Polar alignment of Λ-shaped basic building units within transition metal oxide fluoride materials

A series of pseudosymmetrical structures of formula K₁₀(M ₂O_nF_11-n)₃X (M = V and Nb, n = 2, X = (F₂Cl)_1/3, Br, Br_4/2,I_4/2; M = Mo, n = 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(M ₂O_nF_11-n)₃X. These materials differ in their (non)centrosymmetry despite chemical and structural similarities. The vanadium compounds are centrosymmetric (space groups P3̄m1 or C2/m) while the niobium and molybdenum heterotypes are noncentrosymmetric (Pmn2 ₁). 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.