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.

一系列形如K10(M2OnF11–n)3X（M = V和Nb，n = 2，X = (F2Cl)1/3，Br，Br4/2，I4/2；M = Mo，n = 4，X = Cl，Br4/2，I4/2）的假对称结构，展现了通过使用Λ形的基本构建单元（BBU）来生成极性结构的原理。欲使一种化合物归属于极性空间群，其个别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方法，这得益于这些多面体所包含的二阶Jahn–Teller畸变。然而，对于如何对齐BBU的极性矩，研究投入的关注相对较少。为解决对齐问题，本研究对先前报道的双金属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具有相似性，但其（非）中心对称性却有所不同。钒化合物具有中心对称性（空间群为P3̅m1或C2/m），而铌和钼异构体则不具有中心对称性（Pmn21）。这种对称性的差异源于线性双金属BBU或Λ形双金属BBU的存在及其相关的堆积效应。这些Λ形BBU的形成是金属氧化物氟化物BBU中桥接阴离子周围配位环境的结果。