As the STEM-HAADF and EELS mapping showed, this sample contained a substantial amount of Eu-rich intercrystalline amorphous phase. The theoretic 4.1Eu structures reported imply that a maximum of 10-15% EuO1.5 clusters can be present in the h-ZnO while providing enough ZnO to cover the clusters with one layer of ZnO4 tetrahedra. The ca. 5-10 Å (1-2 ZnO unit cells) thick amorphous inter-crystalline phase, to surround most ZnO:Eu crystals, could contain mainly low coordination Eu-ions and possibly some Zn-ions in higher five or six-coordinations, besides possible four-coordinated Zn-ions. The latter CN4 Zn 2+ -ions would not be possible to distinguish those in the h-ZnO structure. The constraints of the ZnO crystal facets may affect the bonding between the crystalline and amorphous phases and the various ionic radii of the metal ions involved are likely best satisfied by an amorphous Zn-Eu-oxide structure. The Zn 2+ -and Eu 3+ -ions having spherical d 10 and s 2 outer orbital shells without requirements of bond directions may make mixed coordination numbers and wide bondlength distributions facilitated. The different typical ionic sizes may be equalized in an amorphous structure by attaining the very low CN4 for Eu 3+ -ions (84 pm) and CN 5/6 for Zn 2+ -ions (74 pm/ 88 pm) which may allow for a tighter mixing of the ions. It can be mentioned that when looking at DFT generated Eu2 clusters in ZnO containing four-and five-coordinated Eu-ions, the mismatch in size of the Eu-ions and the surrounding crystalline ZnO structure caused a much larger increase in ZnO:Eu unit cell-dimensions associated with an increase in energy. This compared to the larger Eu-oxide clusters which more easily could adjust coordination within the larger cluster occupied cavity and thereby give very low increase in ZnO:Eu unit cell parameters and result in the lowest energies. In the amorphous structure described above there would not necessarily be a strain between the single Eu-ions or oxide clusters and ZnO as with the crystalline ZnO and therefore the formation energy may be lower compared to the crystalline case.