Polar Alignment of A-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 = CI, Br4/2, I4/2) illustrates generation of polar structures with the use of A- 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 d° 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 P3ml 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 A-shaped bimetallic BBUs and related packing effects. These A-shaped BBUs form as a consequence of the coordination environment around the bridging anion of the metal oxide fluoride BBUs. [ABSTRACT FROM AUTHOR]