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

Long-standing uncertainty on the structure type of Na2Ge4O9 has been r esolved. Sodium tetragermanate has been grows by crystallization from a supercooled melt and its single-crystal X-ray structure has been det ermined (R = 0.022). Sodium tetragermanate is trigonal with a = 11.323 4(12), c = 9.6817(9) Angstrom, space group <P(3)over bar>c1, Z = 6, an d D-x = 4.451 g cm(-3). The structure is comprised of a mixed tetrahed ral-octahedral framework with three-membered [Ge3O9] rings of GeO4 tet rahedra interconnected by isolated GeO6 octahedra via shared corners a nd is A(2)Ge(4)O(9)-type. Bond distances and angles for GeO4 tetrahedr a and GeO6 octahedra are very similar to the corresponding values in t he type structure of K2Ge4O9, the two structures differing mainly in t he accommodation of the smaller (medium-large-sized) Na cation, which is now in a 5 + 2 coordination. The structure-composition relationship s 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 s ize of the monovalent cation. We confirm that sodium tetragermanate is a metastable phase at all pressures up to 2 kbar, the stable assembla ge for the Na2Ge4O9 composition being sodium enneagermanate (Na4Ge9O20 ) plus a more sodic phase. (C) 1998 Academic Press.