The defect structure and ionic diffusion processes within
the anion-deficient,
fluorite structured system Ce1–x
Y
x
O2–x/2 have been investigated at high temperatures (873 K–1073 K)
as a function of dopant concentration, x, using a
combination of neutron diffraction studies, impedance spectroscopy
measurements, and molecular dynamics (MD) simulations using interionic
potentials developed from ab initio calculations. Particular attention
is paid to the short-range ion–ion correlations, with no strong
evidence that the anion vacancies prefer, at high temperature, to
reside in the vicinity of either cationic species. However, the vacancy–vacancy
interactions play a more important role, with preferential ordering
of vacancy pairs along the ⟨111⟩ directions, driven
by their strong repulsion at closer distances, becoming dominant at
high values of x. This effect explains the presence
of a maximum in the ionic conductivity in the intermediate temperature
range as a function of increasing x. The wider implications
of these conclusions for understanding the structure–property
relationships within anion-deficient fluorite structured oxides are
briefly discussed, with reference to complementary studies of yttria
and/or scandia doped zirconia published previously.
The highly disordered structure of the delta phase of Bi2O3, which possesses the highest known oxide-ion conductivity, has been studied using neutron powder diffraction. A detailed analysis of data collected at 1033(3) K using Rietveld refinement indicates that the time-averaged structure of delta-Bi2O3 can be described using the accepted model of a disordered, anion-deficient fluorite structure in space group Fm3m. However, reverse Monte Carlo modelling of the total (Bragg plus diffuse) scattering demonstrates that the local anion environment around the Bi3+ resembles the distorted square pyramidal arrangement found within the stable alpha and metastable beta phases at ambient temperature, which is characteristic of the cation's 6s2 lone-pair configuration. Similarities between the structures of the highly disordered delta phase and the ambient temperature metastable beta phase are used to support this assignment and assess the validity of previous structural models based on short-range ordering of vacancies within the cubic lattice of delta-Bi2O3.
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