Functionalization of Borate Networks by the Incorporation of Fluoride: Syntheses, Crystal Structures, and Nonlinear Optical Properties of Novel Actinide Fluoroborates

The boric acid flux reactions of uranyl nitrate with sodium, potassium, rubidium, or thallium fluoride result in the formation of a novel family of uranyl(VI) fluoroborate materials. These compounds are Na[(UO2)B5O8(OH)F]·H2O (NaUBOF-1), K[(UO2)B5O8(OH)F] (KUBOF-1), K11[(UO2)6B24O36F22)](H2BO3) (KUBOF-2), Rb[(UO2)B5O8(OH)F] (RbUBOF-1), and Tl[(UO2)B5O8(OH)F] (TlUBOF-1). A new neptunium(VI) fluoroborate that is isotypic with NaUBOF-1, Na[(NpO2)B5O8(OH)F]·H2O (NaNpBOF-1), was synthesized via the boric acid flux reaction of neptunium(VI) nitrate with sodium fluoride. These new actinide fluoroborates share a common structural motif consisting of a linear actinyl (U(Np)O2 2+) cation surrounded by BO3 triangles and BO4 tetrahedra to create an U(Np)O8 hexagonal bipyramidal environment around uranium or neptunium. The borate anions bridge between actinyl units to create layers. B–F bonds were formed during the reactions to yield BO3F tetrahedral units. The BO3F tetrahedra and additional BO3 triangles extend from the actinyl polyborate layers and are directed approximately perpendicular to the layers. A novel actinyl borate layered topology was found in K11[(UO2)6B24O36F22)](H2BO3) (KUBOF-2). Except for K[(UO2)B5O8(OH)F] (KUBOF-1) and K11[(UO2)6B24O36F22)](H2BO3) (KUBOF-2), all of the other actinide fluoroborate phases adopt noncentrosymmetric space groups. Tl[(UO2)B5O8(OH)F] (TlUBOF-1), which can be obtained as a pure phase, displays second-harmonic generation of 532-nm light from 1064-nm light.