Subsolidus phase relations have been determined for the Bi-Co-Nb-O system in air (750 C to 925 C). Ternary compound formation was limited to pyrochlore (A2B 2O 6O ), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi2Co 2/3Nb 4/3O7 ) suggesting that 8 % to 25 % of the A-sites are occupied by Co. X-ray powder diffraction data confirmed that all Bi-Co-Nb-O pyrochlores form with displacively disordered structures. A structural refinement of the pyrochlore Bi1.508Co 0.928Nb1.431 O7 using single-crystal X-ray diffraction data is reported with the A and some O atoms displaced (0.38 and 0.47 , respectively) from ideal positions to 96g sites and 48f sites, respectively (Fdm (#227), a = 10.551(1) ). 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, slight superparamagnetic effects, and elevated observed moments compared to high-spin, spin-only values for Co2+/3+. At 250 K and 1 MHz the relative dielectric permittivity of the pyrochlore 0.4400:0.2100:0.3500 Bi2O 3:2CoO 1+x:Nb2 O5 was 115 with tan d = 0.06; however, at lower frequencies the sample was conductive which is consistent with the presence of mixed-valent Co. Low-temperature dielectric relaxation such as that observed for Bi1.5Zn 0.92Nb1.5 O6.92 and other bismuth-based pyrochlores was not observed.
Citation: European Journal of Inorganic Chemistry
Pub Type: Journals
Bi-Co-Nb-O, bismuth cobalt niobates, dielectric properties, displacive disorder, phase diagram, phase equilibria, pyrochlore