Enhanced Birefringence in Phosphates via “Two Steps in One”: d0 Transition Metal Regulation and Fluorination Synergistic Strategy

Birefringence is one of the key properties of optoelectronic functional crystals, which play an indispensable role in angle-phase matching and adjusting the polarization of light; materials with large birefringence in ultraviolet (UV) and deep-ultraviolet (DUV) regions are in urgent need. Phosphates exhibit broad transparency in the UV/DUV region; however, they generally have small birefringence due to the nonpolar symmetry of non-π-conjugated group [PO4]. To ameliorate the intrinsic shortcomings of phosphates and improve their birefringence, herein the "Two Steps in One" strategy is proposed. In the first step, by introducing the d0 transition metal (d0-TM) octahedral, a series of phosphates including d0-TM cations (Ti4+, Nb5+, V5+, and Mo6+) with second-order Jahn–Teller (SOJT) distortion effect are screened out from inorganic crystal structure database (ICSD), as compared to phosphates containing alkali metals and alkaline earth metals, the birefringence of the phosphates including d0-TM cations increases of one or two orders of magnitude (ranging from 0.07 to 0.20). To further explore the enhancement of birefringence, the traditional d0-TM octahedron [TiO6] is further fluorinated in step two, resulting in a birefringence improvement from LiTiOPO4 (Δn = 0.17) to F-containing TiPO4F (Δn = 0.20). To verify the effectiveness of our strategy and clarify the inner mechanism, first-principles analysis is conducted. The result shows that [TMO6] distorted octahedral groups and the fluorinated [TMOF] group are the main contributors to excellent birefringence. This study provides a further theoretical basis for exploring birefringent materials.