β-barium Borate

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 was discovered and developed by Chen Chuangtian and others of the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences.

Properties

Barium borate exists in three major crystalline forms: alpha, beta, and gamma. 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 transparent in the range ~190–3300 nm. It can be used for spontaneous parametric down-conversion. Its Mohs hardness is also 4.5. The material exhibits a melting temperature of 1268 K, with anisotropic thermal expansion coefficients: $\alpha_ = 4 \times 10^ K^$ and $\alpha_ = 36 \times 10^ K^$ α₃₃ = 36 × 10⁻⁶ K⁻¹.

Gamma barium borate, γ-BaB₂O₄, discovered recently, was produced by heating beta barium borate 900 °C under 3 GPa of pressure. It was found to have a monoclinic crystal structure.

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.

Although the ambient-pressure α and β crystal phases contain only trigonal, sp² hybridized, boron, BBO glass has around 40% of the boron on tetrahedral, sp³ hybridized, sites. In the liquid state the relative fractions of sp² and sp³ boron are temperature-dependent, with the trigonal planar coordination favored at higher temperatures.

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(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.

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 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.

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.

Barium borate is an inorganic compound, a borate of barium with the chemical formula BaB$*O*$ or Ba(BO$*)*$. It is available in various forms, including hydrates, white powder, or colorless crystals. Its crystalline forms exist in two principal phases: a high-temperature alpha (alpha) phase and a low-temperature beta (beta) phase, abbreviated as BBO. Both phases are birefringent, but beta-BBO is a prominent nonlinear optical (NLO) material, widely used in laser technology for frequency conversion.

Polymorphs of Barium Borate

Barium borate exists in three known crystalline forms: alpha (alpha-BaB$*O*),beta(\beta$-BaB$*O*),andgamma(\gamma$-BaB$*O*$). The distinction between these phases is critical, especially for their optical applications.

Alpha Barium Borate ($\alpha$-BaB$*O*$)

alpha-BaB$*O*$ is the high-temperature phase of barium borate. It forms at temperatures above 925 °C and has a trigonal crystal structure (space group Rbar3c, point group bar3m).

* Symmetry: alpha-BBO possesses centric symmetry. This means its crystal structure has a center of inversion, which inherently prevents it from exhibiting second-order nonlinear optical properties. Therefore, alpha-BBO is not used for frequency conversion.
* Optical Properties: Despite lacking nonlinear optical properties, alpha-BBO is an excellent optical material due to its wide optical transmission window, ranging from approximately 190 nm in the ultraviolet to 3500 nm in the mid-infrared. It exhibits strong birefringence.
* Mechanical Properties: It has good mechanical properties and a Mohs hardness of 4.5. Its damage threshold is relatively high (e.g., 1 GW/cm$^2$ at 1064 nm and 500 MW/cm$^2$ at 355 nm). It also shows low hygroscopicity.
* Applications: alpha-BBO is primarily used for high-power ultraviolet polarization optics. It can serve as a replacement for other birefringent materials like calcite, titanium dioxide, or lithium niobate in components such as Glan–Taylor prisms, Glan–Thompson prisms, and walk-off beam splitters.

Beta Barium Borate (beta-BaB$*O*$)

beta-BaB$*O*$ is the low-temperature phase, and the one extensively used in nonlinear optics. It transforms into the alpha phase upon heating to 925 °C. The difference between the alpha and beta forms lies in the arrangement of the barium ions within the crystal lattice, as well as the orientation of the BO$_$ groups.

* Symmetry: Unlike alpha-BBO, beta-BBO belongs to a non-centrosymmetric trigonal crystal class (space group R3c, point group 3m). This lack of inversion symmetry is a prerequisite for second-order nonlinear optical effects like second harmonic generation.
* Optical Properties: beta-BBO is highly transparent in the range from approximately 189 nm (or 190 nm) in the ultraviolet to 3300 nm in the mid-infrared. It possesses strong negative uniaxial birefringence, which is essential for phase matching.
* Mechanical Properties: Its Mohs hardness is also 4.5.
* Thermal Properties: The material exhibits a melting temperature of 1268 K (1095 °C). Its thermal expansion coefficients are anisotropic: alpha_11=4times10−6 K$^$ and alpha_33=36times10−6 K$^$. This anisotropy must be considered during crystal growth and application. #### Gamma Barium Borate ($\gamma$-BaB$*O*$)

A third polymorph, gamma-BaB$*O*$, was discovered more recently. It is produced by heating beta-BaB$*O*$ to 900 °C under high pressure (around 3 GPa). This phase was found to have a monoclinic crystal structure.

Synthesis and Crystal Growth

Barium borate can be prepared by reacting an aqueous solution of boric acid with barium hydroxide. However, the methods for obtaining the specific polymorphs, particularly high-quality single crystals of beta-BBO for optical applications, are more specialized.

The alpha and beta crystalline phases contain only trigonal, sp2 hybridized boron. However, in the liquid state, the relative fractions of sp2 and sp3 boron are temperature-dependent, with trigonal planar coordination favored at higher temperatures.

Growth of beta-BBO Single Crystals

Large, high-quality beta-BBO single crystals are typically grown from a fluxed melt, primarily using the top-seeded solution growth (TSSG) method. This is necessary because beta-BBO undergoes a phase transition to alpha-BBO at 925 °C and melts incongruently at 1095 °C. This incongruent melting means it decomposes rather than melting uniformly, making conventional melt growth challenging.

* Flux Selection: A common flux mixture consists of barium borate, sodium oxide (Na$_$O), and sodium chloride (NaCl). The flux lowers the growth temperature below the phase transition point, allowing the beta-phase to crystallize directly from the solution.
* Growth Process: The process involves carefully controlling the temperature gradient and growth rate. A seed crystal is introduced into the saturated flux, and the temperature is slowly lowered, causing the crystal to grow.
* Challenges: Key challenges in beta-BBO crystal growth include the potential for flux inclusions within the crystal, which can degrade optical quality, and cracking due to anisotropic thermal expansion. Careful control of the growth parameters is crucial to minimize these defects.

Other Forms of Barium Borate

* Thin Films: Thin films of barium borate can be prepared by methods such as Metalorganic Chemical Vapor Deposition (MOCVD) from precursors like barium(II) hydro-tri(1-pyrazolyl)borate. Different phases can be obtained depending on deposition temperatures. Sol-gel synthesis has also been used to prepare thin films of beta-barium borate.
* Hydrates:
** Barium borate monohydrate is prepared from a solution of barium sulfide and sodium tetraborate. It is a white powder.
** Barium borate dihydrate is precipitated as a white powder from a solution of sodium metaborate and barium chloride at elevated temperatures (90–95 °C) followed by cooling. It loses water above 140 °C.

Nonlinear Optical Properties of beta-BBO

beta-BBO is a negative uniaxial crystal, meaning its ordinary refractive index (n_o) is greater than its extraordinary refractive index (n_e) (n_on_e). It exhibits strong birefringence, allowing for efficient phase matching.

* Transparency Range: beta-BBO is transparent from 189 nm to 3300 nm.
* Nonlinear Coefficients: Its effective nonlinear optical coefficient (d_eff) is high, varying with the specific phase-matching conditions. For instance, d_22 is one of its largest nonlinear coefficients.
* Phase Matching: beta-BBO can be critically phase-matched over a very broad range of wavelengths. This is achieved by carefully orienting the crystal relative to the input laser beam to satisfy the phase-matching condition.
** Type I Phase Matching (ooe): Two ordinary polarized fundamental photons combine to produce an extraordinary polarized second-harmonic photon. beta-BBO is commonly phase-matched for Type I SHG from 409.6 nm to 3500 nm (fundamental wavelength).
** Type II Phase Matching (eoe or oee): One ordinary and one extraordinary polarized fundamental photon combine to produce an extraordinary or ordinary polarized second-harmonic photon. Type II phase matching offers certain advantages in terms of acceptance bandwidths and is often used for OPO applications.
* Angular and Spectral Acceptance: beta-BBO has relatively good angular acceptance, though lower than LBO, and reasonable spectral acceptance.
* Walk-off: Due to its large birefringence, beta-BBO can suffer from significant walk-off effects, where the ordinary and extraordinary beams separate spatially as they propagate through the crystal. This limits the effective interaction length, especially for long crystals or tight focusing.
* Temperature Sensitivity: The temperature sensitivity of the refractive indices in beta-BBO is relatively low, leading to a broad temperature phase-matching bandwidth (e.g., typically around 55 °C for 1064 nm SHG). This characteristic makes it robust against temperature fluctuations in experimental setups.
* Sellmeier Equations: The refractive indices of beta-BBO at 20 °C can be described by Sellmeier equations (where lambda is in µm):
** n_o2=2.7359+frac0.01878lambda2−0.01822−0.01354lambda2
** n_e2=2.3753+frac0.01224lambda2−0.01667−0.01516lambda2

Applications

Nonlinear Optical Applications (beta-BBO)

beta-BBO is one of the most versatile nonlinear optical crystals, widely used for:

* Second Harmonic Generation (SHG): Efficiently converts fundamental laser wavelengths to their second harmonic. It is commonly used for doubling the frequency of Nd:YAG (1064 nm to 532 nm green), Ti:sapphire (800 nm to 400 nm blue/UV), and dye lasers to generate tunable visible and ultraviolet light. Its high damage threshold makes it suitable for high-power pulsed lasers.
* Third Harmonic Generation (THG): Effective for generating the third harmonic of Nd:YAG (1064 nm to 355 nm UV) and other near-infrared lasers. It is preferred for high-power UV generation due to its deep UV transparency and high damage threshold.
* Fourth and Fifth Harmonic Generation (FHG, FiHG): Extends frequency conversion into the deep UV, converting 1064 nm to 266 nm (fourth harmonic) and 213 nm (fifth harmonic). This is critical for applications requiring very short UV wavelengths, such as micromachining, spectroscopy, and lithography.
* Sum Frequency Generation (SFG) and Difference Frequency Generation (DFG): Utilized for mixing two different laser wavelengths to produce new tunable outputs, ranging from the UV to the mid-infrared. SFG is used to generate shorter wavelengths (e.g., UV), while DFG produces longer wavelengths (e.g., IR).
* Optical Parametric Oscillators (OPOs) and Optical Parametric Amplifiers (OPAs): beta-BBO is an excellent crystal for OPOs and OPAs, which produce broadly tunable radiation. When pumped by UV or visible lasers, BBO OPOs can generate tunable light from the visible (e.g., 400 nm) to the near-infrared (e.g., 3000 nm). Its broad phase-matching range and high damage threshold enable both picosecond and femtosecond OPOs/OPAs.
* Quantum Optics: beta-BBO is a fundamental material for generating entangled photon pairs and single photons through spontaneous parametric down-conversion (SPDC). This makes it indispensable in quantum entanglement experiments, quantum cryptography, and quantum computing research.
* Other NLO Applications: It is also used in certain applications involving optical parametric chirped pulse amplification (OPCPA) systems due to its broad gain bandwidth.

Applications of alpha-BBO

As noted, alpha-BBO is not suitable for nonlinear frequency conversion due to its centrosymmetric structure. However, its high birefringence, wide transparency, and high damage threshold make it an ideal material for:

* Polarization Optics: Used in components like Glan-Taylor prisms, Glan-Thompson prisms, and walk-off beam splitters to manage and manipulate polarized light, especially in the UV region.

Other Forms of Barium Borate in Industrial Use

* Hydrates (Monohydrate and Dihydrate): These forms are typically white powders.
** Used as additives in paints, coatings, adhesives, plastics, and paper products.
** Function as flame retardants, mold inhibitors, and corrosion inhibitors.
** Act as white pigments.
** Barium borate is resistant to ultraviolet radiation and can act as a UV stabilizer for polyvinyl chloride.
** Its alkaline properties and the anodic passivation properties of the borate ion enhance anticorrosion performance.
** Commercially available barium metaborate pigment comes in three grades: Grade I (pure barium metaborate), Grade II (compounded with 27% zinc oxide), and Grade III (compounded with 18% zinc oxide and 29% calcium sulfate). Barium borate shows synergistic performance with zinc borate.
* Glass¹ and Ceramics: Barium borate is used as a flux in some barium titanate and lead zirconate (EIA Class 2) dielectric ceramic formulations for ceramic capacitors (typically around 2% concentration). The barium-boron ratio is critical for flux performance, as BaB$*O*$ content can adversely affect performance. Barium-borate-fly ash glasses have also been investigated for radiation shielding, showing superior performance compared to concrete and other barium borate glasses.

Comparison with Other Nonlinear Optical Crystals

BBO is often compared with other prominent NLO crystals such as Lithium Triborate (LBO), Potassium Titanyl Phosphate (KTP), and Potassium Dihydrogen Phosphate (KDP), each with its own advantages and disadvantages:

* BBO vs. LBO:
** Transparency: BBO extends deeper into the UV (down to ~189 nm) than LBO (~160 nm), making it superior for deep UV generation.
** Damage: Threshold: Both have high damage thresholds, but LBO generally exhibits a higher bulk damage resistance, especially for long pulses and high average power, due to its non-hygroscopic nature and different growth mechanisms.
** Hygroscopicity: BBO is slightly hygroscopic and requires careful handling and protective coatings to prevent degradation from moisture, whereas LBO is non-hygroscopic.
** Walk-off and Acceptance Angle: BBO has a larger walk-off angle and narrower acceptance angle compared to LBO. This means BBO is more sensitive to beam divergence and angular alignment. LBO's non-critical phase matching (NCPM) capability (temperature-tuned, zero walk-off) is a significant advantage for CW and quasi-CW high-power applications, a feature BBO generally lacks for common wavelengths.
** Nonlinear Coefficient: BBO generally has higher effective nonlinear coefficients than LBO for many common applications, allowing for higher conversion efficiency in shorter crystals.
* BBO vs. KTP:
** Transparency: BBO has a much broader transparency range, extending into the UV where KTP (transparent from ~350 nm) is opaque.
** Damage Threshold: BBO typically has a higher damage threshold than KTP, making it more suitable for high-peak-power applications. KTP can be prone to "gray tracking" (photodarkening) at high power densities, particularly for green light generation.
** Hygroscopicity: KTP is non-hygroscopic, similar to LBO, making it robust in ambient conditions.
** Applications: KTP is often preferred for high-average-power green light generation (e.g., from Nd:YAG) in moderate power systems due to its low walk-off and high thermal stability. BBO, however, is dominant for UV generation and high-peak-power OPOs.
* BBO vs. KDP:
** Nonlinear Coefficient: BBO has significantly higher nonlinear coefficients (around 6 times) compared to KDP, leading to much higher conversion efficiency.
** Damage Threshold: BBO has a higher damage threshold than KDP.
** Hygroscopicity: KDP is highly hygroscopic and requires hermetic sealing. BBO is less hygroscopic but still requires care.
** Transparency: Both have good UV transparency, but BBO generally extends deeper into the UV.
** Applications: KDP is often used for large aperture, high-energy applications (e.g., in inertial confinement fusion lasers) where crystal size is critical, and its lower nonlinear coefficient is compensated by very long crystal lengths. BBO is preferred for applications requiring high efficiency in smaller crystals.

Hazards

Barium borate carries certain hazards. According to GHS labeling, it is classified with an exclamation mark pictogram, indicating² it is a warning-level substance.

* Hazard statements: H302 (Harmful if swallowed).
* Precautionary statements: P264 (Wash hands thoroughly after handling), P270 (Do not eat, drink or smoke when using this product), P301+P312 (IF SWALLOWED: Call a POISON CENTER/doctor if you feel unwell), P330 (Rinse mouth), P501 (Dispose of contents/container in accordance with local/regional/national/international regulations). It is non-flammable.

References

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