Distinctive modulation of optical anisotropy by halogens in α/β-Cd-P-X (X = Cl, Br, and I)

Birefringent materials are widely applied as photoelectric functional field devices to modulate the polarization of lasers. The introduction of a halogen into the structure of crystals could balance the relationship between the band gap E_g and nonlinear optical (NLO) coefficient owing to their outstanding electronegativity and control the optical anisotropy. In this work, the optical properties of phosphohalides α/β-Cd₂P₃X (X = Cl, Br, I) were studied. It was found that the birefringences of α/β-Cd₂P₃Cl (0.23/0.24 @ 1064 nm) are unexpectedly 8 times larger than those of α/β-Cd₂P₃I (0.04/0.03 @ 1064 nm). To find the optical property origins and explore the contributions of microscopic groups to the optical anisotropy and NLO responses in Cd-P-X (X = Cl, Br, I), the first-principles, real-space atom-cutting, and polarizability anisotropy analysis methods were used. This reveals that the electron distribution is susceptible to halogen electronegativity. Halogen atoms can modulate the polarization anisotropy of the active polyhedron and influence the birefringence significantly, owing to the synergistic effect of the anion size and strong covalent interactions between halogens and metal cations. This work clarifies the optical anisotropy origin mechanism and provides a general strategy for finding promising birefringent crystals in phosphohalide systems.