Centrosymmetric K2SO4·(SbF3)2 and noncentrosymmetric Rb2SO4·(SbF3)2 resulting from cooperative effects of lone pair and cation size

Abstract

By introducing Sb³⁺ cations with a stereochemically active lone pair in the sulfate system, two antimony fluoride sulfates with the same stoichiometric ratio, namely, K₂SO₄·(SbF₃)₂ and Rb₂SO₄·(SbF₃)₂, were successfully synthesized via mild hydrothermal reactions. The cooperative effects of the alkali metal cation size and lone pairs resulted in a different symmetry for these two title compounds. The compound K₂SO₄·(SbF₃)₂ crystallized in a centrosymmetric (CS) space group of P2₁/c, while Rb₂SO₄·(SbF₃)₂ crystallized in the noncentrosymmetric (NCS) space group of P2₁. Though both of these two compounds consisted of similar structural building units, [SbF₃O₂]⁴⁻ tetragonal pyramid complexes and [SO₄]²⁻ tetrahedra, they displayed discrepant helix columns in the crystal structure due to the different steric hindrance forces produced by the alkali metal cations. The alkali metal cations K⁺/Rb⁺ provided a charge balance and connected the different helix columns to form the three-dimensional framework. The detailed characterizations revealed that Rb₂SO₄·(SbF₃)₂ was a potential ultraviolet nonlinear optical material. In addition, theoretical calculations were performed.