Synthesis and characterization of hydrated alkali metal calcium nitrates ACa(NO ₃ ) ₃ ·2H ₂ O (A = NH ₄ , Rb, Cs) as short ultraviolet birefringent materials

The development of robust strategies for designing high-performance ultraviolet (UV) birefringent materials has become a critical challenge. The combination of alkali metal and Ca²⁺ cations, which do not exhibit d-d or f-f transitions, with [NO₃]⁻ anions that possess high polarizability anisotropy results in the synthesis of three hydrated alkali metal calcium nitrates, ACa(NO₃)₃·2H₂O (A = NH₄, Rb, Cs). The title compounds were synthesized at room temperature using the water solution-evaporation technique, featuring a unique chain-like structure composed of [CaO₉] polyhedra and [NO₃]⁻ units. The compounds exhibit notable optical properties, including short UV absorption edges (220-235 nm) and moderate birefringence (0.077-0.086 at 546 nm). Detailed structural investigation and theoretical analyses reveal that the birefringence is primarily due to the planar triangular arrangement of the [NO₃]⁻ groups. This research not only increases the structural diversity within inorganic materials but also provides a key approach for further studies of all-inorganic birefringent optical materials in the UV region.