Phase Formation of Co and Cr Co-Doped Bismuth Niobate with Pyrochlore Structure

Abstract

The formation mechanisms of pyrochlore-type Bi2Co1/2Cr1/2Nb2O9+Δ (space group Fd-3m, a = 10.4838(8) Å), in the temperature range from 400 to 1050 °C were studied by employing X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy. An extensive reaction between the binary metal oxides was found to begin at temperatures above 550 °C, following the transition of monoclinic α-Bi2O3 into a tetragonal β-Bi2O3 polymorph. The synthesis process occurs in several stages when Bi-rich intermediate products (Bi6CrO12, Bi6Cr2O15, and Bi5Nb3O15) transform into bismuth-depleted BiNbO4 and a chromium–cobalt spinel is formed. The formation of a single pyrochlore phase occurs at the final reaction stage at 1050 °C via the doping of bismuth ortho-niobate, BiNbO4, by the transition metal cations. The observed mechanism is essentially similar to the mechanism of tantalate-based phases except for the formation of Bi5Nb3O15 at the intermediate reaction stages.