Thus, the real space investigation confirmed the presence of Eu atoms within the ZnO nano-crystals. However, the model with 4-8 Eu atom sized oxide clusters present within the ZnO, as discussed in previous work for a 5Eu sample heated at 600 o C 22 was not unequivocally confirmed. The Eu-rich amorphous inter-crystalline phase revealed was most significant for the 20Eu sample, but was also present to some extent in the 5Eu sample. The rather homogenous thickness and nature of this ZnO:Eu grain-boundary phase showing 3-4 times higher relative Eu concentration than the ZnO:Eu crystals indicate that it was formed by expulsion of Eu-oxide from the highly Eu-doped metastable ZnO:Eu crystals during heat-treatment. The 20Eu sample is expected from a structural point of view to be able to include at most 10-15% Eu as 4 to 8 Eu atom sized clusters in the ZnO crystals, as there would not be enough ZnO to cover the clusters otherwise. The XRD and IR data indicated that the sample to have at least some material with a different doping mode besides that observed for the 5Eu and 10Eu sample. This difference might be connected to the Eu-rich ZnO:Eu crystal boundary phase observed mainly in the 20Eu sample, but also to some extent, in the 5Eu. The presence of this phase in the 5Eu sample may be due to the higher temperature used in this study compared to the previous one; 650 o C in the present study, compared to 600 o C in the previous study. It was previously shown by TEM that the first very rare 2-3 nm sized Eu2O3 crystals formed at the ZnO:Eu sponge surface upon heating at 700 o C. A more easy adaptation of the Euoxygen-coordination polyhedral and cluster is expected in the thin amorphous inter ZnO:Eu-crystal boundaries, not the least where ZnO:Eu crystals with misaligned lattice directions meet. For comparison, the 5-10 Å thickness in the 20Eu sample corresponds to ca. 2-4 Eu/ZnOx units. The inter-crystalline phase may also include ZnO.