Phase formation and properties in the system Bi₂O ₃:2CoO_1+x:Nb₂O₅

Subsolidus phase relations have been determined for the Bi–Co–Nb–O system in air (750–925 °C). Ternary compound formation was limited to pyrochlore (A2B2O6O′), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi2Co2/3Nb4/3O7) suggesting that about 8 to 25 % of the A sites are occupied by Co. X-ray powder diffraction data confirmed that all Bi–Co–Nb–O pyrochlores exhibit displacive behavior. A structural refinement of the pyrochlore Bi1.56Co0.96Nb1.48O7 using single-crystal X-ray diffraction data is reported with the A and O′ atoms displaced (0.34 Å and 0.45 Å, respectively) from ideal positions to96g sites and 32e sites, respectively [Fd3equation imagem (#227), a = 10.551(1) Å]. The displacive structural behavior is similar to that found in analogous Bi–M–Nb–O pyrochlores (M = Zn, Fe, Mn). Bi–Co–Nb–O pyrochlores exhibited overall paramagnetic behavior with negative Curie–Weiss temperature intercepts indicating weak antiferromagnetic interactions. At 250 K and 1 MHz the relative dielectric permittivity of the pyrochlore 0.4400:0.2100:0.3500 Bi2O3:2CoO1+x:Nb2O5 was ca. 115 with tan δ = 0.06; however, at lower frequencies the sample was conductive. Low-temperature dielectric relaxation such as that observed for Bi1.5Zn0.92Nb1.5O6.92 and other bismuth-based pyrochlores was not observed. Consideration of bond lengths, effective magnetic moments, and dielectric properties suggests that the average oxidation state of Co in the pyrochlore phases is close to but slightly higher than 2.0. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)