Barium borate

Barium borate
	Other names barium diborate, barium boron oxide, barium metaborate
Identifiers
CAS number	13701-59-2 Y,  26124-86-7 (monohydrate), 23436-05-7 (dihydrate)
ChemSpider	3642855 Y
EC number	237-222-4
Jmol-3D images	{{#if:[Ba+2].[O-]B=O.[O-]B=O|Image 1
SMILES [Ba+2].[O-]B=O.[O-]B=O
InChI InChI=1S/2BO2.Ba/c2*2-1-3;/q2*-1;+2 Y Key: QBLDFAIABQKINO-UHFFFAOYSA-N Y InChI=1/2BO2.Ba/c2*2-1-3;/q2*-1;+2 Key: QBLDFAIABQKINO-UHFFFAOYAX
Properties
Molecular formula	BaB2O4 or Ba(BO2)2
Molar mass	222.95
Appearance	white powder or colorless crystals
Odor	odorless
Density	3.85 g/cm3[1]
Melting point	1095 °C[2]
Solubility in hydrochloric acid	soluble
Refractive index (nD)	ne = 1.5534, no = 1.6776
Structure
Crystal structure	Rhombohedral, hR126[3]
Space group	R3c, No. 161
Lattice constant	a = 1.2529 nm, c = 1.274 nm
Hazards
MSDS	MSDS
Flash point	Non-flammable
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Barium borate is an inorganic compound, a borate of barium with a chemical formula BaB₂O₄ or Ba(BO₂)₂. It is available as a hydrate or dehydrated form, as white powder or colorless crystals. The crystals exist in the high-temperature α phase and low-temperature β phase, abbreviated as BBO; both phases are birefringent, and BBO is a common nonlinear optical material.

Properties

Crystal structure of BBO viewed nearly perpendicular to the c-axis. Colors: green – Ba, pink – B, red – O

BBO viewed along the c-axis.

Barium borate exists in two major crystalline forms, alpha and beta; the low-temperature beta phase converts into the alpha phase upon heating to 925 °C. β-Barium borate (BBO) differs from the α form by the positions of the barium ions within the crystal. Both phases are birefringent, however the α phase possesses centric symmetry and thus does not have the same nonlinear properties as the β phase.^[4]

Alpha barium borate, α-BaB₂O₄ is an optical material with a very wide optical transmission window from about 190 nm to 3500 nm. It has good mechanical properties and is a suitable material for high-power ultraviolet polarization optics.^[5] It can replace calcite, titanium dioxide or lithium niobate in Glan-Taylor prisms, Glan-Thompson prisms, walk-off beam splitters and other optical components. It has low hygroscopicity and its Mohs hardness is 4.5. Its damage threshold is 1 GW/cm² at 1064 nm and 500 MW/cm² at 355 nm.^[1]

Beta barium borate, β-BaB₂O₄, is a nonlinear optical material which is transparent in the range ~190–3300 nm. It can be used for Spontaneous parametric down-conversion. Its Mohs hardness is also 4.5.^[1][2]

Barium borate has strong negative uniaxial birefringence and can be phase-matched for type I (ooe) second harmonic generation from 409.6 to 3500 nm. The temperature sensitivity of the indices of refraction is low, leading to an unusually large (55 °C) temperature phase-matching bandwidth.^[2]

Synthesis

Barium borate can be prepared by reaction of an aqueous solution of boric acid with barium hydroxide. The prepared γ-barium borate contains water of crystallization that can not be completely removed by drying at 120 °C. Dehydrated γ-barium borate can be prepared by heating to 300–400 °C. Calcination at about 600–800 °C causes complete conversion to the β form. BBO prepared by this method does not contain trace amounts of BaB₂O₂^[6]

BBO crystals for nonlinear optics can be grown from fluxed melt of barium borate, sodium oxide and sodium chloride.^[7]

Thin films of barium borate can be prepared by MOCVD from barium(II) hydro-tri(1-pyrazolyl)borate. Different phases can be obtained depending on deposition temperatures.^[8] Thin films of beta-barium borate can be prepared by sol-gel synthesis.^[9]

Barium borate monohydrate is prepared from the solution of barium sulfide and sodium tetraborate. It is a white powder. It is used as an additive to e.g. paints as flame retardant, mold inhibitor, and corrosion inhibitor. It is also used as a white pigment.

Barium borate dihydrate is prepared from the solution of sodium metaborate and barium chloride at 90–95 °C. After cooling to room temperature, white powder is precipitated. Barium borate dihydrate loses water at above 140 °C. It is used as a flame retardant for paints, textiles, and paper.^[10]

Applications

BBO is a popular nonlinear optical crystal. Barium borate is a bactericide and fungicide.^[11] It is added to paints, coatings, adhesives, plastics, and paper products.

Barium borate is resistant to ultraviolet radiation. It can act as UV stabilizer for polyvinyl chloride.^[12]

The solubility of barium borate is a disadvantage when used as a pigment. Silica-coated powders are available. The alkaline properties and the anodic passivation properties of the borate ion enhance the anticorrosion performance. Commonly available barium metaborate pigment comes in three grades; Grade I is a barium metaborate itself, grade II is compounded with 27% zinc oxide, and grade III is compounded with 18% of zinc oxide and 29% calcium sulfate. Barium borate shows synergistic performance with zinc borate.^[13]

Barium borate is used as a flux in some barium titanate and lead zirconate EIA Class 2 dielectric ceramic formulations for ceramic capacitors, in amount of about 2%. The barium-boron ratio is critical for flux performance; BaB₂O₂ content adversely affects the performance of the flux.^[6][14]

Barium borate-fly ash glass can be used as radiation shielding. Such glasses are superior in performance to concrete and to other barium borate glasses.^[15]

References