BaB4O5F4 with reversible phase transition featuring unprecedented fundamental building blocks of [B16O21F16] in the α-phase and [B4O6F4] in the β-phase

The first fluorooxoborate with reversible phase transition, BaB₄O₅F₄ with a large band gap and moderate birefringence, features the unprecedented fundamental building blocks of [B₁₆O₂₁F₁₆] in the α-phase and [B₄O₆F₄] in the β-phase.