Structure ofA2Ge4O9-Type Sodium Tetragermanate (Na2Ge4O9) and Comparison with Other Alkali Germanate and Silicate Mixed Tetrahedral–Octahedral Framework Structures
Structure ofA2Ge4O9-Type Sodium Tetragermanate (Na2Ge4O9) and Comparison with Other Alkali Germanate and Silicate Mixed Tetrahedral–Octahedral Framework Structures
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DOI：
10.1006/jssc.1998.7839
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发表时间：
1998-11
影响因子：
5.4
通讯作者：
M. Fleet;S. Muthupari
中科院分区：
文献类型：
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作者：
M. Fleet;S. Muthupari
解决了长期以来对Na 2Ge 4 O 9结构类型的不确定性。用过冷熔体结晶法生长了四锗酸钠，测定了其单晶X射线结构（R=0.022）。四锗酸钠是三角晶系，a =11.3234（12），c=9.6817（9）A，空间群P3 c1，Z=6，Dx =4.451 g cm−3。该结构由四面体-八面体混合骨架组成，GeO 4四面体的三元[Ge 3 O 9]环与GeO 6八面体通过共享角相互连接，为A2 Ge 4 O 9型。GeO 4四面体和GeO 6八面体的键距和键角与K2 Ge 4 O 9型结构中的相应值非常相似，这两种结构的不同之处主要在于较小（中-大尺寸）Na阳离子的容纳，它现在处于5+2配位。锗酸盐和硅酸盐的水铁矿型、A_2Ge_4O_9型和Na_2Si_4O_9型结构的结构-组成关系主要取决于T-O距离和一价阳离子的大小。我们确认，钠tetragermanate是一个亚稳相，在所有的压力高达2千巴，稳定的组合物为钠enneagermanate（Na 4Ge 9 O20）加上更多的钠相。
Long-standing uncertainty on the structure type of Na2Ge4O9has 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 witha=11.3234(12),c=9.6817(9) A, space groupP3c1,Z=6, andDx=4.451 g cm−3. The structure is comprised of a mixed tetrahedral–octahedral framework with three-membered [Ge3O9] rings of GeO4tetrahedra interconnected by isolated GeO6octahedra via shared corners and isA2Ge4O9-type. Bond distances and angles for GeO4tetrahedra and GeO6octahedra are very similar to the corresponding values in the type structure of K2Ge4O9, the two structures differing mainly in the accommodation of the smaller (medium–large-sized) Na cation, which is now in a 5+2 coordination. The structure–composition relationships of wadeite-type,A2Ge4O9-type, and Na2Si4O9-type structures of germanates and silicates depend largely on theT–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 Na2Ge4O9composition being sodium enneagermanate (Na4Ge9O20) plus a more sodic phase.