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, alpha- and beta-Ba[VFO2(IO3)(2)] and alpha- and beta-Ba-2[VO2F2(IO3)(2)]IO3, based on the distinct structure-directing properties of oxide-fluoride anions. alpha- and beta-Ba[VFO2(IO3)(2)] contain the trans-[VFO2(IO3)(2)](2-) polyanion and crystallize in the nonpolar space groups Pbcn and P2(1)2(1)2(1). In contrast, alpha- and beta-Ba-2[VO2F2(IO3)(2)]IO3 contain the cis-[VO2F2(IO3)(2)](3-) Lambda-shaped polyanion and crystallize in the polar space groups Pna2(1) and P2(1), respectively. Detailed structural analyses show that the variable polar orientation of trans-[VFO2(IO3)(2)](2-) polyanions is the main cause of the nonpolar structures in alpha- and beta-Ba[VFO2(IO3)(2)]. However, the Lambda-shaped configuration of cis-[VO2F2(IO3)(2)](3-) polyanions can effectively guarantee the polar structures. Further property measurements show that polar alpha- and beta-Ba-2[VO2F2(IO3)(2)]IO3 possess excellent NLO properties, including the large SHG responses (similar to 9 X KDP), wide visible and mid-IR transparent region (similar to 0.5-10.5 mu m), and high thermal stability (up to 470 degrees C). Therefore, combining cis-directing oxide-fluoride anions and iodates is a viable strategy for the effective design of polar iodates.