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]
| 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] |
Fluorooxoborate is one of a series of anions or salts that contain boron linked to both oxygen and fluorine. Several structures are possible, rings, or chains. They contain [BOxF4−x](x+1)− units BOF32− BO2F23−, or BO3F14−. In addition there can be borate BO3 triangles and BO4 tetrahedrons. These can then be linked by sharing oxygen atoms, and when they do that, the negative charge is reduced. They are distinct from the fluoroborates in which fluorine is bonded to the metals rather than the boron atoms. For example, KBBF, KBe2BO3F2 is a fluoroborate and has more fluorine and oxygen than can be accommodated by the boron atom.
The borate fluorides or fluoroborates are compounds containing borate or complex borate ions along with fluoride ions that form salts with cations such as metals. They are in the broader category of mixed anion compounds. They are not to be confused with tetrafluoroborates (BF4) or the fluorooxoborates which have fluorine bonded to boron.
The borate carbonates are mixed anion compounds containing both borate and carbonate ions. Compared to mixed anion compounds containing halides, these are quite rare. They are hard to make, requiring higher temperatures, which are likely to decompose carbonate to carbon dioxide. The reason for the difficulty of formation is that when entering a crystal lattice, the anions have to be correctly located, and correctly oriented. They are also known as borocarbonates. Although these compounds have been termed carboborate, that word also refers to the C=B=C5− anion, or CB11H12− anion. This last anion should be called 1-carba-closo-dodecaborate or monocarba-closo-dodecaborate.
The iodate fluorides are chemical compounds which contain both iodate and fluoride anions (IO3− and F−). In these compounds fluorine is not bound to iodine as it is in fluoroiodates.
The sulfate fluorides are double salts that contain both sulfate and fluoride anions. They are in the class of mixed anion compounds. Some of these minerals are deposited in fumaroles.
A selenite fluoride is a chemical compound or salt that contains fluoride and selenite anions. These are mixed anion compounds. Some have third anions, including nitrate, molybdate, oxalate, selenate, silicate and tellurate.
The borosulfates are heteropoly anion compounds which have sulfate groups attached to boron atoms. Other possible terms are sulfatoborates or boron-sulfur oxides. The ratio of sulfate to borate reflects the degree of condensation. With [B(SO4)4]5- there is no condensation, each ion stands alone. In [B(SO4)3]3- the anions are linked into a chain, a chain of loops, or as [B2(SO4)6]6− in a cycle. Finally in [B(SO4)2]− the sulfate and borate tetrahedra are all linked into a two or three-dimensional network. These arrangements of oxygen around boron and sulfur can have forms resembling silicates. The first borosulfate to be discovered was K5[B(SO4)4] in 2012. Over 75 unique compounds are known.
The borophosphates are mixed anion compounds containing borate and phosphate anions, which may be joined together by a common oxygen atom. Compounds that contain water or hydroxy groups can also be included in the class of compounds.
The boroselenites are heteropoly anion chemical compounds containing selenite and borate groups linked by common oxygen atoms. They are not to be confused with the boroselenates with have a higher oxidation state for selenium, and extra oxygen. If selenium is replaced by sulfur, it would be a borosulfite. Boroselenites are distinct from selenoborates in which selenium replaces oxygen in borate, or perselenoborates which contain Se-Se bonds as well as Se-B bonds. The metal boroselenites were only discovered in 2012.
The borate chlorides are chemical compounds that contain both borate ions and chloride ions. They are mixed anion compounds. Many of them are minerals. Those minerals that crystallise with water (hydrates) may be found in evaporite deposits formed when mineral water has dried out.
The borate bromides are mixed anion compounds that contain borate and bromide anions. They are in the borate halide family of compounds which also includes borate fluorides, borate chlorides, and borate iodides.
Selenogallates are chemical compounds which contain anionic units of selenium connected to gallium. They can be considered as gallates where selenium substitutes for oxygen. Similar compounds include the thiogallates and selenostannates. They are in the category of chalcogenotrielates or more broadly chalcogenometallates.
Sulfidostannates, or thiostannates are chemical compounds containing anions composed of tin linked with sulfur. They can be considered as stannates with sulfur substituting for oxygen. Related compounds include the thiosilicates, and thiogermanates, and by varying the chalcogen: selenostannates, and tellurostannates. Oxothiostannates have oxygen in addition to sulfur. Thiostannates can be classed as chalcogenidometalates, thiometallates, chalcogenidotetrelates, thiotetrelates, and chalcogenidostannates. Tin is almost always in the +4 oxidation state in thiostannates, although a couple of mixed sulfides in the +2 state are known,
Arsenidosilicates are chemical compounds that contain anions with arsenic bonded to silicon. They are in the category of tetrelarsenides, pnictidosilicates, or tetrelpnictides. They can be classed as Zintl phases or intermetallics. They are analogous to the nitridosilicates, phosphidosilicates, arsenidogermanates, and arsenidostannates. They are distinct from arsenate silicates which have oxygen connected with arsenic and silicon, or arsenatosilicates with arsenate groups sharing oxygen with silicate.
Selenidogermanates are compounds with anions with selenium bound to germanium. They are analogous with germanates, thiogermanates, and telluridogermanates.
Iodate nitrates are mixed anion compounds that contain both iodate and nitrate anions.
Tellurogermanates or telluridogermanates are compounds with anions with tellurium bound to germanium. They are analogous with germanates, thiogermanates and selenidogermanates.
Selenite sulfates are mixed anion compounds containing both selenite (SeO3), and sulfate (SO4) anions.
A fluorooxoiodate or fluoroiodate is a chemical compound or ion derived from iodate, by substituting some of the oxygen by fluorine. They have iodine in the +5 oxidation state. The iodine atoms have a stereochemically active lone-pair of electrons. Many are non-centrosymmetric, and are second harmonic generators (SHG) of intense light shining through them. They are under investigation as materials for non-linear optics, such as for generating ultraviolet light from visible or infrared lasers.
Iodate sulfates are mixed anion compounds that contain both iodate and sulfate anions. Iodate sulfates have been investigated as optical second harmonic generators, and for separation of rare earth elements. Related compounds include the iodate selenates and chromate iodates.