Birefringence Disparity Induced by Synergistic Effects of Stereochemically Active Lone Pairs

Birefringent crystals with significant optical anisotropy have played a pivotal role in laser technology and scientific research by modulating and controlling light polarization. In this study, we have successfully synthesized three new birefringent materials with mixed-valence antimony, namely, KSb3O6, RbSb3O6, and α-Sb2O4, through the introduction of Sb3+ with stereochemically active lone pairs (SCALP) to the total oxygen system using a high-temperature solution method. To the best of our knowledge, K/RbSb3O6 represents the first alkali metal mixed-valence Sb-based oxide. Interestingly, despite their similar sandwich structure, these materials exhibit vastly different levels of birefringence (almost 10 times difference). It is worth noting that α-Sb2O4 demonstrates the largest experimental birefringence (0.201 at 546 nm) among non-π-conjugated Sb-based oxides to date, which can be attributed to the synergistic effect of SCALP group distortion and arrangement. These findings hold valuable implications for guiding future efforts in designing and synthesizing large birefringent materials.