As discussed above, the distribution of Eu and Zn atoms in the 5Eu and 20Eu nano-sponges were obtained from micrometre to nanometre scale by the HAADF-STEM and EELS studies. To get a closer insight into the local atomic structure, the inter-atomic distances were investigated with ePDF. Figure shows the calculated rPDF of a standard h-ZnO structure (PDF card 00-036-1451) and the experimental rPDF of the 5Eu and 20Eu samples. The most obvious differences between the 5Eu and 20 Eu samples and the h-ZnO rPDF were the peaks found at r = 2.1-2.8 Å, which were absent in the calculated h-ZnO rPDF. The peaks at longer distances showed significant variations between the samples, but as these peaks originate from several different atom-pairs having similar distances, these peaks were more difficult to analyse in detail. Tentative assignments of the atom-pairs contributing to the composite peaks in the range r = 1.5-5. The ZnO:Eu samples and calculated h-ZnO rPDF showed a clear Zn-O peak at 1.97 Å (5Eu) -2.02 Å (20Eu), which was overlapped by the broad Eu-O peak in the latter case. This peak fits the Zn-O distances given as 1.974-1.989 Å for h-ZnO (PDF card: 00-036-1451) well. The Zn 2+ -ions of the h-ZnO are all tetrahedral four-coordinated by oxygen ions and the Zn-O distances also match the sum of the ionic radii of CN4 Zn 2+ -and O 2-ions (60 + 140 pm). A clear difference between the two ZnO:Eu samples could be observed in the Eu-O peaks. The 5Eu sample showed a single Eu-O peak at 2.2 -2.7 Å, with a maximum at 2.51 Å. These Eu-O distances are in fair agreement with those obtained in the theoretic 4.1Eu structures containing oxide clusters with 4 or 8 Eu atoms. The Euoxide clusters contain Eu 3+ -ions coordinated mainly by 5 or 6 oxygen atoms, but in some cases a four-coordinated Eu-ion is present. This led to very unusual low average Eu CN:s of 5.2-5.5 for the theoretic structures fitting the experimental data quite well and yielding the lowest energy. The 20Eu sample showed a much wider Eu-O peak covering distances from ca. 2.1 to 2.8 Å, with the peak shape at the longest distances of the band being similar to that of the 5Eu sample and a maximum at 2.40 Å. This broad band could tentatively be deconvoluted into serval peaks corresponding to different Eu-O coordination environments obtained from the modelled 4.1Eu structures (Figure ). The shortest Eu-O distances at ca. 2.1-2.4 Å fit the 2.22-2.26 Å found for four-coordinated Eu-ions in the theoretic Eu-oxide clusters , as well as the extrapolated sum of four-coordinated Eu-ions (0.84 Å) and oxygen-ions (1.40 Å) 28 of 2.24 Å. The peak component at 2.3-2.5 Å, was supplemented by smaller component at 2.1-2.6 Å, to fit the rPDF of the 20Eu in this region. These two combined peak components are in good agreement with the 2.12-2.45 Å obtained for five-coordinated theoretic clusters in 4.1Eu structures, as well as the bond distance of 2.29 Å obtained from the extrapolated ionic radii of five-coordinated Eu 3+ -ions (0.89 Å) and O 2-ions (140 pm). The longest Eu-O bonds were in the range 2.4-2.6 Å, which is in line with the ca. 2.50 Å obtained from the six-coordinated Eu 3+ -ions in the theoretical 4.1Eu structures. This yields an Eu 3+ ionic radius of 1.10 Å, which is longer than the reported experimentally obtained ionic radius of the Eu-ion (0.94 Å). The difference in ionic radii found for six-coordinated Eu-ions in the literature and the present and previously obtained experimental and theoretic structures may stem from the former being close packed crystalline structures while the latter are part of non-periodic Eu-oxide clusters containing differently coordinated Eu-ions adapting their coordination sphere to fit the ZnO lattice which show very little change due to the dopant clusters.