The structure of a barium niobium silicon oxide with the probable composition Ba6+xNb14Si4O47 (x ? 0.23)

Reaction of BaO, Nb₂O₅, and Nb in mole ratios of 2.4:1.6:1 in an evacuated silica capsule at 1250°C produces a mixture of at least two products, one of which has the probable composition $\text{Ba}{}_{\mathrm{6+x}}\text{Nb}{}_{14}\text{Si}{}_4\text{O}{}_{47}\text{(x ? 0.23)}$. This compound has an hexagonal unit cell of dimensions a = 9,034 ± 0.004 Å, c = 27.81 ± 0.02 Å, probable space group $\text{P6}{}_3\text{mcm}\text{, Z = 2}$. Its structure has been determined from 942 independent reflections collected by a counter technique and refined by least squares methods to a conventional R value of 0.062. The basic structure consists of strings of four NbO₆ octahedra sharing opposite corners, each string joined to the next by edge sharing of the end octahedra, so that the c axis corresponds to the length of a strand of seven corner-linked octahedra. Chains of three such strands are formed by corner sharing between the strands. The chains in turn are joined by NbO₆ octahedra and Si₂O₇ groups in which the SiOSi linkage is linear. Barium atoms are in sites between the chains coordinated by 13 oxygen atoms. A second site, 15 coordinated, probably has a small amount of barium as well; the fractional occupancy for barium in this site is 0.076.