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

Abstract

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[VFO₂(IO₃)₂] and α- and β-Ba₂[VO₂F₂(IO₃)₂]IO₃, based on the distinct structure-directing properties of oxide-fluoride anions. α- and β-Ba[VFO₂(IO₃)₂] contain the trans-[VFO₂(IO₃)₂]^2- polyanion and crystallize in the nonpolar space groups Pbcn and P2₁2₁2₁. In contrast, α- and β-Ba₂[VO₂F₂(IO₃)₂]IO₃ contain the cis-[VO₂F₂(IO₃)₂]^3- Λ-shaped polyanion and crystallize in the polar space groups Pna2₁ and P2₁, 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 α- and β-Ba[VFO₂(IO₃)₂]. However, the Λ-shaped configuration of cis-[VO₂F₂(IO₃)₂]^3- polyanions can effectively guarantee the polar structures. Further property measurements show that polar α- and β-Ba₂[VO₂F₂(IO₃)₂]IO₃ 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.