Neutron power diffraction structure and electrical properties of the defect pyrochlores Pb/sub 1. 5/M/sub 2/O/sub 6. 5/ (M = Nb, Ta)

Journal Article · Thu Dec 01 00:00:00 EST 1988 · J. Solid State Chem.; (United States)

DOI:https://doi.org/10.1016/0022-4596(88)90255-1· OSTI ID:6258650

Beech, F; Jordan, M; Catlow, C R.A.; Santoro, A; Steele, B C.H.

Power neutron diffraction and Rietveld analysis were used to investigate the crystal structures of the defective pyrochlores Pb/sub 1.5/Nb/sub 2/O/sub 6.5/ and Pb/sub 1.5/Ta/sub 2/O/sub 6.5/. Both materials crystallize with the symmetry of space group Fd3m, with lattice parameters a = 10.5647(2) and a = 10.5558(2) /Angstrom/, respectively. No evidence has been observed of oxygen or lead vacancy ordering in these compounds. This result is interpreted in terms of a model in which all lead present in the structure has sevenfold pyramidal coordination and forms domain separated by regions of lead vacancies with hexagonal or bipyramidal configurations of the oxygen atoms. This model, built on the assumption that the driving force in the formation of this type of defect pyrochlore is the coordination of lead, leads us to conclude that the system Pb/sub 1+x/M/sub 2/O/sub 6+x/ (M = Nb, Ta) may exist over a range of compositions with 0.33 less than or equal to x less than or equal to 0.6, and may also explain results obtained in other studies of related materials. The electric measurements show that both compounds are predominantly electronic conductors and that the ionic contribution to the total conductivity is very small even at the highest temperatures used in the study.

• Research Organization: Univ. College London (England)
• OSTI ID: 6258650
• Journal Information: J. Solid State Chem.; (United States), Vol. 77:2
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
LEAD OXIDES
CRYSTAL STRUCTURE
NIOBIUM OXIDES
TITANIUM OXIDES
ELECTRIC CONDUCTIVITY
EXPERIMENTAL DATA
LATTICE PARAMETERS
NEUTRON DIFFRACTION
POWDERS
SPACE GROUPS
CHALCOGENIDES
COHERENT SCATTERING
DATA
DIFFRACTION
ELECTRICAL PROPERTIES
INFORMATION
LEAD COMPOUNDS
NIOBIUM COMPOUNDS
NUMERICAL DATA
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
REFRACTORY METAL COMPOUNDS
SCATTERING
TITANIUM COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
360202* - Ceramics
Cermets
& Refractories- Structure & Phase Studies
360204 - Ceramics
Cermets
& Refractories- Physical Properties