A fast salt-based synthesis route to nano-crystalline ZnO:Eu (0-15% EuO_1.5) sponges built of ~10 nm sized ZnO:Eu crystallites is presented. The phase-development from 200 to 900 ^oC using TG, DSC, XRD, IR spectroscopy, SEM, TEM, XPS, and EXAFS is described. The ZnO:Eu sponges obtained at up to 600 ^oC showed only h-ZnO with unit cell-dimensions very close to the un-doped ZnO. In the range 700-1000 ^oC, phase separation took place yielding c-Eu₂O₃ nano-crystals on the ZnO:Eu sponge surface. The ZnO:Eu sponges obtained at 600 ^oC were elementally homogenous while containing high concentrations of the large, alio-valent Eu³⁺-ions. The nature and even existence of such high doping levels is debated and was thus of particular interest. The dopant structure was studied using EXAFS and DFT. It was found that the ZnO structure could be close to unchanged while containing 5% EuO_1.5 for structures containing Eu-oxide clusters of 4 or 8 Eu³⁺-ions. These structures showed excellent agreement with all other experimental data. Structures containing smaller Eu-oxide clusters with two Eu³⁺-ions were less stable and deviated from the experimental data, but were more stable than those with random Eu³⁺ ions. These structures were highly unexpected, and may help in the understanding of doped optic and magnetic semiconductor oxides.
 
Europium-doped ZnO nanosponges – controlling optical properties and photocatalytic activity, R. Marin, F. Oussta, S. Naim Katea, S. Prabhudev, G. A. Botton, G. Westin, E. Hemmer, J. Mater. Chem. C. 2019, 7, 3909.

Fast, low-cost synthesis of ZnO:Eu nanosponges and the nature of Ln doping in ZnO, S. Naim Katea, P. Broqvist, J. Kullgren, E. Hemmer, G. Westin, Inorg. Chem. 2020, 59, 7584.

Entrapped molecule-like europium-oxide clusters in zinc oxide with nearly unaffected host structure, S. Mukherjee, S. Naim Katea, E.M. Rodrigues, C. Segre, E. Hemmer, P. Broqvist, H. Rensmo, G. Westin, Small, 2022, 2203331.