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

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posted on 06.01.2014 by Michael Holland, Martin D. Donakowski, Eric A. Pozzi, Andrew M. Rasmussen, Thanh Thao Tran, Shannon E. Pease-Dodson, P. Shiv Halasyamani, Tamar Seideman, Richard P. Van Duyne, Kenneth R. Poeppelmeier
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 Pm1 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.