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 [Fd3$\bar {S}$m (#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)

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Equilibria E 4000Phase Formation and Properties in the System Bi2O3:2CoO1+x:Nb2O5. -Subsolidus phase relations are determined for the Bi-Co-Nb-O system in air (750-925°C).As revealed by single crystal XRD the pyrochlore Bi1.56Co0.96Nb1.48O7 existing in the title system crystallizes in the space group Fd3m. The displacive structural behavior is similar to that of analogous Bi-M-Nb-O pyrochlores (M: Zn, Fe, Mn). Bi-Co-Nb-O pyrochlores exhibit overall paramagnetic behavior with negative Curie-Weiss temperature intercepts indicating weak antiferromagnetic interactions. -(VANDERAH, T. A.; SIEGRIST, T.; LUFASO*, M. W.; YEAGER, M. C.; ROTH, R. S.; NINO, J. C.; YATES, S.; Eur.

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The corrosion behavior of MgONd 2 Zr 2 O 7 cercer composites was investigated under hydrothermal conditions at 300 C and saturation pressure. It was found that corrosion resistance was microstructure dependent in terms of MgO contiguity and homogeneity. The inhomogeneous composites completely dissolved in less than 1 h showing poor hydration resistance similar to MgO. The homogeneous composites exhibited an improved hydration resistance and the corrosion was primarily limited to the surface. The normalized mass loss (NML) was proportional to the corrosion time and the NML rate increased with MgO volume fraction. The corrosion process followed an Arrhenius relationship and the apparent activation energy was calculated to be $3941 kJ/mol. It is suggested that the mass loss of the MgONd 2 Zr 2 O 7 composites is mainly governed by hydration of MgO and the NML rate is enhanced by contiguity of MgO. A desirable microstructure with enhanced corrosion resistance is viewed to have minimum contiguity of MgO, good homogeneity, and large MgO grain size (minimizing grain boundaries).