Order–disorder transformations induced by composition and temperature change in (SczYb1−z)2Ti2O7 pyrochlores, prospective fuel cell materials

The structures and site occupancies of Sc–Yb titanate solid solutions with the A2B2O7 pyrochlore structure type were determined by Rietveld analysis of powder-diffraction profiles obtained from a pulsed neutron source. The results are the first in situ studies of order–disorder transformations in pyrochlore that are induced by temperature increase (25<T<1500 °C) and the first examination of the structural changes caused by substitution of a smaller cation in the eight-coordinated A3+ site. Solid solution was found to extend to z<0.4. Decreases in the average value of the ionic radius RA and increase in the temperature (20<T<1500 °C) were found to cause mixing of the occupancy of the cation sites (anti-site defects) and partial occupancy of the normally vacant oxygen site (anion Frenkel defects). This behavior is analogous to the disorder previously shown to result from temperature increase or substitution of larger cations in the six-coordinated B site, which, as in the present compositions, serve to reduce the difference between the average value of RA and RB.