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

摘要

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.