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首页> 外文期刊>Crystal growth & design >Structures of α-K_3MoO_3F_3 and α-Rb_3MoO_3F_3: Ferroelectricity from anion ordering and noncooperative octahedral tilting
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Structures of α-K_3MoO_3F_3 and α-Rb_3MoO_3F_3: Ferroelectricity from anion ordering and noncooperative octahedral tilting

机译：α-K_3MoO_3F_3和α-Rb_3MoO_3F_3的结构：阴离子排序和非合作八面体倾斜产生的铁电

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摘要

The room temperature crystal structures of α-K_3MoO _3F_3 and α-Rb_3MoO_3F_3 have been solved via combined Rietveld refinements of synchrotron and neutron powder diffraction data. These two compounds are part of a broader family of A2BMO3F3 compounds that have been studied for their dielectric properties, but until now the complex crystal structures of the ferroelectric phases of these compounds were not known. At room temperature and below, these two isostructural compounds are tetragonal with I41 space group symmetry and unit cell parameters of a = 19.38613(3) ?, c = 34.86739(8) ? for α-K_3MoO_3F_3 and a = 20.0748(4) ?, c = 36.1694(1) ? for α-Rb3MoO 3F3. Their structures are related to the cubic double perovskite structure but are considerably more complicated due to noncooperative octahedral tilting and long-range orientational ordering of the polar MoO _3F_3 ~(3-) units. The pattern of octahedral tilting is equivalent to that seen in the α-K_3AlF_6 structure, which has I4_1/a symmetry, but orientational ordering of MoO_3F_3 ~(3-) units lowers the symmetry to I4 _1. The polar space group symmetry is consistent with earlier reports of ferroelectricity in these compounds. Hence orientational ordering of the MoO_3F_3 ~(3-) units is directly responsible for the ferroelectric behavior.
机译：通过同步加速器和中子粉末衍射数据的Rietveld精炼，解决了α-K_3MoO_3F_3和α-Rb_3MoO_3F_3的室温晶体结构。这两种化合物是A2BMO3F3化合物的更广泛家族的一部分，已经对其介电性能进行了研究，但是直到现在，这些化合物的铁电相的复杂晶体结构还是未知的。在室温及低于室温的情况下，这两个同构化合物为四边形，具有I41空间群对称性，其晶胞参数为a = 19.38613（3）？，c = 34.86739（8）？对于α-K_3MoO_3F_3和a = 20.0748（4）？，c = 36.1694（1）？对于α-Rb3MoO3F3。它们的结构与立方双钙钛矿结构有关，但由于不合作的八面体倾斜和极性MoO _3F_3〜（3-）单元的长距离取向有序而变得更加复杂。八面体倾斜的模式与在具有I4_1 / a对称性的α-K_3AlF_6结构中看到的模式相同，但是MoO_3F_3〜（3-）单元的取向顺序将对称性降低到I4 _1。极性空间基团的对称性与这些化合物中铁电的早期报道是一致的。因此，MoO_3F_3〜（3-）单元的取向顺序直接导致铁电行为。

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