Polar Alignment of Λ‑Shaped Basic Building Units within Transition Metal Oxide Fluoride Materials
journal contributionposted on 06.01.2014, 00:00 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 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.