From Holodirected to Hemidirected Coordination Activated by Oxygenation Strategy: A Facile Route to Long Wave Infrared Birefringent Crystal

Long wave infrared (LWIR) birefringent crystals are essential for infrared optical applications but such materials are rarely reported due to the requirements of large birefringence and wide infrared transmission range. This study proposes a facile route to create new LWIR birefringent crystal by oxygenation strategy in halides with cations containing lone pair electrons. Given the tendency of such cations to form holodirected geometry, a divalent oxygen was introduced to substitute the monovalent halogen to decrease the coordination number and activate the lone pair electrons. Our efforts in Rb⁺-Sb³⁺-Cl^- system result three new structures, namely, Rb₁₃Sb₈Cl₃₇, Rb₃Sb₂OCl₇ and Rb₂Sb₂OCl₆. The holodirected SbCl₆ in Rb₁₃Sb₈Cl₃₇ has been successfully translated to hemidirected SbOCl₄ in Rb₃Sb₂OCl₇ and Rb₂Sb₂OCl₆, which are the first examples in alkali metal antimony(III) oxyhalides. The birefringence of Rb₂Sb₂OCl₆ reached to 0.191@550 nm, which is 11.2 times that of Rb₁₃Sb₈Cl₃₇ (0.017@550 nm). Large-sized crystal of Rb₂Sb₂OCl₆ have been successfully grown (6×6×2 mm³). It can exhibit good transmission performance in the range of 0.4-13.5 μm, indicating its potential as a promising LWIR birefringent crystal. Our research not only opens up a new material system - alkali metal antimony(III) oxyhalides, but also provides a new strategy to create LWIR birefringent crystals.