K2SO4(NH3SO3)2 and Cs2SO4(NH3SO3)3: two deep-UV transparent sulfamate-sulfates with moderate birefringence

Abstract

In this work, polar tetrahedral [NH₃SO₃] and non-polar tetrahedral [SO₄]²⁻ groups were combined to synthesize two deep-UV transparent crystals K₂SO₄(NH₃SO₃)₂ and Cs₂SO₄(NH₃SO₃)₃ based on the solution evaporation method. Due to the presence of two types of tetrahedral groups with wide bandgap and alkali metal cations without d–d or f–f transitions, both crystals exhibit a deep-UV cutoff edge, approximately 190 nm. Significantly, the birefringence values of K₂SO₄(NH₃SO₃)₂ and Cs₂SO₄(NH₃SO₃)₃ are 0.0123 and 0.0153 @546 nm, respectively, which are remarkably improved compared to those of many conventional sulfate crystals. In addition, theoretical calculations were performed to analyze the relationships between the structures and optical properties.