From BaAlBO3F2 to BaAlB3O6F2 and BaAl2(B3O6)2F2: the enhancement of birefringence and band gap by extending the π-conjugated system combined with [Al–O/F] functional groups

The investigation of birefringent materials is predominantly focused on π-conjugated systems; however, a large band gap or deep ultraviolet (DUV) transmittance cannot be guaranteed. In this work, we propose an extended π-conjugated strategy to increase the birefringence while keeping the large band gap. In the scheme, two new fluoroaluminoborates, BaAlB3O6F2 and BaAl2(B3O6)2F2, are synthesized by combining the [BO3] group with the [AlO4F2] group and the [B3O6] group with the [AlO3F] group through a rational structural design. The experimental results indicate that both BaAlB3O6F2 and BaAl2(B3O6)2F2 exhibit shorter UV cutoff wavelengths (<200 nm) and larger birefringence (Δn > 0.07@546 nm). Notably, this is a new finding in the Ba–Al–B–O–F system since the discovery of the well-known nonlinear optical crystal BaAlBO3F2. In comparison with BaAlBO3F2, BaAlB3O6F2 and BaAl2(B3O6)2F2 exhibit a markedly elevated birefringence (0.0418 vs. 0.087/0.105 at 546 nm). Furthermore, BaAl2(B3O6)2F2 exhibits the largest band gap (7.87 eV) of all known F-containing aluminoborates. This work not only identifies two potential DUV birefringent materials, but also proposes a novel design approach for reconciling the conflicting properties between birefringence and bandgap.