An Unexpected Crystal‐Chemical Principle for the Pyrochlore Structure

Vanderah, Terrell A.; Levin, Igor; Lufaso, Michael W.

doi:10.1002/ejic.200500234

Cited by 117 publications

(108 citation statements)

References 52 publications

Supporting

Mentioning

101

Contrasting

Order By: Relevance

“…In addition to extensive compositional ranges, pyrochlores exhibit a remarkable variety of exploitable physical properties, hence the considerable technical importance of this class of solids. [29] Bi-Co-Nb-O pyrochlores have been previously reported to form at the stoichiometries Bi 2 Co 2/3 Nb 4/3 O 7 , [31] [32] however, under the conditions of the present study none of these compositions are included in the pyrochlore singlephase field: Composition Bi 2 Co 2/3 Nb 4/3 O 7 formed a three-phase mixture as described in Figure 1 Compositions, nominal formulas, and refined unit cell parameters for single-phase Bi-Co-Nb-O pyrochlores (all dull dark greenish gray in color) prepared in the present study are given in Table 1. [34] The trends in unit cell parameters show a strong dependence on Bi concentration.…”

Section: Phase Formationmentioning

confidence: 97%

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

Vanderah

Siegrist²,

Lufaso

et al. 2006

Eur J Inorg Chem

Self Cite

View full text Add to dashboard Cite

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)

show abstract

Section: Phase Formationmentioning

confidence: 97%

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

Vanderah

Siegrist²,

Lufaso

et al. 2006

Eur J Inorg Chem

Self Cite

View full text Add to dashboard Cite

show abstract

“…4,5 This disorder has been attributed to the active lone pair of the Bi cation 6 and appears in the infrared ͑IR͒ spectrum of BZN as additional modes from those predicted by the ideal pyrochlore structure. [7][8][9] The additional IR modes were also observed by Chen et al 7 in three other bismuth pyrochlores Bi 3/2 ZnTa 3/2 O 7 ͑BZT͒, Bi 3/2 MgNb 3/2 O 7 ͑BMN͒, and Bi 3/2 MgTa 3/2 O 7 ͑BMT͒.…”

Section: Introductionmentioning

confidence: 95%

Raman study of phonon modes in bismuth pyrochlores

et al. 2010

View full text Add to dashboard Cite

have been measured at room temperature. The frequencies of the Raman modes, obtained from first-principles calculations, for Bi 2 Ti 2 O 7 are presented for comparison. The spectra of the four samples are similar and agree well with the first-principles calculations. Each bismuth pyrochlore shows more than the six modes expected for the ideal pyrochlore structure. The analysis shows that many of the additional modes could be explained as the relaxation of the selection rules due to the displacive disorder. The Raman modes are assigned by reference to spectra of other pyrochlore materials, comparison to infrared data, and the ab initio calculations.

show abstract

“…Instead, pyrochlore compounds with lone-pair active A cations frequently accommodate the offcentering through incoherent, local displacements. Vanderah, Levin, and Lufaso [4] have recently demonstrated that the A 2 O 0 sublattice of the pyrochlore can tolerate a large degree of substitutional as well as displacive disorder on the A-cation site. Indeed for certain pyrochlore compositions, the phase becomes stable only when such substitutional disorder is present on the A-site [4].…”

Section: Introductionmentioning

confidence: 99%

“…Vanderah, Levin, and Lufaso [4] have recently demonstrated that the A 2 O 0 sublattice of the pyrochlore can tolerate a large degree of substitutional as well as displacive disorder on the A-cation site. Indeed for certain pyrochlore compositions, the phase becomes stable only when such substitutional disorder is present on the A-site [4]. Local static disorder, rather than coherent distortions of the structure can be advantageous in device applications where structural phase transitions are contraindicated.…”

Section: Introductionmentioning

confidence: 99%

Displacive disorder in three high-k bismuth oxide pyrochlores

Melot

Rodriguez

Proffen

et al. 2006

Materials Research Bulletin

View full text Add to dashboard Cite

We use time-of-flight neutron powder diffraction to examine static displacive disorder in three different pyrochlore A 2 B 2 O 6 O 0 compounds with Bi on the A site. The compounds (Bi 1.5 Zn 0.5 )(Nb 1.5 Zn 0.5 )O 6 O 0 (BZN), (Bi 1.5 Zn 0.5 )(Ta 1.5 Zn 0.5 )O 6 O 0 (BZT), and (Bi 1.5 Zn 0.5 )(Sb 1.5 Zn 0.5 )O 6 O 0 (BZS), are of interest -particularly BZN -for their high dielectric constants in the absence of any phase transition from the cubic high temperature phase. The local structures of the three compounds is characterized by displacive disorder from the ideal pyrochlore positions for both the A and O 0 sites, with the precise nature of the disorder being quite similar. However the extent of displacive disorder is different, despite the B-O networks being nearly identical in the three compounds. The reported dielectric constants of the three compounds are related to the extent of local displacement, and BZN, with the largest extent of local atomic displacement of A and O 0 , is also reported to have the largest dielectric constant at 1 MHz. The dielectric constants are also related to the magnitudes of the thermal parameters of the different ions. The strongest correlation is found to involve the thermal parameter on the B site (Nb, Ta, or Sb). #

show abstract

An Unexpected Crystal‐Chemical Principle for the Pyrochlore Structure

Cited by 117 publications

References 52 publications

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

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

Raman study of phonon modes in bismuth pyrochlores

Displacive disorder in three high-k bismuth oxide pyrochlores

Contact Info

Product

Resources

About

An Unexpected Crystal‐Chemical Principle for the Pyrochlore Structure

Cited by 117 publications

References 52 publications

Phase Formation and Properties in the System Bi2O3:2CoO1+x:Nb2O5

Phase Formation and Properties in the System Bi2O3:2CoO1+x:Nb2O5

Raman study of phonon modes in bismuth pyrochlores

Displacive disorder in three high-k bismuth oxide pyrochlores

Contact Info

Product

Resources

About

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

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