Deep-ultraviolet (deep-UV, wavelength below 200 nm) coherent light plays an important role in a number of advanced scientific instruments. KBe₂BO₃F₂ (KBBF) is the benchmark deep-UV NLO material, but it contains highly toxic beryllium and suffers a strong layering tendency, hindering its commercial applications.

Recent studies led by Prof. LUO Junhua and ZHAO Sangen from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences obtained a new beryllium-free deep-UV NLO material, K₃Ba₃Li₂Al₄B₆O₂₀F (KBLABF), by the substitution of toxic Be for Al & Li elements. KBLABF preserves the double-layered structure of the notable Sr₂Be₂B₂O₇ (Nature, 373, 322，1995).

In the structure of KBLABF, the [Li₂Al₄B₆O₂₀F]_∞ double-layers maintains the favorable arrangement of the NLO-active [BO₃]^3- groups in the structures of KBBF and Sr₂Be₂B₂O₇, thereby preserving the desirable optical propeties of the latter: a deep-UV transparent window, a powder SHG response (1.5 × KDP) comparable to that of KBBF, and phase-matchability. Furthermore, the [Li₂Al₄B₆O₂₀F]_∞ double-layers are tightly bridged via Ba―O bonds.

As a result, KBLABF exhibits no layering growth tendency, which is confirmed by the as-grown KBLABF single crystal with a thickness of about 8 mm in the c direction. More importantly, KBLABF eliminates the structural instability of Sr₂Be₂B₂O₇, which is confirmed by the small structural converge factor, obtainment of large-sized KBLABF single crytal, and phonon dispersion calculations (by co-authors Prof. LIN Zheshuai et al from Technical Institute of Physics and Chemistry, Chinese Academy of Sciences).

These attributes make KBLABF an attractive candidate for the next-generation deep-UV NLO materials. Moreover, the substitution of Be for Al & Li in beryllium borates provides a new opportunity to design beryllium-free deep-UV NLO materials with good performance.

The study entitled “Designing a Beryllium-Free Deep-Ultraviolet Nonlinear Optical Material without a Structural Instability Problem” has been published in J. Am. Chem. Soc. (2016, 138, 2961-2964).

The substitution of Be for Al & Li from Sr₂Be₂B₂O₇ led to a beryllium-free deep-UV NLO material, K₃Ba₃Li₂Al₄B₆O₂₀F, without a structural instability problem (Image by Prof. LUO Junhua's group)