C. Heremans et al., FAST-ION CONDUCTING Y-2(ZRYTI1-Y)(2)O-7 PYROCHLORES - NEUTRON RIETVELD ANALYSIS OF DISORDER-INDUCED BY ZR SUBSTITUTION, Journal of solid state chemistry, 117(1), 1995, pp. 108-121
The structure of Y-2(ZryTi1-y)O-7 solid solutions progressively change
s, with increasing y, from an ordered pyrochlore structure A(2)B(2)O(7
) square, space group Fd3m, to a deflect-fluorite structure (A, B)(4)
(O(0.875)square(0.125))(8), space group Fm3m, at y = 0.90. The anion a
rray consists of three independent sites O(1), O(2), and O(3), occupyi
ng positions 48f, 8a and 8b, respectively, of which 8b is unoccupied i
n 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 pyrclc
hlore superstructure intensity data provided weighted profile residual
s 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 displac
ed 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 b
y Y for y less than or equal to 0.45 and is predominantly Y at y = 0.6
0. The substituted Zr4+ thus replaces Ti4+ in the six-coordinated B si
te for most of the solid-solution series. Complete mixing of all three
cation species occurs abruptly over the narrow compositional range 0.
60 < y less than or equal to 0.90. The disordered structural states ca
nnot be described by a single-order parameter. The O(1) ion is displac
ed 0.46 Angstrom toward the vacant O(3) site in the ordered pyrochlore
at y = 0. The displacement relaxes to the ideal position of the fluor
ite structure with a quadratic dependence on y. Increase of the anisot
ropic temperature-factor coefficients with y indicates general softeni
ng of the structure with increased disorder. The array of O(1) ions fo
rms a continuous path for migration of anions through the structure. T
he magnitude and composition dependence of the reported change of ioni
c conductivity with y may be satisfactorily explained by the variation
of the product of charge-carrier concentration and vacancy concentrat
ion N(N - 1), where N is taken as the site occupancy of O(1). (C) Acad
emic Press, Inc.