; Yang, D.; Wang, Y., Band Gap Modulation and Nonlinear Optical Properties of Quaternary Tellurates Li2GeTeO6. Dalton Trans. 2022, 51, 8955–8959). Through an interplay between far-infrared spectroscopy and density functional theory calculations, we further demonstrate that LGT remarkably extends the THz transparency window to 10.76 THz. Quantitative mode analysis reveals that the suppression of THz absorption originates from the near-complete cancellation of Born charge vectors associated with the opposing translational motions of Ge4+ and Te6+ ions in a simulated THz mode at 4.85 THz. Nevertheless, the Ge4+ and Te6+ cations are not Jahn–Teller active as the Nb5+ cations, consequently leading to an order-of-magnitude reduction of the nonlinear optical effect. Despite an intrinsic trade-off between absorption suppression and nonlinearity, this study elucidates the structure–property relationships of LN family materials and provides guidance for the rational design of next-generation high-performance and broadband THz source materials.