Assisting the Effective Design of Polar Iodates with Early Transition-Metal Oxide Fluoride Anions

Polar materials are of great technical interest but challenging to effectively synthesize. That is especially true for iodates, an important class of visible and mid-IR transparent nonlinear optical (NLO) materials. Aiming at developing a new design strategy for polar iodates, we successfully synthesized two sets of polymorphic early transition-metal (ETM) oxide-fluoride iodates, α- and β-Ba[VFO 2 (IO 3 ) 2 ] and α- and β-Ba 2 [VO 2 F 2 (IO 3 ) 2 ]IO 3, based on the distinct structure-directing properties of oxide-fluoride anions. α- and β-Ba[VFO 2 (IO 3 ) 2 ] contain the trans -[VFO 2 (IO 3 ) 2 ] 2– polyanion and crystallize in the nonpolar space groups Pbcn and P 2 1 2 1 2 1 . In contrast, α- and β-Ba 2 [VO 2 F 2 (IO 3 ) 2 ]IO 3 contain the cis -[VO 2 F 2 (IO 3 ) 2 ] 3– Λ-shaped polyanion and crystallize in the polar space groups Pna 2 1 and P 2 1, respectively. Detailed structural analyses show that the variable polar orientation of trans -[VFO 2 (IO 3 ) 2 ] 2– polyanions is the main cause of the nonpolar structures in α- and β-Ba[VFO 2 (IO 3 ) 2 ]. However, the Λ-shaped configuration of cis -[VO 2 F 2 (IO 3 ) 2 ] 3– polyanions can effectively guarantee the polar structures. Further property measurements show that polar α- and β-Ba 2 [VO 2 F 2 (IO 3 ) 2 ]IO 3 possess excellent NLO properties, including the large SHG responses (∼9 × KDP), wide visible and mid-IR transparent region (∼0.5–10.5 μm), and high thermal stability (up to 470 °C). Therefore, combining cis -directing oxide-fluoride anions and iodates is a viable strategy for the effective design of polar iodates.