The YPrO<sub>3+δ</sub> system
is a nearly ideal model system for the investigation of oxide defect
creation and annihilation in oxide ion conductor related phases with
potential applications as solid state electrolytes in solid oxide
fuel cells. The formation, structure, high temperature reactivity,
and magnetic susceptibility of phase pure YPrO<sub>3+δ</sub> (0 ≤ δ ≤ 0.46) are reported. The topotactic
reduction and oxidation of the YPrO<sub>3+δ</sub> system was
investigated by powder X-ray <i>in situ</i> diffraction
experiments and revealed bixbyite structures (space group: <i>Ia</i>3̅) throughout the series. Combined neutron and
X-ray data clearly show oxygen uptake and removal. The research provides
a detailed picture of the Y<sup>3+</sup>/Pr<sup>3+</sup>/Pr<sup>4+</sup> sublattice evolution in response to the redox chemistry. Upon oxidation,
cation site splitting is observed where the cation in the (<sup>1</sup>/<sub>4</sub>, <sup>1</sup>/<sub>4</sub>, <sup>1</sup>/<sub>4</sub>) position migrates along the body diagonal to the (<i>x</i>, <i>x</i>, <i>x</i>) position. Any oxygen in
excess of YPrO<sub>3.0</sub> is located in the additional 16<i>c</i> site without depopulating the original 48<i>e</i> site. The <i>in situ</i> X-ray diffraction data and thermal
gravimetric analysis have revealed the reversible topotactic redox
reactivity at low temperatures (below 425 °C) for all compositions
from YPrO<sub>3</sub> to YPrO<sub>3.46</sub>. Magnetic susceptibility
studies were utilized in order to further confirm praseodymium oxidation
states. The linear relation between the cubic unit cell parameter
and oxygen content allows for the straightforward determination of
oxygen stoichiometry from X-ray diffraction data. The different synthesis
strategies reported here are rationalized with the structural details
and the reactivity of YPrO<sub>3+δ</sub> phases and provide
guidelines for the targeted synthesis of these functional materials
