BiNb3O9, a metastable perovskite phase with Bi/vacancy ordering: Crystal structure and dielectric properties

The perovskite, BiNb3O9, is a metastable phase in the Bi2O3:Nb2O5 system that forms only when cooled from a liquid phase. Crystals of BiNb3O9 prepared in this way display pseudocubic 2a×2a×2a diffraction patterns, due to non-merohedral twinning of a tetragonal a×a×2a cell, with the doubled axis oriented along all three pseudocubic axes. The structure was refined using data collected on a twinned crystal to R1=0.034 for 318 observed reflections. BiNb3O9 has tetragonal symmetry, P4/mmm with a=3.9459(3) Å, c=7.8919(6) Å. Partial ordering of Bi atoms and vacancies occurs, with alternate (0 0 1) planes having 28% and 42% Bi occupancies. The Bi atoms are displaced from the A-site special positions by up to 0.4 Å. The compound exhibits a relatively high permittivity value of ∼230 at room temperature, and shows a sharp increase with increasing temperature towards an apparent diffuse phase transition above ∼180 °C, with an associated frequency dependent peak in the dielectric loss.