Crystals with birefringent properties play an important role in modulating light polarization in the optical communication and laser industries. Today a lot of affords are applied to obtain a material that is not only birefringent but also is able to transmit a light in the deep ultraviolet (DUV) region. This could be achieved with wide band gap. Therefore, the ideal crystal for modern laser technologies must have large birefringent coefficient (Δn > 0.08) and a huge band gap (E_g > 4.43 eV). The lack of materials that fit these criteria underscores the importance of exploring novel DUV crystals with substantial birefringence.
Borates of rare-earth elements have already proven themselves as a stable material with promising optical properties. Among the main borates building blocks, the best optical characteristics has the [BO₃]^3- that make a main contribution to the Δn and E_g at the same time. Besides of the borate’s anion itself, the groups arrangement also contributes to the optical properties: the more ordered anions are the higher birefringent will be. There are only two main difficulties related with borates’ crystal synthesis: 1) to obtain a crystal we must apply high pressure; 2) [BO₃]^3-are not stable upon compression and transform to [BO₄]^5- around 10 GPa.
In the present study, we have applied methods of single-crystal X-ray diffraction in laser-heated diamond anvil cell to synthesize the novel type of lanthanum hydroxyl borate with unique arrangement of [BO₃]^3- groups.
The new compound La₂B₂O₅(OH)₂ was synthesized by the reaction of lanthanum hydroxide and boron oxide at extreme conditions (30 GPa and ~2400 K). The single-crystal X-ray structure determination revealed: P-3c, a = 6.555(2) Å, c = 17.485(8) Å, Z = 6, R₁ = 0.056. The three-dimensional structure consists of discrete planar [BO₃]^3- groups and three crystallographically different La ions: one is surrounded by 9, one by 10, and one by 12 oxygen anions. The band gap was estimated using ab initio calculations to be 4.64 eV at ambient pressure and 5.26 eV at 30 GPa. The current work describes the novel high-pressure/high-temperature lanthanum hydroxyl borate that can be used as a potential DUV birefringent material.

Olga Barkova

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