Cs-3[(BOP)(2)(B3O7)(3)]: A Deep-Ultraviolet Nonlinear Optical Crystal Designed by Optimizing Matching of Cation and Anion Groups

Deep-ultravioletnonlinear optical (DUV NLO) crystals are indispensablefor solid-state lasers to produce coherent light with wavelengthsshorter than 200 nm, yet their structure design still faces greatchallenges because two groups of conflicting properties must be satisfiedsimultaneously, i.e., "large second harmonic generation(SHG) response-large band gap" and "large birefringence-weak growth anisotropy". Clearly,hitherto, no crystal can perfectly satisfy these properties, includingKBe(2)BO(3)F(2). Herein, we design a newmixed-coordinated borophosphate Cs-3[(BOP)(2)(B3O7)(3)] (CBPO) by optimizing the matchingof cation and anion groups, which unprecedentedly achieves a balancefor two groups of contradictions concurrently for the first time.In the structure of CBPO, it has the coplanar and pi-conjugatedB(3)O(7) groups, which can make it possess a largeSHG response (3 x KDP) and large birefringence (0.075@532 nm).Then, terminal O atoms of these B3O7 groupsare connected by BO4 and PO4 tetrahedra, whicheliminates all dangling bonds and blue shifts the UV absorption edgeto the DUV region (165 nm). More importantly, owing to the judiciousselection of cations, the size of cations and void of anion groupsis a perfect match, which makes CBPO possess a very stable three-dimensionalanion framework and thus reduces the crystal growth anisotropy. A CBPO single crystal with a size of up to 20 x 17x 8 mm(3) has been successfully grown, through whicha DUV coherent light has also been achieved in Be-free DUV NLO crystalsfor the first time. These indicate CBPO will be the next generationof DUV NLO crystals.