The formation of substoichiometric mixed zirconium-yttrium oxides by electroreduction of cubic 9.5YSZ is investigated. A strongly oxygen depleted phase with the stoichiometry (Zr,Y) \(_{8.6}\) O is observed, forming belt-shaped features below the sample surface in the heavily reduced region close to the cathode. It is embedded in another oxygen depleted phase with the stoichiometry (Zr,Y) \(_2\) O. The electroreduction is performed by drawing a DC current through a single crystal with circular platinum electrodes. The new phases, which are possibly metastable and not yet reported in literature, are identified by STEM investigations and EDX, which is used to measure the composition. The found composition indicates Zr and Y with an oxidation state of +I, respectively, between +I and 0. The (Zr,Y) \(_{8.6}\) O phase has a significant distortion of the original cubic symmetry. The (Zr,Y) \(_2\) O phase exhibits a slight, the (Zr,Y) \(_{8.6}\) O phase a strong decrease of the molar volume compared to unreduced YSZ. A chequerboard-like structure on the surface of the single crystal can be most probably explained by strain relaxation due to dislocation gliding. Misfit dislocations can be found in the interface between the substoichiometric phases. The induced strain due to the volume contraction may also be responsible for the known deterioration of the mechanical properties after reduction.
