For
the composition–structure–property relationship,
material properties can often be changed or even adjusted through
some specific structural evolutions. Here, we report four antimony
phosphates, K3SbP2O9, K2NaSbP2O8, α-NaSb3P2O10, and β-NaSb3P2O10, obtained by using the high-temperature solution method. The four
compounds were in diverse space groups with isolated PO4 groups. The presence of Sb3+ cations with lone pairs
not only induces the crystal structure symmetry transformation but
also significantly enhances the birefringence. α-NaSb3P2O10 exhibits a significantly increased birefringence
which is about 60 times that of K3SbP2O9 due to the introduction of lone pairs in metal cations. The
useful strategies will have significant meaning for the discovery
of new materials with large birefringence in the future.
For traditional birefringent materials, the anion groups play the leading role in birefringence, while the alkali‐metal and alkaline‐earth‐metal cations are nearly inactive. This work proposes a strategy to activate the functions of the cation groups and uses the hydroxyborate systems for illustration. The experimental verification is carried out in the same anionic frame by replacing the traditional metal cations with the birefringence‐active units including planar groups of protonation [C(NH2)3], [C3N2H5], and [CN4H7]. The structures of four new hydroxyborates, α‐Rb2[B4O5(OH)4]·H2O, β‐Rb2[B4O5(OH)4]·H2O, [C(NH2)3]2[B4O5(OH)4]·H2O, and (CN4H7)B5O6(OH)4 are obtained for the first time, and [C(NH2)3]2[B4O5(OH)4]·H2O exhibits a markedly enlarged birefringence, which is about 2.75 times that of α‐Rb2[B4O5(OH)4]·H2O theoretically because the activated cations appear. This strategy breaks the dominant status of anions in the design of UV/deep ultraviolet materials with large birefringence and has significant implications for the rational design and discovery of materials with large birefringence in the future.
The enhancement mechanism of birefringence is very important to modulate optical anisotropy and materials design. Herein, the different cations extending from alkaline-earth to alkaline-earth, d 10 electron configuration, and 6s 2 lone pair cations are highlighted to explore the influence on the birefringence. A flexible fluorooxoborate framework from AEB 4 O 6 F 2 (AE = Ca, Sr) is adopted for UV/ deep-UV birefringent structures, namely, M II B 4 O 6 F 2 (M II = Be, Mg, Pb, Zn, Cd). The maximal enhancement on birefringence can reach 46.6 % with the cation substitution from Ca, Sr to Be, Mg (route-I), Pb (route-II), and Zn, Cd (route-III). The influence of the cation size, the stereochemically active lone pair, and the binding capability of metal cation polyhedra is investigated for the hierarchical improvement on birefringence. Significantly, the BeB 4 O 6 F 2 structure features the shortest UV cutoff edge 146 nm among the available anhydrous beryllium borates with birefringence over 0.1 at 1064 nm, and the PbB 4 O 6 F 2 structure has the shortest UV cutoff edge 194 nm within the reported anhydrous lead borates that hold birefringence larger than 0.1 at 1064 nm. This work sheds light on how metal cation polyhedra modulate birefringence, which suggests a credible design strategy to obtain desirable birefringent structures by cation control.
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