Synthesis and characterization of hydrated alkali metal calcium nitrates ACa(NO3)3·2H2O (A = NH4, Rb, Cs) as short ultraviolet birefringent materials

Abstract

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.