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 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 ultravioletpolarizationoptics. 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 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 Isecond-harmonic generation from 409.6 to 3500 nm. The temperature sensitivity of the indices of refraction is low, leading to an unusually large temperature phase-matching bandwidth.
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 BBO crystals for nonlinear optics can be grown from fluxed melt of barium borate, sodium oxide and sodium chloride. Thin films of barium borate can be prepared by MOCVD from barium hydro-triborate. 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.
Applications
BBO is a popular nonlinear optical crystal. Quantum linked photons are producible with beta barium borate. Barium borate is a bactericide and fungicide. 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. 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 zirconateEIA Class 2 dielectricceramic 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. 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.
