Crystal Structures, Phase Transition, and Optical Properties of Two Rare Earth Borate Fluorides: α- and β-BaGdBO₃F₂

Borate materials possess diverse crystal structures and exhibit outstanding physical properties, making them highly suitable for optical applications. In this work, two new compounds, α-BaGdBO₃F₂ and β-BaGdBO₃F₂, were synthesized by the high-temperature solid solution method and a temperature-controlled phase transition from α-BaGdBO₃F₂ to β-BaGdBO₃F₂ was observed. In the crystal structures of α-BaGdBO₃F₂ and β-BaGdBO₃F₂, the [BO₃] basic building unit and the [GdO₆F₂] polyhedra are interconnected to form a complex three-dimensional network. The title compounds exhibit good thermal stability, a short ultraviolet cutoff of 274 nm, and a moderately large birefringence (0.072 and 0.059@ 546 nm). The DFT calculations also reveal that β-BaGdBO₃F₂ could be a potential nonlinear optical (NLO) material. The results indicate that α-BaGdBO₃F₂ and β-BaGdBO₃F₂ are promising candidates for UV birefringent and NLO crystal materials.