Syntheses, Structures, and Characterization of Alkali-Metal Molybdates, A₃Cl[Mo₂O₇] (A = K, Rb and Cs)

Three new alkali-metal molybdates, A₃Cl[Mo₂O₇] (A = K, Rb, and Cs), have been successfully synthesized via a combined ionic and polyanionic substitution strategy with the antiperovskite Na₃Cl[MoO₄] as a structural template. Interestingly, all three compounds crystallize in the <i>P</i>6₃/<i>mmc</i> space group and adopt an unexpected honeycomb-like structure composed of alternating infinite [ClA₃]_∞ layers and Mo₂O₇ dimers. Comparative structural analysis with the template Na₃Cl[MoO₄] reveals that the substitution of isolated MoO₄ tetrahedra with <i>T</i>_<i>d</i> symmetry to corner-shared Mo₂O₇ dimers with <i>C</i>_3<i>v</i> symmetry is the key driving force for the transformation of structural framework from antiperovskite to honeycomb-like structure. Furthermore, theoretical calculations and optical property measurements show that A₃Cl[Mo₂O₇] (A = K, Rb, and Cs) compounds possess broad transparency windows spanning the visible to mid-infrared regions (0.3-10.5 μm) and significant birefringence (Δ<i>n</i> = 0.135-0.151 at 1064 nm). These results not only show that A₃Cl[Mo₂O₇] would be potential middle infrared optical functional crystals but also provide some insights for the design of new antiperovskite structures.