The structure of a barium niobium silicon oxide with the probable composition [formula omitted

Abstract

Reaction of BaO, Nb 2O 5, 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 Ba 6+x Nb 14Si 4O 47 (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 P6 3 mcm , 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 NbO 6 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 NbO 6 octahedra and Si 2O 7 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.