Polar Alignment of Λ‑Shaped Basic Building Units within Transition Metal Oxide Fluoride Materials

A series
of pseudosymmetrical structures of formula K10(M2OnF11–n)3X (M = V and Nb, n = 2, X = (F2Cl)1/3, Br, Br4/2,I4/2; M
= Mo, n = 4, X = Cl, Br4/2, I4/2) illustrates generation of polar structures with the use of Λ-shaped
basic building units (BBUs). For a compound to belong to a polar space
group, dipole moments of individual species must be partially aligned.
Incorporation of d0 early transition metal polyhedral BBUs
into structures is a common method to create polar structures, owing
to the second-order Jahn–Teller distortion these polyhedra
contain. Less attention has been spent examining how to align the
polar moments of BBUs. To address alignment, we present a study on
previously reported bimetallic BBUs and synthesized compounds K10(M2OnF11–n)3X. These materials differ in their (non)­centrosymmetry
despite chemical and structural similarities. The vanadium compounds
are centrosymmetric (space groups P3̅m1 or C2/m) while the
niobium and molybdenum heterotypes are noncentrosymmetric (Pmn21). The difference in symmetry occurs owing
to the presence of linear, bimetallic BBUs or Λ-shaped bimetallic
BBUs and related packing effects. These Λ-shaped BBUs form as
a consequence of the coordination environment around the bridging
anion of the metal oxide fluoride BBUs.