Polar Alignment of Λ‑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 = 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.

一系列形似伪对称结构的化合物，其化学式为K10(M2OnF11–n)3X（M为V和Nb，n=2，X为(F2Cl)1/3、Br、Br4/2、I4/2；M为Mo，n=4，X为Cl、Br4/2、I4/2），展示了通过采用Λ形基本构建单元（BBU）来生成极性结构的方法。一个化合物若要属于极性空间群，其单个物种的偶极矩必须部分对齐。将d0早期过渡金属多面体BBU融入结构中是创建极性结构的一种常见手段，这归因于这些多面体所包含的二阶Jahn–Teller畸变。然而，在探讨如何对齐BBU的极性矩方面，所投入的关注相对较少。为了解决这一问题，本研究对先前报道的二金属BBU及其合成的化合物K10(M2OnF11–n)3X进行了探讨。尽管这些材料在化学和结构上相似，但它们的（非）中心对称性却存在差异。钒化合物是中心对称的（空间群为P3̅m1或C2/m），而铌和钼异构体则非中心对称（Pmn21）。这种对称性的差异源于线性二金属BBU或Λ形二金属BBU的存在以及相关的堆积效应。这些Λ形BBU的形成是金属氧化物氟化物BBU桥接阴离子周围配位环境的结果。