Birefringence\nDisparity Induced by Synergistic Effects\nof Stereochemically Active Lone Pairs

Birefringent crystals with significant optical anisotropy\nhave\nplayed a pivotal role in laser technology and scientific research\nby modulating and controlling light polarization. In this study, we\nhave successfully synthesized three new birefringent materials with\nmixed-valence antimony, namely, KSb<sub>3</sub>O<sub>6</sub>, RbSb<sub>3</sub>O<sub>6</sub>, and α-Sb<sub>2</sub>O<sub>4</sub>, through\nthe introduction of Sb<sup>3+</sup> with stereochemically active lone\npairs (SCALP) to the total oxygen system using a high-temperature\nsolution method. To the best of our knowledge, K/RbSb<sub>3</sub>O<sub>6</sub> represents the first alkali metal mixed-valence Sb-based\noxide. Interestingly, despite their similar sandwich structure, these\nmaterials exhibit vastly different levels of birefringence (almost\n10 times difference). It is worth noting that α-Sb<sub>2</sub>O<sub>4</sub> demonstrates the largest experimental birefringence\n(0.201 at 546 nm) among non-π-conjugated Sb-based oxides to\ndate, which can be attributed to the synergistic effect of SCALP group\ndistortion and arrangement. These findings hold valuable implications\nfor guiding future efforts in designing and synthesizing large birefringent\nmaterials.