Understanding the local structure of Eu3+- and Y3+-stabilized zirconia: insights from luminescence and X-ray absorption spectroscopic investigations

This study combines bulk structural and spectroscopic investigations of Eu3+- or Y3+/Eu3+ co-doped tetragonal and cubic zirconia polymorphs to gain an in-depth understanding of the solid solution formation process. Our bulk structural characterizations show that the dopant is homogenously distributed in the ZrO2 host structure resulting in an increase of the bulk symmetry with increasing dopant substitution (from 8 to 26 mol%). The local site symmetry around the Eu3+ dopant, however, determined with luminescence spectroscopy (TRLFS), remains low in all samples. Results obtained with X-ray pair distribution function and X-ray absorption spectroscopy show that the average coordination environment in the stabilized zirconia structures remains practically unchanged. Despite this very constant average dopant environment, site-selective TRLFS data show the presence of three nonequivalent Eu3+ environments in the ZrO2 solid structures. These Eu3+ environments are assumed to arise from Eu3+ incorporation at superficial sites, which increase in abundance as the size of the crystallites decrease, and incorporation on two bulk sites differing in the location of the oxygen vacancies with respect to the dopant cation.