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[(UO 2 )B 5 O 8 (OH)F]·H 2 O ( NaUBOF-1 ), K[(UO 2 )B 5 O 8 (OH)F] ( KUBOF-1 ), K 11 [(UO 2 ) 6 B 24 O 36 F 22 )](H 2 BO 3 ) ( KUBOF-2 ), Rb[(UO 2 )B 5 O 8 (OH)F] ( RbUBOF-1 ), and Tl[(UO 2 )B 5 O 8 (OH)F] ( TlUBOF-1 ). A new neptunium(VI) fluoroborate that is isotypic with NaUBOF-1, Na[(NpO 2 )B 5 O 8 (OH)F]·H 2 O ( 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)O 2 2+ ) cation surrounded by BO 3 triangles and BO 4 tetrahedra to create an U(Np)O 8 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 BO 3 F tetrahedral units. The BO 3 F tetrahedra and additional BO 3 triangles extend from the actinyl polyborate layers and are directed approximately perpendicular to the layers. A novel actinyl borate layered topology was found in K 11 [(UO 2 ) 6 B 24 O 36 F 22 )](H 2 BO 3 ) ( KUBOF-2 ). Except for K[(UO 2 )B 5 O 8 (OH)F] ( KUBOF-1 ) and K 11 [(UO 2 ) 6 B 24 O 36 F 22 )](H 2 BO 3 ) ( KUBOF-2 ), all of the other actinide fluoroborate phases adopt noncentrosymmetric space groups. Tl[(UO 2 )B 5 O 8 (OH)F] ( TlUBOF-1 ), which can be obtained as a pure phase, displays second-harmonic generation of 532-nm light from 1064-nm light.