Crystallographic features of ammonium fluoroelpasolites: dynamic orientational disorder in crystals of (NHb>4b>)b>3b>HfFb>7b> and (NHb>4b>)b>3b>Ti(Ob>2b>)Fb>5b>

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• 作者：Anatoly A. Udovenko ; Alexander A. Karabtsov and Natalia M. Laptash
• 关键词：fluoroelpasolites ; ammonium ; dynamic orientational disorder ; electron-density profiles ; vibrational spectra
• 刊名：Acta Crystallographica Section B
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：73
• 期：1
• 页码：1-9
• 全文大小：1510K
• ISSN：2052-5206

文摘

A classical elpasolite-type structure is considered with respect to dynamically disordered ammonium fluoro-(oxofluoro-)metallates. Single-crystal X-ray diffraction data from high quality (NHb>4b>)b>3b>HfFb>7b> and (NHb>4b>)b>3b>Ti(Ob>2b>)Fb>5b> samples enabled the refinement of the ligand and cationic positions in the cubic (Z = 4) structure. Electron-density atomic profiles show that the ligand atoms are distributed in a mixed (split) position instead of 24e. One of the ammonium groups is disordered near 8c so that its central atom (N1) forms a tetrahedron with vertexes in 32f. However, a center of another group (N2) remains in the 4b site, whereas its H atoms (H2) occupy the 96k positions instead of 24e and, together with the H3 atom in the 32f position, they form eight spatial orientations of the ammonium group. It is a common feature of all ammonium fluoroelpasolites with orientational disorder of structural units of a dynamic nature.