Bi3Nb17O47: a potentially ferroelectric crystal structure of the tungsten bronze type

Abstract

The crystal structure of Bi₃Nb₁₇O₄₇ has been determined from three-dimensional X-ray data and refined to R 0·092 for 1278 visually estimated independent reflections. The crystals are orthorhombic with a= 12·516 ± 0·005, b= 37·14 ± 0·02, c= 3·922 ± 0·002 Å, space group Pba2, and Z= 2. Bi₃Nb₁₇O₄₇ belongs to a family of A_x(MO)_yMO₃ compounds, which are based on the tetragonal bronze structure, with (MO) in some pentagonal tunnels. The bismuth atoms are statistically arranged among a number of sites in the remaining pentagonal and diamond shaped tunnels, and tend towards a one-sided four-co-ordination. The structure is potentially ferroelectric.