Crystal Structure of BaGe[Ge3O9]and Its Relation to Benitoite

BaGe[Ge3O9] is trigonal, space group P3, with lattice constants a = 11.61, c = 4.74 Å, and Z = 3. The structure was established by three-dimensional Patterson and electron density syntheses. Three-dimensional least-squares refinement resulted in a final R value of 6.8 percent (observed data only). The previously proposed structural relationship of this compound with benitoite, BaTiSi3O9, has been confirmed. The structure can be considered as composed of Ge3O9 rings, in which the Ge is tetrahedrally coordinated, linked through octahedrally coordinated Ge atoms to form a three-dimensional Ge-O network. All Ge polyhedra are linked by corner sharing. The Ba ions occupy positions in channels of the network.

Radial Distribution Study of Vitreous Barium Borosilicate

An X-ray diffraction study of a barium borosilicate glass consisting of 24 mole percent barium oxide, 40 mole percent boric oxide, and 36 mole percent silicon dioxide has been performed. Resulting atomic radial distribution functions give the following average interatomic distances: Si-O, 1.6 A; Ba-O, 2.8 A; Ba-Ba, 4.7 A; and Ba-Ba, 6.8 A. From the 4.7 A Ba-Ba separation a Ba-O-Ba bond angle of about 115° has been calculated. The observed average barium separations are in partial agreement with that predicted by Levin and Block on the basis of a structural interpretation of immiscibility data. A proposed coordination change by Levin and Block for the barium atoms in the system has been confirmed, but the details of the coordination change mechanism have not. Combining the results of the radial distribution study and immiscibility data on the barium borosilicate modifier-rich liquid at maximum barium oxide content has indicated that approximately 16.75 mole percent barium oxide is involved in the 4.7 A separation and 8.25 mole percent is associated with the 6.8 A separation. A mechanism which allows the composition of the modifier-rich liquids in the ternary system to be calculated has been presented. The calculated composition has been found to agree well with the experimental value.