Ba3Mg3(BO3)3F3 polymorphs with reversible phase transition and high performances as ultraviolet nonlinear optical materials.

Nonlinear optical (NLO) materials are the vital components of future photoelectric technologies as they can broaden the tunable wavelength range supplied by common laser sources. However, the necessary prerequisites for a practical NLO material are rather strict. Accordingly, considerable efforts have been focused on finding potential NLO materials. Here we report two asymmetric beryllium-free borates Pna21- and P[Formula: see text]2m-Ba3Mg3(BO3)3F3 featuring NLO-favorable 2∞[Mg3O2F3(BO3)2] layered structures. The reversible phase transition among two polymorphs was demonstrated by multiple experimental tests. The optical measurements reveal that Pna21-Ba3Mg3(BO3)3F3 possesses the optical properties required for ultraviolet NLO applications. Remarkably, Pna21-Ba3Mg3(BO3)3F3 has a large laser damage threshold, a deep-ultraviolet cutoff edge, a favorable anisotropic thermal expansion as well as the capacity of insolubility in water. These optical properties can be comparable or superior to that of commercial NLO material β-BaB2O4, which make Pna21-Ba3Mg3(BO3)3F3 a promising ultraviolet NLO material.