Phase Formation and Properties in the System Bi₂O₃:2CoO_1+x:Nb₂O₅.

Abstract

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.

Stability Phase‐Fields and Pyrochlore Formation in Sections of the Bi2O3–Al2O3–Fe2O3–Nb2O5 System

“…As seen in Fig. 2, the pyrochlore and fluorite‐related Bi 3 NbO 7 patterns are related but distinctly different; furthermore, the cubic unit cell parameter for a Bi–Al–Nb–O pyrochlore should be less than 10.5 Å (not 10.9 Å), according to the values observed for other Bi–M–Nb–O pyrochlores 9–11,13 . Another report of a Bi 2 AlNbO 7 pyrochlore by Zou et al , 17 with a =10.7171 Å, is questionable and raises concern: The data in Fig.…”

“…Many of the pyrochlores in these systems exhibit dielectric behavior similar to that of the Zn system, including relative permittivities on the order of 120, as well as low‐temperature dielectric relaxation. These pyrochlores are also characterized by exhibiting displacive disorder within their crystal structures, 9–11,13 which apparently gives rise to dielectric relaxation 14,15 . A number of other bismuth‐niobate‐based pyrochlores have been reported in earlier studies, including in the Bi–Al–Nb–O system 16,17 .…”

“…In addition to low processing temperatures (<1000°C), high relative dielectric permittivity, relatively low dielectric losses, and temperature stability, 6 Bi–Zn–Nb–O pyrochlore films display a large electric‐field tunability of the dielectric constant at 1 MHz, 7,8 a property needed for voltage‐tunable capacitors and filters capable of multifrequency operation. The processing of Bi–Zn–Nb–O pyrochlore ceramics, especially as thin films, is complicated by the relative volatility of ZnO, and several other Bi–M–Nb–O systems (M=Fe, 9 Mn, 10 Co, 11 Ni, 6,12 Mg, 6 Cu 6 ) have been characterized with regard to the formation and properties of pyrochlore‐type phases. Many of the pyrochlores in these systems exhibit dielectric behavior similar to that of the Zn system, including relative permittivities on the order of 120, as well as low‐temperature dielectric relaxation.…”

Structural Disorder in (Bi, M)2(Fe, Mn, Bi)2O6+x (M = Na or K) Pyrochlores Seen from Reverse Monte Carlo Analysis of Neutron Total Scattering

“…The crystallographic disorder revealed previously by Rietveld refinement of (Na 0.60 Bi 1.40 )­(Fe 1.06 Mn 0.17 Bi 0.77 )­O 6.87 and (K 0.24 Bi 1.51 )­(Fe 1.07 Mn 0.15 Bi 0.78 )­O 6.86 pyrochlores (henceforth referred to as Na- and K-pyrochlore, respectively) was accounted for by off-center A and O′ positions modeled using the 96 h and 32 e positions in the Fd 3̅ m space group, respectively (Supporting Information Figure S1), commonly used to account for disorder in pyrochlores when analysis of Bragg scattering is performed. ,− The incorporation of multiple metals on both the A and B sites, and the mixing of bismuth across both cation sites add further complexity to the complete description of the structures of our materials. The PDFs derived from both pyrochlores indicate that their structures are very similar (Figure ), displaying sharp, resolved features out to distances well beyond 25 Å, showing that the extended structure of these materials resembles a crystalline pyrochlore, consistent with our previous analysis and implying that disorder is present only on a local length scale.…”

Disordered Structures and Dielectric Properties of Ni-Doped Bismuth Magnesium Niobate Pyrochlores

“…However, the cubic pyrochlore structure has a great deal of tolerance for introducing different ions on the A site. Considerable doping modification for enhancing the dielectric properties of bismuth-based pyrochlores are studied, such as Ca 2+ , Nd 3+ , Cu 2+ , and Co 2+ doping modifications. − …”