Fluorooxoborate Explained

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 [BO_xF_4−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.

Common properties are a wide range of transparency from deep ultraviolet (DUV) to near infrared (NIR); non-linear optical response, meaning that a high-intensity light will have some power converted to double the frequency; birefringence. The transparency deep into the ultraviolet corresponds to a large band gap, whereby it takes a lot of energy to shift an electron in the material.

Compared to borates, fluorooxoborates are more likely to have lower dimensional structures such as layers or chains, as there are fewer oxygen connections available. Fluorooxoborates are more likely to be noncentrosymmetric as adding one fluorine atom to a boron reduces symmetry.

Liquid state

When dissolved, the nature of the fluorooxoborate ions present depends on the boron to oxygen ratio. At the lowest oxygen levels BF₄⁻ exists, and is converted by oxygen to F₃BOBF₃²⁻. With boron to oxygen ratios near 1, the cyclic B₃O₃F₆³⁻ ion predominates.^[1]

Compounds

B₁

BaBOF₃ contains a one-dimensional chain of -OBF₂O-⁻ and extra F⁻

KBe₂BO₃F₂ crystallises in layers with alanine. It has a monoclinic form with space group P2₁ a=8.5800 b=4.9668 c =9.4146 Å, and β =116.563°.^[2]

B₂

SnB₂O₃F₂ crystal system rhombohedral space group P3₁m 250 nm UV cutoff Z = 1, a = 4.5072 Å, c = 4.7624 Å; stable up to 325 °C. Above this it decomposes to BF₃ boric oxide and Sn₃[B₃O₇]F.^[3]

PbB₂O₃F₂ contains two-dimensional sheets of BFOB pairs connected by four oxygen atoms to adjacent units. crystal system rhombohedral space group P3₁m 220 nm UV cutoff

BaB₂O₃F₂ contains a double chain of -BO-O-BF-^[4] The formula weight is 244.96. The crystal system is monoclinic space group P2₁. Unit cell dimensions are a=4.455 Å b=4.265 Å c=9.239 Å β=91.104° V=175.5 Å³ Z=2 density =4.635 g/cm3. Band gap 7.00 eV. It is stable up to 610 °C, but above that decomposes, giving off BF₃ and forming BaB_4O7.^[4]

BiB₂O₄F contains one-dimensional chains.^[4]

BaCdBe₂(BO₃)₂F₂ to NaMgBe₂(BO₃)₂F have ultraviolet absorption edges below 200 nm.^[5]

B₃

Na₃B₃O₃F₆ contains isolated rings.^[4]

KB₃O₄F₂ has a monoclinic crystal with space group P2₁/n with unit cell parameters a=4.6437 b=17.243 c=6.4301 β=102.660°. It contains one-dimensional [B₃O₄F₂]_∞ chains which cause a high birefringence.^[6]

K₃B₃O₃F₆ is monoclinic, space group P2₁/n, a=9.76 b=6.931 c=11.86 β=91.78 Z=12 volume=802 ų density=2.582. Melts at 432 °C.^[7]

Li₂B₃O₄F₃ is an ionic conductor due to its large channels. It contains linear chains of B₃O₄F₃ rings.^[4] Its crystal structure is orthorhombic with space group P2₁2₁2₁. Unit cell a=4.891, b=8.734, and c=12.301 Å. The subunit is a boroxine ring with tetrahedra BO₂F₂ and BO₃F and a BO₃ triangle.^[8] K₂B₃O₄F₃, KNaB₃O₄F₃, and KCsB₃O₄F₃ form a similar structure but with space group Pbcn.^{[9] [10]}

Cs₃B₃O₃F₆ can be produced by heating CsBF₄, CsF, and H₃BO₃ together at 300 °C. Crystals are orthorhombic with space group Pbcn, and with unit cell dimensions a=10.66 b=12.74 c=7.47 Å, and unit cell volume 1014 ų. The density is 3.884. It contains rings of B₃O₃F₆^3-.^[11]

Rb[B₃O₃F₂(OH)₂], Ph₄P[B₃O₃F₂(OH)₂] and Ph₃MeP[B₃O₃F₂(OH)₂] all have high birefingence. The Rb salt has unit cell a=14.569 b=6.084 c=9.828 β=129.825°.^[12]

NaB₃O₄F(OH) contains one-dimensional chains of B₃O₄F(OH). It has a monoclinic crystal structure with space group P2₁/_c number 14 with unit cell sizes a=5.7958 Å b=8.7348 Å c=9.5409 Å, and β=94.128°.^[13] [C(NH₂)₃][B₃O₃F₂(OH)₂] and [C(NH₂)₃]₂[B₃O₃F₄(OH)], guanidinium salts are non-linear optical materials.^[14] Ammonium, rubidium, and caesium salts also exist: NH₄[B₃O₃F₄(OH)], Rb[B₃O₃F₄(OH)] and Cs[B₃O₃F₄(OH)]. Related substances include K_2.3Cs_0.7B₃O₃F₆ (KCsBOF), and Cs₃[B₃O₃(OH)₃]Cl₃.^[15]

B₄

NH₄B₄O₆F, (ABF) RbB₄O₆F (RBF), CsB₄O₆F, (CBF) For the rubidium and ammonium compounds the structure is orthorhombic. It contains borate sheets with fluorine pointing up and down into the between layers that contains the cations NH₄, Rb or Cs. The units in the sheet are a BO₃ triangle joined to a boroxine ring with an extra fluorine (B₃O₆F). Two oxygen atoms from the ring, and two from the triangle share with the adjacent units.^[4] The ammonium salt is stable to 300 °C, rubidium to 453 °C, and caesium to 609 °C.

KB₃O₄F₂ has a monoclinic crystal with space group P2₁/n with unit cell parameters a=4.6437 b=17.243 c=6.4301 β=102.660°. It contains one-dimensional [B₃O₄F₂]_∞ chains which cause a high birefringence.

KB₄O₆F is a hypothetical substance, predicted to have a short ultraviolet cut-off at 161 nm.^[16]

NaB₄O₆F contains [B4O6F]∞. It produces a second harmonic from light. UV cut off is 180 nm. The structure contains stacked layers, that contain B₃O₃ rings joined with BO₃ triangles. A larger ring, part of 3 small rings and three triangles, with B₉O₉ surrounds sodium atoms. A fluorine sticks out from each ring towards the sodium in the adjacent sheet. The crystal structure is monoclinic with space group C2. Formula weight 181.23; a = 11.39 Å b = 6.521 Å c = 8.030(6) Å β = 114.18° V=544.2 Z=4 Density=2.212; It decomposes by losing BF₃ when heated to 400 °C; It is pyroelectric, but not ferroelectric ^[17]

CaB₄O₆F₂ SrB₄O₆F₂ BaB₄O₆F₂ The structure contains stacked layers, that contain B₃O₃ rings joined with BO₃ triangles. A larger ring, part of 3 small rings and three triangles, with B₉O₉ surrounds the alkaline earth atoms. Two fluorine atoms stick out from each ring towards the metal in the adjacent sheet.^[17] Ca: formula weight 314.58; monoclinic P21/n, a=6.6384  Å, b=7.6733  Å, c=11.3385  Å, β=91.281°, V=579.31, Z=4, density=3.609^[18]

Cs₄B₄O₃F₁₀ contains tetrafluoroborate and six-membered B₃O₃F₆ rings. Its crystals are monoclinic and it has space group P2₁/c. It melts at 293 °C, the lowest for any fluorooxoborate compound.^[19]

B₅

CaB₅O₇F₃, Formula weight =263.13 Orthorhombic Cmc2₁ a = 9.93 Å b = 8.40 Å c = 7.97 Å Volume= 664 ų Z=4 Density=2.631; The structure contains a repeating pattern of alternate tiles of double rings B₅O₉F₃. This has 4 protruding oxygen atoms that are shared with neighbours. A large B₉O₉ ring encloses the calcium atom. These form sheets that are stacked on each other in the b direction. It is stable up to 640 °C.^[20]

SrB₅O₇F₃ The crystal structure is orthorhombic with space group Cmc2₁. The subunits are double rings B₅O₉F₃ which are fused at a boron and oxygen atom. There is a fluorine attached to this boron, and also to the boron atoms connected to the ring bridging oxygen. Four oxygen atoms connect from the outer boron atoms on the rings to join up with adjacent subunits to make a two-dimensional sheet. Optically it is a negative biaxial crystal. The birefringence is 0.070 at 1084 nm increasing to 0.075 at 400 nm. The ultraviolet cutoff limit is below 180 nm.^[21]

BaB₅O₈F·xH₂O with x≈0.17 Contains two boroxine rings interconnected at a common boron: B₃O₅²⁻ and B₃O₄F⁻. This substance is transparent from 180 nm to 1000 nm (UV to NIR). Optically it is a positive biaxial. The birefringence is around 0.06 in the visible light region of the spectrum, but increases sharply in the UV t 0.093 at 200 nm. Crystal structure: Orthorhombic, Pbca a = 11.399 Å b = 9.429 Å c = 13.467(4) Å Volume 1447.4 ų Z=8, Calculated density = 3.133 g·cm^‒3[22]

PbB₅O₈F^[22] This compound has a building block consisting of B₅O₁₀F⁶⁻ with a double boroxine ring with one shared boron. Each of the other boron atoms has a side oxygen connection that is shared with other of the building blocks. One of those boron atoms has a fluorine attached. The building block connect into two interpenetrating three-dimensional structures. The crystal structure is orthorhombic with space group Pbca; MW 408.24; a=10.885, b=9.108, c=13.576 Å, Z=8 Volume= 1345.9 Å³ Density = 2.938 ;It is a positivebiaxial crystal; Birefringence is 0.0685 at 1064 nm (NIR) to 0.0737 at 400 nm. The band gap is 5.23.^[23]

PbB₅O₇F₃ has a large birefringence and is a second harmonic generator.

Li₂Na_0.9K_0.1B₅O₈F₂ contains a two dimensional sheet composed of units which are a pair of borixine rings fused at one boron atom. Each ring has an extra fluorine atom, and shares two oxygen bridges to adjacent units.^[17]

B₆

LiB₆O₉F contains a unit with two boroxine rings linked with a shared oxygen. One ring has an extra fluorine, and each connects to two other units through shared oxygen.^[17] Crystal structure orthorhombic space group Pna21, a = 7.6555 Å, b = 8.5318 Å, c = 10.7894 Å, Z = 4^[24]

Li₂B₆O₉F₂ has a three dimensional network which is composed of units with fused boroxine ring pairs connected to a BF₂O₂ bridge.^[17] monoclinic space group Cc formula weight 260.74 a = 4.821 b = 16.149 c = 10.057 β = 92.003 V=782.5 Z=4 density=2.213. It is a lithium ion conductor.^[25]

Na₂B₆O₉F₂^[20] contains a unit with two boroxine rings linked with a shared oxygen. Each ring has an extra fluorine, and each connects to two other units through shared oxygen.^[17] Formula weight 292.84; monoclinic P2₁/c; a=8.196 b=13.001 c=7.896 β=90.750° V =841.3, Z=4.^[26]

K₃B₆O₉F₃(KBF), two dimensional sheets composed of units that have a pair of boroxine rings fused at one boron, one links to a tetrahedron BF₂O₂ bridge unit. The other ring has an extra fluorine atom and bridges via two oxygens to adjacent units.^[17] Crystal structure is monoclinic, space group P21/c formula weight 383.15; unit cell a=7.3898 b= 14.2142 c=10.2551Å β=93.419° Volume=1075.3 Z=4, density=2.367 namd gap=6.98 eV.^[27]

K₃Ba₃Li₂Al₄B₆O₂₀F has formula weight 1054.98; It has hexagonal structure space group P2c unit cell a=8.7547 c= 16.434 Å V=1090.8 Z=2^[28]

B₇

Na₃B₇O₁₁F₂ Contains a pair of boroxine rings interlinked by another ring with OBF₂O. The outer pair of rings is linked with extra oxygen atoms at each end to form a ladder-shaped structure. It has orthorhombic structure with space group Pnma; formula weight 717.28 a=9.2659 b= 16.3431 c= 6.6326 Å Volume 1004.40 ų Z= 2Density 2.372. It is transparent from 161 to 2500 nm.^[29]

B₈

CsKB₈O₁₂F₂^[20] It contains borate sheets with fluorine pointing up and down into the between layers which alternate with potassium in one layer and caesium in the other.^[17]

CsRbB₈O₁₂F₂^[20] It contains borate sheets with fluorine pointing up and down into the between layers which contain mixed rubidium and caesium.^[17]

B₁₀

K₃NaB₁₀O₁₆F₂ has band gap 6.93 eV and birefringence Δn = 0.047 at 1064 nm. It has unit cell parameters a=12.0020 b=12.3563 c=11.9778 Å β=117.839° and space group C2/c.^[30]

CaB₁₀O₁₄F₆

SrB₁₀O₁₄F₆^[29]

B₁₁

K₅B₁₁O₁₈F₂ has a B₁₁O₂₁F₂ building block and contains 14-membered rings. Crystals are triclinic with a=6.6221 b=10.320 Å c=14.046 α=100.086° β=95.406° γ=108.192°.

B₁₂

Na₂BaB₁₂O₁₈F₄ is transparent to under 190 nm. Its structure has two interpenetrating 3D networks made from BO₃ and BO₃F units. Crystals are orthorhombic, with space group pbcn, and unit cell dimensions: a = 12.9697 Å b = 7.9476 Å c = 16.2560 Å.^[31]

K₆B₁₂O₁₉F₄ has disorganised BO₃ and BO₄ units. It has space group P n m a with unit cell a = 15.291Å b = 7.707Å c = 8.672Å.^[32]

B₁₃

The compounds K₁₀B₁₃O₁₅F₁₉ and Rb₁₀B₁₃O₁₅F₁₉ contain B₁₀O₁₂F₁₃⁷⁻ and B₃O₃F₆³⁻ units.^[33]

Further reading

• Su . Hongkang . Yan . Ziting . Hou . Xueling . Zhang . Min . Fluorooxoborates: A precious treasure of deep-ultraviolet nonlinear optical materials . Chinese Journal of Structural Chemistry . March 2023 . 42 . 3 . 100027 . 10.1016/j.cjsc.2023.100027. 256609289 .

Notes and References

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