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K3B3O4(OH)4·2H2O: A UV Nonlinear Optical Crystal with Isolated [B3O4(OH)4](3-) Anion Groups.

A potential ultraviolet (UV) nonlinear optical (NLO) material, K3B3O4(OH)4·2H2O, was successfully synthesized by hydrothermal methods. The compound crystallizes into the Cmc21 space group and exhibits isolated [B3O4(OH)4](3-) anion groups connected by O-H···O hydrogen bonds. The UV-vis diffuse reflectance spectrum shows that K3B3O4(OH)4·2H2O has a wide transparency range with an absorption edge below 190 nm. Powder second harmonic generation (SHG) measurements using 1064 nm radiation revealed a moderate efficiency, 0.8 × KDP. Additional particle size vs SHG efficiency measurements indicate that K3B3O4(OH)4·2H2O is type I phase-matchable. Our calculated results show that the borate groups as well as the waters of hydration determine the NLO properties of K3B3O4(OH)4·2H2O.