# Giant birefringence

When values of birefingence 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$_x$Cl$_{3−x}$ have fairly high birefringence that varies significantly in the optical spectrum.
Some transition metal oxyhalides: MoOCl$_4$, WOCl$_4$, have birefringence in the giant category and MoO$_2$Br$_2$, WOBr$_4$, NbOBr$_2$, and NbOI$_2$ are predicted to have birefringence over 0.6 at 1065 nm.

## List

| substance | formula | birefringence | band gap eV | comment |
| guanidinium hydrogen squarate | C(NH$_2$)$_3$(HC$_4$O$_4$) | 0.313@546 nm | | |
| | NbSe$_2$I$_2$ | 0.313 | | |
| | LiBF$_2$C$_2$O$_4$ | 0.317@546 nm | | |
| barium cyanurate | Ba$_3$(C$_3$N$_3$O$_3$)$_2$ | 0.32@800 nm | | |
| pentazinc dicyanurate tetrahydroxide | Zn$_5$(OH)$_4$(C$_3$N$_3$O$_3$)$_2$ | 0.32@400 nm | | |
| magnesium tetrazinc dicyanurate tetrahydroxide | MgZn$_4$(OH)$_4$(C$_3$N$_3$O$_3$)$_2$ | 0.32@400 nm | | |
| pyridinium antimony oxalate difluoride hydrate | [C(NH$_2$)$_3$]Sb(C$_2$O$_4$)F$_2$·H$_2$O | 0.323@546 nm | | |
| | Cs$_2$Sb$_2$(C$_2$O$_4$)$_2$-F$_4$·H$_2$O | 0.325@546 nm | | |
| | β-(C$_3$H$_7$N$_6$)$_2$Cl$_2$·H$_2$O | 0.33–0.38@550 nm | | |
| | (C$_3$H$_7$N$_6$)F·H$_2$O | 0.33–0.38@550 nm | | |
| scandium diiodate nitrate | Sc(IO$_3$)$_2$(NO$_3$) | 0.348 at 546 nm | | |
| potassium indium tetra(iso-cyamelurate) octadecahydrate | K$_{0.5}$In$_{0.5}$(H$_2$C$_6$N$_7$O$_3$)$_2$·9H$_2$O | 0.35@1064 nm | 4.05 eV | |
| calcium squarate | CaC$_4$O$_4$ | 0.35@1064 | | |
| tristrontium dicyanurate | β-Sr$_3$(C$_3$N$_3$O$_3$) | 0.35 | | |
| cerium difluoride sulfate | CeF$_2$(SO$_4$) | 0.361 | | |
| | Na$_4$Ba$_3$(S$_2$)$_4$S$_3$ | 0.37 at 1064 nm | | |
| guanidinium hydrogen oxalate hydrate | [C(NH$_2$)$_3$]HC$_2$O$_4$·H$_2$O | 0.371@532 nm | | |
| | Cs$_2$Pb$_4$Br$_{10}$ | 0.392 @ 550 nm | | |
| | RbNH$_4$(H$_2$C$_3$N$_3$O$_3$)$_2$·2H$_2$O | 0.40 @ 1064 nm | 5.24 | |
| dipotasium hydrogen trithiocyanate hemihydrate | K$_4$(HC$_3$N$_3$S$_3$)$_2$·H$_2$O | 0.402 @550 nm | | |
| | K$_{1.03}$(NH$_4$)$_{0.97}$(I$_5$O$_{12}$)(IO$_3$) | 0.405 @546 nm | | |
| LCHCY hydroisocyanurate | Li$_2$Ca(H$_2$C$_3$N$_3$O$_3$)$_4$·6H$_2$O | 0.407@800 nm | | |
| guanidinium dihydrogen cyanurate | C(NH$_2$)$_3$(H$_2$C$_3$N$_3$O$_3$) | 0.419@400 nm | | UV cutoff 238 nm |
| | (NH$_4$)$_2$(I$_5$O$_{12}$)(IO$_3$) | 0.431 @546 nm | | |
| tripotassium cyamelurate dihydrate | K$_3$C$_6$N$_7$O$_3$·2H$_2$O | 0.446@1064 nm | | |
| | Al$_4$(P$_2$S$_6$)$_3$ | 0.47 @ 2050 nm | | |
| sodium hydrogen squarate hydrate | NaHC$_4$O$_4$·H$_2$O | 0.52 at 1064 nm | | |
| | HgB$_2$S$_4$ | 0.52 at 1064 nm | | |
| tricaesium tricyanomelaminate hydrate | Cs$_3$C$_6$N$_9$•H$_2$O | 0.52@550 nm | | |
| | CrSbSe$_3$ | 0.56 at 650 nm | | |
| | Cs$_2$S$_6$ | 0.58@1064 nm | | |
| trithiocyanurate | Cs$_2$Mg(H$_2$C$_3$N$_3$S$_3$)$_4$·8H$_2$O | 0.58@800 nm | | UV cutoff 374 nm |
| | ZrTe$_5$ | 0.58 @ 942 nm | | |
| | Ba$_2$HgTe$_5$ | 0.643@2090 nm | 1.28 | |
| mercury hexathiodiphosphate | Hg$_2$P$_2$S$_6$ | 0.65@546 nm, 0.50 @ 1064 nm, 0.48 @2050 nm | | |
| | Ba$_6$Sb$_6$O$_2$S$_{13}$ | 0.66 at 2050 nm | | black; thermal conductivity of 0.25 W m$^{−1}$ K$^{−1}$ at 700 K |
| | Sn$_2$PO$_4$I | 0.664@546 nm | | |
| | Na$_2$BP$_2$ | 0.68 | | |
| hexagonal boron nitride | h-BN | 0.7 | | |
| | BaTiSe$_3$ | 0.7 | | |
| | BaTiS$_3$ | 0.76 | | |
| vanadium dioxide | VO$_2$ | >0.9 | | in thin film |
| sodium rhodizonate | Na$_2$C$_6$O$_6$ | 1.35@2500 | 1.6 | brown |
| molybdenum ditelluride | MoTe$_2$ | 1.54 mid IR | | |
| tungsten disulfide | WS$_2$ | 1.95 | | refractive indexes 4.96, 3.01 |
| | Sr$_{9/8}$TiS$_3$ | 2.1 in mid IR | | n$_e$ = 4.5 n$_o$ = 2.4 |