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

Abstract

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.