Structures of 伪-K3MoO3F3 and 伪-Rb3MoO3F3: Ferroelectricity from Anion Ordering and Noncooperative Octahedral Tilting

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• 作者：Allyson M. Fry ; Patrick M. Woodward
• 刊名：Crystal Growth & Design
• 出版年：2013
• 出版时间：December 4, 2013
• 年：2013
• 卷：13
• 期：12
• 页码：5404-5410
• 全文大小：472K
• 年卷期：v.13,no.12(December 4, 2013)
• ISSN：1528-7505

文摘

The room temperature crystal structures of 伪-K₃MoO₃F₃ and 伪-Rb₃MoO₃F₃ 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) 脜, c = 34.86739(8) 脜 for 伪-K₃MoO₃F₃ and a = 20.0748(4) 脜, c = 36.1694(1) 脜 for 伪-Rb₃MoO₃F₃. 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 MoO₃F₃^{3鈥?/sup> units. The pattern of octahedral tilting is equivalent to that seen in the 伪-K₃AlF₆ structure, which has I4₁/a symmetry, but orientational ordering of MoO₃F₃3鈥?/sup> units lowers the symmetry to I4₁. The polar space group symmetry is consistent with earlier reports of ferroelectricity in these compounds. Hence orientational ordering of the MoO₃F₃3鈥?/sup> units is directly responsible for the ferroelectric behavior.}