Therefore, the possibility of increasing the Eu-oxide doping level, even beyond 5% was explored to find out how much of Eu-oxide clusters that can be accommodated within the ZnO structure and what happens when this dopant level is exceeded. X-ray diffraction (XRD) and IR spectroscopy indicated a change in doping mode between 10 and 20 at.% Eu. SEM, XRD, and IR spectroscopy, used here are techniques describing the average structure but do not provide information on the local structural changes on a very fine nano-or atomic-scale required to understand these complex materials. At these high doping levels, it is not unlikely that more than one kind of Eu-doped ZnO is present. Advanced HRTEM techniques were applied to examine the nano-crystalline ZnO:Eu sponges doped with 5 at.% and 20 at.% Eu at scales from entire sponge particles, via the ca. 10-20 nm sized ZnO:Eu crystals and grain-boundaries to atomic scale. HAADF-STEM imaging, of which the contrast strongly relies on the sample thickness and atomic number (Z), was used together with electron diffraction to reveal the structure at atomic scale. To gain a comprehensive understanding of the local atomic structure and correlations between europium, zinc, and oxygen atoms, as well as structural ordering and a possible secondary phase, electron pair-distribution-function (PDF) was used to reveal the atom-atom distances in a chosen area. PDF shows the probability of finding an atom at a certain distance r from a given atom. This function can be mathematically described as follows: