* ~a-B is the torsional angle about the A-B bond, in which the other two atoms required to define the bond are those attached to either end of the bond and are in the ring in question. JOHNSON, C. K. (1965) The crystal structure and the chemical composition of Ba6Ti~7040, a new compound in the system BaO-TiO2, have been established by single-crystal methods. Hexabarium 17-titanate crystallizes in space group C2/c with a= 9"883, b= 17"08, c = 18"92/~, ,8= 98°42, Z= 4. The 2094 observed Fhe~ were used in a combination of Patterson and Fourier methods to determine the crystal structure and to refine it to R=0.06. The structure can be described as a three-dimensional framework of TiO6 coordination octahedra. The most prominent features of this framework are ribbons of octahedra of composition Ti3Ol0 which extend in the [rl0] direction at height z=0, and in the [110] direction at z= ½. The ribbons are joined by clusters of octahedra of composition TillO40. The voids in this framework are filled by the Ba atoms. An alternate way of looking at the crystal structure is to regard it as a hexagonally close-packed array of oxygen and barium atoms with the titanium atoms filling the energetically most favorable octahedral voids. The direction of packing is [103] and the stacking sequence is ,4BCAC,4BCBCABABCACABCBCAB. Ba6Til7040 is another example in the short list of titanates which can be described as close-packed arrays, while most belong to the Wadsley-Anderson type phases with one cell edge of about 4/~. The details of the atomic arrangement show appreciable distortions of the bond lengths and angles. The individual bond distances Ti-O vary from 1"77 to 2.43/~. The mean of all Ti-O bond lengths formed by one oxygen atom can be predicted by using the expression dTi_o = (1.552 + 0"2000/~, where ( is the sum of the electrostatic valences received by this oxygen atom. This is in accord with the extended electrostatic valence rule..
IntroductionCompounds in the system BaO-TiO2 have been extensively studied in recent years. So far the crystal structures of Ba2TiO4 (Bland, 1961), cubic BaTiO3 (Megaw, 1947), hexagonal BaTiO3 (Burbank & Evans, 1948), BaTi205 (Harrison, 1956), BaTi409 (Lukaszewicz, 1957) and BaTisOll (Tillmanns, 1969) have been described. Rase & Roy (1955) report another compound in this system of composition BaTi307 and two more are mentioned by Kwestroo (1960): BazTisO~z and Ba2Ti9020. The X-ray powder patterns for these three compounds are not indexed and single-crystal work has not been undertaken.