Structure ofA2Ge4O9-Type Sodium Tetragermanate (Na2Ge4O9) and Comparison with Other Alkali Germanate and Silicate Mixed Tetrahedral–Octahedral Framework Structures
✍ Scribed by M.E Fleet; S Muthupari
- Book ID
- 102974837
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 515 KB
- Volume
- 140
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
Long-standing uncertainty on the structure type of Na 2 Ge 4 O 9 has been resolved. Sodium tetragermanate has been grown by crystallization from a supercooled melt and its single-crystal X-ray structure has been determined (R ؍ 0.022). Sodium tetragermanate is trigonal with a ؍ 11.3234(12), c ؍ 9.6817(9) A > , space group P3 c1, Z ؍ 6, and D x ؍ 4.451 g cm ؊3 . The structure is comprised of a mixed tetrahedral-octahedral framework with three-membered [Ge 3 O 9 ] rings of GeO 4 tetrahedra interconnected by isolated GeO 6 octahedra via shared corners and is A 2 Ge 4 O 9 -type. Bond distances and angles for GeO 4 tetrahedra and GeO 6 octahedra are very similar to the corresponding values in the type structure of K 2 Ge 4 O 9 , the two structures differing mainly in the accommodation of the smaller (medium-largesized) Na cation, which is now in a 5 ؉ 2 coordination. The structure-composition relationships of wadeite-type, A 2 Ge 4 O 9type, and Na 2 Si 4 O 9 -type structures of germanates and silicates depend largely on the T-O distance and the size of the monovalent cation. We confirm that sodium tetragermanate is a metastable phase at all pressures up to 2 kbar, the stable assemblage for the Na 2 Ge 4 O 9 composition being sodium enneagermanate (Na 4 Ge 9 O 20 ) plus a more sodic phase.