Periodic Tendril Perversion and Helices in the AMoO₂F₃ (A = K, Rb, NH₄, Tl) Family

Although compounds of the formula AMoO₂F₃ (A = K, Rb, Cs, NH₄, Tl) have been known for decades, crystal structures have only been reported for CsMoO₂F₃ and NH₄MoO₂F₃. The three compounds (Rb/NH₄/Tl)MoO₂F₃ are isostructural and crystallize in the centrosymmetric space group C2/c (No. 15). The compounds contain the MoO₂F₃^- anionic chain, composed of corner-sharing MoO₂F₄ octahedra, with Mo⁶⁺ coordinated by two cis bridging fluoride anions that are trans to terminal oxide anions. The MoO₂F₃^- chain has a very unusual and complex chain structure; a single chain contains alternating zigzag and helical sections. These helical regions alternate in chirality along the chain, and thus the chains exhibit periodic tendril perversion. To the best of the authors' knowledge, no other materials with a similar chain structure have been reported. On the other hand, KMoO₂F₃ is noncentrosymmetric and chiral, crystallizing in the enantiomorphic space group P2₁2₁2₁ (No. 19). KMoO₂F₃ also contains the MoO₂F₃^- anionic chain. However, the chain is helical, with only one enantiomer present, resulting in a chiral, noncentrosymmetric structure.