BaCe_1-xPd_xO_3-δ (0 ≤ x ≤ 0.1): Redox controlled ingress and egress of palladium in a perovskite

We demonstrate using a combination of X-ray and neutron diffraction and X-ray photoelectron spectroscopy that Pd²⁺ ions can be substituted for Ce in perovskite BaCeO₃ and that, under oxidizing conditions, BaCe_1-xPd_xO_3-δ (0 ≤ x ≤ 0.1) compositions can be prepared. Neutron diffraction has helped verify that δ ≈ x, implying that Ce⁴⁺ ions are substituted by Pd²⁺ and that a vacancy is concurrently created on the oxygen sublattice. The structure of the host compound and models for Pd substitution have also been studied using density functional theory, which has provided a detailed local description of the structure. The Pd²⁺-containing perovskite phases extrude elemental face-centered cubic palladium when heated in a reducing atmosphere. This elemental palladium is re-absorbed as ions into the perovskite lattice upon heating in flowing oxygen. Evidence for such cyclable ingress and egress of palladium under redox conditions is presented. A curious morphological change that results from the redox cycling of BaCe_1-xPd _xO_3-δ is the formation of perovskite nanowires, which we believe to arise from regrowth of the perovskite around particles of elemental palladium.