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.[1]

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 CsPbBrxCl3-x have fairly high birefringence that varies significantly in the optical spectrum.[2]

Some transition metal oxyhalides: MoOCl4, WOCl4, have birefringence in the giant category and MoO2Br2, WOBr4, NbOBr2, and NbOI2 are predicted to have birefringence over 0.6 at 1065 nm.[3]

List

edit
substance formula birefringence band gap eV comment reference
guanidinium hydrogen squarate C(NH2)3(HC4O4) 0.313@546 nm [4]
NbSe2I2 0.313 [5]
LiBF2C2O4 0.317@546 nm [6]
barium cyanurate Ba3(C3N3O3)2 0.32@800 nm [7]
pentazinc dicyanurate tetrahydroxide Zn5(OH)4(C3N3O3)2 0.32@400 nm [8]
magnesium tetrazinc dicyanurate tetrahydroxide MgZn4(OH)4(C3N3O3)2 0.32@400 nm [8]
pyridinium antimony oxalate difluoride hydrate [C(NH2)3]Sb(C2O4)F2·H2O 0.323@546 nm [9]
Cs2Sb2(C2O4)2-F4·H2O 0.325@546 nm [10]
β-(C3H7N6)2Cl2·H2O 0.33–0.38@550 nm [11]
(C3H7N6)F·H2O 0.33–0.38@550 nm [11]
scandium diiodate nitrate Sc(IO3)2(NO3) 0.348 at 546 nm [12]
potassium indium tetra(iso-cyamelurate) octadecahydrate K0.5In0.5(H2C6N7O3)2·9H2O 0.35@1064 nm 4.05 eV [13]
calcium squarate CaC4O4 0.35@1064 [14]
tristrontium dicyanurate β-Sr3(C3N3O3) 0.35 [14]
cerium difluoride sulfate CeF2(SO4) 0.361 [15]
Na4Ba3(S2)4S3 0.37 at 1064 nm [16]
guanidinium hydrogen oxalate hydrate [C(NH2)3]HC2O4·H2O 0.371@532 nm [17]
Cs2Pb4Br10 0.392 @ 550 nm [18]
RbNH4(H2C3N3O3)2·2H2O 0.40 @ 1064 nm 5.24 [19]
dipotasium hydrogen trithiocyanate hemihydrate K4(HC3N3S3)2·H2O 0.402 @550 nm [20]
K1.03(NH4)0.97(I5O12)(IO3) 0.405 @546 nm [21]
LCHCY hydroisocyanurate Li2Ca(H2C3N3O3)4·6H2O 0.407@800 nm [22]
guanidinium dihydrogen cyanurate C(NH2)3(H2C3N3O3) 0.419@400 nm UV cutoff 238 nm [23]
(NH4)2(I5O12)(IO3) 0.431 @546 nm [21]
tripotassium cyamelurate dihydrate K3C6N7O3·2H2O 0.446@1064 nm [24]
Al4(P2S6)3 0.47 @ 2050 nm [25]
sodium hydrogen squarate hydrate NaHC4O4·H2O 0.52 at 1064 nm [26]
HgB2S4 0.52 at 1064 nm [27]
tricaesium tricyanomelaminate hydrate Cs3C6N9•H2O 0.52@550 nm [28]
CrSbSe3 0.56 at 650 nm [29]
Cs2S6 0.58@1064 nm [16]
trithiocyanurate Cs2Mg(H2C3N3S3)4·8H2O 0.58@800 nm UV cutoff 374 nm [23]
ZrTe5 0.58 @ 942 nm [30]
Ba2HgTe5 0.643@2090 nm 1.28 [31]
mercury hexathiodiphosphate Hg2P2S6 0.65@546 nm,

0.50 @ 1064 nm,

0.48 @2050 nm

[25]
Ba6Sb6O2S13 0.66 at 2050 nm black; thermal conductivity of 0.25 W m−1 K−1 at 700 K [32]
Sn2PO4I 0.664@546 nm [33]
Na2BP2 0.68 [14]
hexagonal boron nitride h-BN 0.7 [34]
BaTiSe3 0.7 [35]
BaTiS3 0.76 [35][36]
vanadium dioxide VO2 >0.9 in thin film [37]
sodium rhodizonate Na2C6O6 1.35@2500 1.6 brown [14]
molybdenum ditelluride MoTe2 1.54 mid IR [38]
tungsten disulfide WS2 1.95 refractive indexes 4.96, 3.01 [39]
Sr9/8TiS3 2.1 in mid IR ne = 4.5 no = 2.4 [40]

References

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