The enhancement of birefringence resulted from anionic dimensionality changes through adjusting cationic density

Single crystals of a new series of alkaline metal molybdates, LiNa₃(MoO₄)₂ and LiAMo₃O₁₀ (A = K⁺, Rb⁺, Cs⁺), have been successfully synthesized through conventional high-temperature solid-state reactions. The compounds are structurally characterized by single-crystal X-ray diffraction and exhibit three-dimensional (3D) crystal structures consisting of 0D or 1D Mo−O framework. Interestingly, when Mo−O dimensionality changes from 0D of LiNa₃(MoO₄)₂ to 1D of LiAMo₃O₁₀, the birefringence of LiAMo₃O₁₀ is significantly enhanced to as large as BaTeMo₂O₉ and Na₂TeW₂O₉. The differences of bandgaps induced by anionic-group dimensionality are also discussed based on structural and theoretical analyses, which illuminate that the bandgap tends to be relatively broad for compounds with isolated MoO₄ tetrahedra in alkaline metal molybdate family. In addition, the dimensional reduction formalism was applied in the alkaline metal molybdate system to explore the changes of Mo−O framework along with cation/Mo ratio for the first time. - Highlights: • A series of alkaline metal molybdates were prepared through solid-state reactions. • Dimensional reduction formalism is firstly used in alkaline metal molybdates. • Birefringence of Li(K, Rb, Cs)Mo₃O₁₀ is enhanced via adjusting cationic density. • Birefringence of Li(K, Rb, Cs)Mo₃O₁₀ is comparable with BaTeMo₂O₉ and Na₂TeW₂O₉. • The bandgap of compounds with isolated MoO₄ unit is broader in alkali molybdates.