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

A series of pseudosymmetrical structures of formula K₁₀(M₂O_<i>n</i>F_{11–<i>n</i>})₃X (M = V and Nb, <i>n</i> = 2, X = (F₂Cl)_1/3, Br, Br_4/2,I_4/2; M = Mo, <i>n</i> = 4, X = Cl, Br_4/2, I_4/2) 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(M₂O_<i>n</i>F_{11–<i>n</i>})₃X. These materials differ in their (non)­centrosymmetry despite chemical and structural similarities. The vanadium compounds are centrosymmetric (space groups <i>P</i>3̅<i>m</i>1 or <i>C</i>2/<i>m</i>) while the niobium and molybdenum heterotypes are noncentrosymmetric (<i>Pmn</i>2₁). 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