Borate

An oxygen-containing anion of boron. borates can contain trigonal $\mathrm{B}{\mathrm{O}}_3$ and tetrahedral $\mathrm{B}{\mathrm{O}}_4$ units joined by bridging oxygen.

Barium borate refers to barium-containing borate compounds, notably $\mathrm{Ba}{\mathrm{B}}_2{\mathrm{O}}_4$. Different structural phases can have different optical behavior.

Beta-barium borate is a nonlinear optical crystal used for frequency conversion. The chemical formula alone is therefore insufficient: the crystal phase and orientation determine which optical processes the material supports.

The low-temperature crystalline phase of barium borate, used as a nonlinear optical crystal. A properly phase-matched and compensated source can use it for spontaneous parametric down-conversion; the choice of collected modes and compensation determines the resulting polarization entanglement.

The anion $[{\mathrm{B}}_3{\mathrm{O}}_6{]}^{3-}$ formed from three trigonal $\mathrm{B}{\mathrm{O}}_3$ units sharing corners in an alternating six-membered B–O ring. Each boron also has one terminal oxygen.