Birefringent Properties of Two Sb(III)−Nb(V) Oxyfluorides: Rb4(SbF4)2(NbOF5) and Cs8Sb4Nb5O5F35

Balancing a wide optical band gap with large birefringence remains a key challenge in the design of short-wavelength birefringent materials. Herein, two Sb(III)−Nb(V) oxyfluorides, Rb4(SbF4)2(NbOF5) (I) and Cs8Sb4Nb5O5F35 (II), were investigated in terms of their crystal structures and optical properties. Compound I contains spatially isolated Sb- and Nb-centered units, whereas II features multiple inequivalent Sb−O/F and Nb−O/F coordination environments assembled into finite heterometallic aggregates. Both compounds exhibit relatively wide optical band gaps of 4.60 and 4.46 eV, respectively, while their birefringences reach 0.078 and 0.154 at 546 nm. Structural and theoretical analyses show that II contains a highly asymmetric [SbF3O] unit with pronounced local polarizability anisotropy, together with differentiated Nb−O/F coordination environments and a distinct organization of the anisotropic Sb- and Nb-centered units. These structural characteristics are consistent with its larger birefringence. These findings provide insight into the relationship between local coordination environments and optical properties in Sb(III)−Nb(V) oxyfluorides.