When values of birefringence are very high, the property is termed giant birefringence which more generically is called giant optical anisotropy. Values for giant birefringence exceed 0.3. Much bigger numbers (over 2.0) are termed "colossal birefringence". These are achieved using nanostructures.
Some oxides, for example borate or iodate can have high birefringence. Also compounds containing C=O bonds have higher levels. These include oxalates, squarates and cyanurates. One trade-off is with band gap. If the band gap is small, then the material is not transparent to visible light, but can be transparent for infrared. Chalgogenides may have high birefringence, but only in the infrared. Halide perovskites such as CsPbBr<sub>x</sub>Cl<sub>3âÂÂx</sub> have fairly high birefringence that varies significantly in the optical spectrum.
Some transition metal oxyhalides: MoOCl<sub>4</sub>, WOCl<sub>4</sub>, have birefringence in the giant category and MoO<sub>2</sub>Br<sub>2</sub>, WOBr<sub>4</sub>, NbOBr<sub>2</sub>, and NbOI<sub>2</sub> are predicted to have birefringence over 0.6 at 1065 nm.