Fast-ion conducting Y{sub 2}(Zr{sub y}Ti{sub 1-y}){sub 2}O{sub 7} pyrochlores: Neutron Rietveld analysis of disorder induced by Zr substitution
- Massachusetts Institute of Technology, Cambridge, MA (United States)
- National Institute of Standards and Technology, Gaithersburg, MD (United States)
The structure of Y{sub 2}(Zr{sub y}Ti{sub 1-y}){sub 2}O{sub 7} solid solutions progressively changes, with increasing y, from an ordered pyrochlore structure A{sub 2}B{sub 2}O{sub 7}{four_gradient}, space group Fd3m, to a deflect-fluorite structure (A,B){sub 4} (O{sub 0.875}{four_gradient}{sub 0.125}){sub 8}, space group Fm3m, at y = 0.90. The anion array consists of three independent sites O(1), O(2), and O(3), occupying positions 48f, 8a and 8b, respectively, of which 8b is unoccupied in a fully ordered pyrochlore. Rietveld powder-profile analysis of data collected with 1.5453-{angstrom} thermal neutrons was used to determine the structural state of four samples with increasing Zr content (y = 0.30, 0.45, 0.60, and 0.90). Refinements that employed only the pyrochlore superstructure intensity data provided weighted profile residuals that ranged 8.06 to 8.67% compared with expected values of 7.13 to 7.87% derived from counting statistics. The onset of disorder at y = 0.30 is marked by filling of the vacant 8b site with oxygen ions displaced from the nearest-neighbor anion shell - i.e., O(1) in 48f. Only for y > 0.45 does O(2) participate in the disorder. Mixing of the occupancy of cation sites A and B begins only with under occupancy of the O(2) site. The eight-coordinated A site, position 16c, is occupied solely by Y for y {le} 0.45 and is predominantly Y at y = 0.60. The substituted Zr{sup 4+} thus replaces Ti{sup 4+} in the six-coordinated B site for most of the solid-solution series. Complete mixing of all three cation species occurs abruptly over the narrow compositional range 0.60 < y {le} 0.90.
- DOE Contract Number:
- FG02-86ER45261
- OSTI ID:
- 171918
- Journal Information:
- Journal of Solid State Chemistry, Vol. 117, Issue 1; Other Information: PBD: Jun 1995
- Country of Publication:
- United States
- Language:
- English
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