barium diborate, barium boron oxide, barium metaborate
|BaB2O4 or Ba(BO2)2|
|Appearance||white powder or colorless crystals|
|Melting point||1,095 °C (2,003 °F; 1,368 K)|
|Solubility in hydrochloric acid||soluble|
Refractive index (nD)
|ne = 1.5534, no = 1.6776|
|Crystal structure||Rhombohedral, hR126|
|Space group||R3c, No. 161|
|Lattice constant||a = 1.2529 nm, c = 1.274 nm|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
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.
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.
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. 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.
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.
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.
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
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. Thin films of beta-barium borate can be prepared by sol-gel synthesis.
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.
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.
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.
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