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Periodic Tendril Perversion and Helices in the AMoO2F3 (A = K, Rb, NH4, Tl) Family
journal contribution
posted on 2020-03-19, 18:51 authored by Justin
C. Hancock, Matthew L. Nisbet, Weiguo Zhang, P. Shiv Halasyamani, Kenneth R. PoeppelmeierAlthough 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.