It was therefore interesting to investigate in detail how high Eu-doping levels the ZnO:Eu nano-sponges can be accommodated without phase separation detectable with XRD, and what structures form when exceeding the maximum Eu-doping limit. Herein, XRD and IR spectroscopy were used to find out the temperature limits for the presence of carbonate (lowest T) and phase separation (highest T) versus Eu-oxide concentration. Considering the doping model with 4 to 8 Eu membered non-periodic oxide clusters in crystalline h-ZnO given in literature, it should only be possible to reach 10-15% EuO1.5 in the ZnO before there is not enough ZnO to cover each cluster with one layer of ZnO. However, XRD and IR spectroscopy describe the materials as an average over the volume of the material which does not allow for obtaining important details on where the Eu resides, its oxidation state, the Zn-and Eu-to-oxygen coordination spheres, and the presence of more than one phase. Therefore, an extensive study using TEM/STEM in real-space and reciprocal-space was used to provide the detailed structural features of the ZnO:Eu nano-sponges. The STEM images and spectroscopy showed that the Eu was present in the ZnO crystals, but also formed an Eu(III)-oxide rich inter-crystalline amorphous phase, especially pronounced with the higher Eu-doping levels. The ePDF revealed atom-atom pair distances within the ZnO:Eu sponges, which further proved the existence of Eu-oxide within the ZnO as well as the Eu-rich inter-crystalline boundary phase.