Face-sharing octahedra in Cs3Al2F9 and Cs2AlF5. Issue 2 (11th March 2015)

• Record Type: Journal Article
• Authors: Le Bail, A.; Smrčok, L'.
• Is Part Of: Powder diffraction. Volume 30:Issue 2(2015)
• Page Start: 130
• Page End: 138
• Publication Date: 2015-03-11
• Subjects: Powder metallurgy -- Periodicals; Materials -- Analysis -- Periodicals; 620.1127
• DOI: 10.1017/S0885715614001444
• Languages: English
• ISSNs: 0885-7156

Abstract

<abstract abstract-type="normal"> <title> <x content-type="archive" xml:space="preserve">Abstract</x> </title> <p>The structure of Cs<sub>2</sub>AlF<sub>5</sub> obtained by thermal dehydration of Cs<sub>2</sub>AlF<sub>5</sub>H<sub>2</sub>O is determined <italic>ab initio</italic> from powder diffraction data, space group <italic>Pmn</italic>2<sub>1</sub>, <italic>a</italic> = 6.36216 (17) Å, <italic>b</italic> = 12.7523 (4) Å, <italic>c</italic> = 11.4102 (3) Å, and <italic>Z</italic> = 6. Contrarily to most A<sub>2</sub>MF<sub>5</sub> compounds presenting MF<sub>5</sub><italic>cis</italic> or <italic>trans</italic>-chains of corner-sharing MF<sub>6</sub> octahedra, Cs<sub>2</sub>AlF<sub>5</sub> is characterized by the rare occurrence of the face-sharing anion Al<sub>2</sub>F<sub>9</sub><sup>3−</sup> combined with an isolated AlF<sub>6</sub><sup>3−</sup> octahedron, the sum leading to Al<sub>3</sub>F<sub>15</sub><sup>6−</sup>. The structure of Cs<sub>3</sub>Al<sub>2</sub>F<sub>9</sub> [space group <italic>P</italic>6<sub>3</sub>/<italic>mmc</italic>, <italic>a</italic> = 6.2535 (2) Å, <italic>c</italic> = 14.7193 (6) Å, <italic>Z</italic> = 2] is confirmed to be isostructural with Cs<sub>3</sub>Fe<sub>2</sub>F<sub>9</sub>, built up from the same M<sub>2</sub>F<sub>9</sub><sup>3−</sup> dimers (<italic>M</italic> = Fe, Al). Both crystal structures are optimized by energy minimization density funtional theory (DFT) in the solid state using a hybrid PBE0 functional, and their relations with the hexagonal perovskites and elpasolites are discussed.</p> </abstract>