Discovery of crystals functionalized by optically active
units
is of great significance to laser science and industry since these
species have the potential for frequency conversion and light modulation.
This spurred researchers on to find new optical crystals with such
functionality, especially in new unexplored systems. Herein, inspired
by the idea of introducing different optically active units into one
structure, difluoro(oxalato)borate was defined as a new source of
short-wavelength ultraviolet (UV) optical crystals. Based on this,
three difluoro(oxalato)borates with [BF2C2O4] bifunctional units were synthesized and characterized. Among
them, NaBF2C2O4 shows a large nonlinear
optical effect (1.1 × KDP), a wide band gap (Eg = 4.48 eV), and a large birefringence (Δnexp. = 0.162@546.1 nm), making it a potential
short-wavelength UV nonlinear optical crystal. Among them, LiBF2C2O4 and Sr(BF2C2O4)(BF4), with giant birefringence (Δncal. = 0.317 and 0.130@546 nm) as well as wide
band gaps (5.54 and 4.76 eV), were characterized as short-wavelength
UV birefringent crystals. Theoretical analysis clarifies that [BF2C2O4] constructed by both π- and
non-π-conjugated units shows balanced microscopic properties
and that it can be regarded as bifunctional units to cause a considerable
nonlinear optical effect and birefringence simultaneously. Our study
confirms the feasibility of exploring optical crystals with balanced
performance in the difluoro(oxalato)borate system.