Total scattering analysis of cation coordination and vacancy pair distribution in Yb substituted δ-Bi₂O₃

Abstract

Reverse Monte Carlo (RMC) modelling of neutron total scattering data, combined with conventional Rietveld analysis of x-ray and neutron data, has been used to describe the cation coordination environments and vacancy pair distribution in the oxide ion conducting electrolyte Bi3YbO6. The thermal variation of the cubic fluorite unit cell volume, monitored by variable temperature x-ray and neutron experiments, reveals significant curvature, which is explained by changes in the oxide ion distribution. There is a significant increase in tetrahedral oxide ion vacancy concentration relative to δ-Bi2O3, due to the creation of Frenkel defects associated with the Yb(3+) cation. The tetrahedral oxide ion vacancy concentration increases from room temperature to 800 °C, but little change is observed in the vacancy pair distribution with temperature. The vacancy pair distributions at both temperatures are consistent with a favouring of [100] vacancy pairs.

New Insights into Complex Materials Using Reverse Monte Carlo Modeling

“…In contrast to the case for pure δ-Bi 2 O 3 , the angular distribution functions calculated from RMC configurations provide direct evidence for the ordering of vacancies, predominantly in the <110> direction. Two Ybdoped Bi oxides were examined in a similar fashion (40,41), and again distinct differences between dopant-O and Bi-O correlations were found. In these materials, vacancy ordering along the <100> direction was favored energetically, although the <110> pairs were more numerous (due to being statistically the most likely).…”

Defect structure in δ-Bi5PbY2O11.5

“…The change in slope in the neutron data at around 450 C, is a feature common in d-phase bismuth oxide based systems and has been previously correlated with changes in oxide-ion/vacancy distributions. 29,[48][49][50] 3.2 Local structure analysis M-O and O-O pair correlation functions, g ij (r), derived from the RMC analyses at 20, 500 and 700 C are shown in Fig. 6.…”

Oxide ion distribution, vacancy ordering and electrical behaviour in the Bi3NbO7–Bi3YbO6pseudo-binary system

“…This trend is similar to that seen in the Nb/Y analogue 36 and can be associated with the change in the degree of distortion of the average dopant polyhedron, with more distortion of the niobate polyhedra (distorted niobate trigonal anti-prisms in the x ¼ 0.0 composition 23 ), compared to the less distorted ytterbate polyhedra (a distorted octahedral coordination for x ¼ 1.0 (ref. 34)). The oxide ion content in the 48i/24d sites does not vary signicantly with composition in this system.…”

“…Neutron powder diffraction data were obtained on the Polaris diffractometer at the ISIS Facility, Rutherford Appleton Laboratory for the x ¼ 0.2, 0.4 and 0.6 compositions. Data were collected on back-scattering (130-160 ), 90 (85-95 ), low-angle (28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42) and very low angle (13-15 ) detectors, over the respective time of ight ranges 1.0 to 20 ms, 0.8 to 19.2 ms, 0.5 to 20 ms and 2.0 to 18.6 ms. Room temperature measurements were made with the sample contained in a cylindrical vanadium can (standard 11 mm diameter can for x ¼ 0.2 and 0.4 and a thin walled 8 mm diameter can for x ¼ 0.6) located in front of the back-scattering detectors. For the x ¼ 0.2 and 0.4 compositions, data collections of ca.…”

“…In our own recent studies on the Bi 2 O 3 -Yb 2 O 3 system, using reverse Monte Carlo (RMC) modelling of total neutron scattering data, favouring of h100i vacancy ordering was observed. 33,34 A number of studies have examined co-doping of bismuth oxide, which has been observed to have the advantage of stabilising the cubic d-phase at lower levels of substitution than singledoping. 35 This has been explained in terms of an increased congurational entropy contribution.…”