skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Local Structure of Displacively Disordered Pyrochlore Dielectrics

Abstract

Local cation environments in the pyrochlore dielectrics A{sub 2}B{sub 2}O6O' = (Bi,M){sub 2}(M,Nb){sub 2}O{sub 6}O' (M = Zn, Fe) were determined from the extended X-ray absorption fine structure. The A cations and O' ions are displaced from their ideal positions, yielding the effective (4 + 2 + 2) coordination for the A cations. The A-cation displacements are directed toward a pair of the framework O atoms; the specific displacement directions are determined by the nature of the A cations (i.e., Bi, Zn, Fe) and the local Nb/Zn (or Nb/Fe) B-site configurations. Local correlations among the atomic displacements and both [A{sub 4}O'] and [A{sub 2}B{sub 2}O] configurations provide near-ideal bond valence sums for all constituent species in these pyrochlore structures.

Authors:
; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
930311
Report Number(s):
BNL-81021-2008-JA
Journal ID: ISSN 0897-4756; CMATEX; TRN: US200822%%1463
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Chemistry of Materials; Journal Volume: 19
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ABSORPTION; ATOMIC DISPLACEMENTS; ATOMS; CATIONS; CORRELATIONS; DIELECTRIC MATERIALS; FINE STRUCTURE; IONS; PYROCHLORE; VALENCE; national synchrotron light source

Citation Formats

Krayzman,V., Levin, I., and Woicik, J. Local Structure of Displacively Disordered Pyrochlore Dielectrics. United States: N. p., 2007. Web. doi:10.1021/cm062429g.
Krayzman,V., Levin, I., & Woicik, J. Local Structure of Displacively Disordered Pyrochlore Dielectrics. United States. doi:10.1021/cm062429g.
Krayzman,V., Levin, I., and Woicik, J. Mon . "Local Structure of Displacively Disordered Pyrochlore Dielectrics". United States. doi:10.1021/cm062429g.
@article{osti_930311,
title = {Local Structure of Displacively Disordered Pyrochlore Dielectrics},
author = {Krayzman,V. and Levin, I. and Woicik, J.},
abstractNote = {Local cation environments in the pyrochlore dielectrics A{sub 2}B{sub 2}O6O' = (Bi,M){sub 2}(M,Nb){sub 2}O{sub 6}O' (M = Zn, Fe) were determined from the extended X-ray absorption fine structure. The A cations and O' ions are displaced from their ideal positions, yielding the effective (4 + 2 + 2) coordination for the A cations. The A-cation displacements are directed toward a pair of the framework O atoms; the specific displacement directions are determined by the nature of the A cations (i.e., Bi, Zn, Fe) and the local Nb/Zn (or Nb/Fe) B-site configurations. Local correlations among the atomic displacements and both [A{sub 4}O'] and [A{sub 2}B{sub 2}O] configurations provide near-ideal bond valence sums for all constituent species in these pyrochlore structures.},
doi = {10.1021/cm062429g},
journal = {Chemistry of Materials},
number = ,
volume = 19,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • To understand the origin of the spin-glass state in molybdate pyrochlores, the structure of Tb{sub 2}Mo{sub 2}O{sub 7} is investigated using two techniques: the long-range lattice structure was measured using neutron powder diffraction (NPD), and local structure information was obtained from the extended x-ray absorption fine structure (EXAFS) technique. While the long-range structure appears generally well ordered, enhanced mean-squared site displacements on the O(1) site and the lack of temperature dependence of the strongly anisotropic displacement parameters for both the Mo and O(1) sites indicate some disorder exists. Likewise, the local structure measurements indicate some Mo-Mo and Tb-O(1) nearest-neighbor disordermore » exists, similar to that found in the related spin-glass pyrochlore, Y{sub 2}Mo{sub 2}O{sub 7}. Although the freezing temperature in Tb{sub 2}Mo{sub 2}O{sub 7}, 25 K, is slightly higher than in Y{sub 2}Mo{sub 2}O{sub 7}, 22 K, the degree of local pair distance disorder is actually less in Tb{sub 2}Mo{sub 2}O{sub 7}. This apparent contradiction is considered in light of the interactions involved in the freezing process.« less
  • The structural characterisation of Nd{sub 2}Zr{sub 2}O{sub 7} prepared via a precursor route was performed using a combination of local and average structure probes (neutron total scattering, X-ray and neutron diffraction). We present the first total scattering and reverse Monte Carlo (RMC) modelling study of Nd{sub 2}Zr{sub 2}O{sub 7}, which provides compelling evidence for the adoption of a disordered fluorite-type structure by Nd{sub 2}Zr{sub 2}O{sub 7} prepared by a low-temperature precursor route. Annealing the material at high temperatures leads to a transformation to a pyrochlore-type structure; however, Rietveld refinement using powder neutron diffraction data shows that the oxygen sublattice retainsmore » a degree of disorder. - Graphical abstract: Display Omitted - Highlights: • The first total scattering and RMC modelling study of Nd{sub 2}Zr{sub 2}O{sub 7}. • Demonstration that the synthetic route influences the crystal structure adopted. • Insight into the importance of total scattering in studies of complex superstructures, especially for nano-sized materials.« less
  • Compositions, local atomic structures, porosity, and vibrational dynamics of carbons obtained by pyrolyzing epoxy novolak resins at 650 and 1,000 C are investigated using several neutron scattering techniques. The ultimate goal is to understand the origin of the very high capacity for Li uptake exhibited by these materials. Neutron radial distribution function analysis and incoherent inelastic scattering show that the structural motif is a planar hexagonal graphene fragment {approximately}10 {angstrom} on a side, with edge carbons terminated by single hydrogen atoms and random stacking between fragments. Small-angle neutron scattering reveals substantial porosity on a length scale comparable to the sizemore » of the graphene fragments. Coupled with computer simulations, these results are consistent with two proposed mechanisms for unusually high Li capacity, one analogous to conventional intercalation but with Li on both sides of isolated graphene fragments, the other involving bonding of Li to H-terminated edge carbons.« less
  • The power of the atomic pair density function method to study the local atomic structure of dispersed materials is discussed for three examples (I) supercapacitor hydrous ruthenia, (II) electroctalyst platinum-iron phosphate and (III) nanoparticle gold catalyst. Hydrous ruthenia appears to be amorphous, but was found to be nanocomposite with RuO{sub 2} nanocrystals supporting electronic and hydrous boundaries protonic conductivity. A platinum-iron phosphate electrocatalyst, that exhibits activity for the oxygen reduction reaction has platinum in a non-metallic state. In catalysts comprised of gold nanoparticles supported on TiO{sub 2}, atomic correlations in the second atomic shell were observed suggesting interaction with themore » support that could modify gold chemical activity.« less
  • The local structure around Ag ions in silver borate glasses g-Ag{sub 2}O{center_dot}nB{sub 2}O{sub 3} (n=2,4) was studied by x-ray absorption spectroscopy at the Ag K edge for temperatures from 77 to 450 K. Extended x-ray absorption fine structure (EXAFS) analysis based on cumulant expansion or multishell Gaussian model fails for these systems. Therefore, the radial distribution functions (RDFs) around Ag ions were reconstructed using a method based on the direct inversion of the EXAFS expression. The RDFs consist of about eight atoms (oxygens and borons), exhibit a relatively weak temperature dependence, and indicate the presence of strong static disorder. Twomore » main components can be identified in RDFs, located at about 2.3-2.4 A and 2.5-3.4 A, respectively. The chemical types of atoms contributing to the RDF were determined via a simulation of configurationally averaged x-ray absorption near-edge structure (XANES) and EXAFS signals. The immediate neighborhood of Ag contains mostly oxygens while borons dominate at larger distances. The combination of EXAFS and XANES techniques allowed us to determine a more complete structural model than would be possible by relying solely on either EXAFS or XANES alone.« less