Structure of A(2)Ge(4)O(9)-type sodium tetragermanate (Na2Ge4O9) and comparison with other alkali germanate and silicate mixed tetrahedral-cathedral framework structures

摘要

Long-standing uncertainty on the structure type of Na2Ge4O9 has been resolved. Sodium tetragermanate has been grows 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) Angstrom, space group c1, Z = 6, and D-x = 4.451 g cm(-3). The structure is comprised of a mixed tetrahedral-octahedral framework with three-membered [Ge3O9] rings of GeO4 tetrahedra interconnected by isolated GeO6 octahedra via shared corners and is A(2)Ge(4)O(9)-type. Bond distances and angles for GeO4 tetrahedra and GeO6 octahedra 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, A(2)Ge(4)O(9)-type, and Na2Si4O9-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 Na2Ge4O9 composition being sodium enneagermanate (Na4Ge9O20) plus a more sodic phase. (C) 1998 Academic Press. [References: 24]