Optical spectroscopy and defect disorder of multi-site of Eu dopant in NaCaPO4 structure was studied by computational and experimental methods. The prediction of intrinsic disorder and the symmetry and detailed geometry of multi-site of Eu dopant was made by atomistic modeling. The crystal field parameters are then calculated and used to obtain the 7F1 sublevels of the Eu3+ ion, which we used to discuss the local symmetry of the multi-site of Eu3+. The calculations suggest that the most energetically feasible configurations involve the substitution of Eu3+ and Eu2+ ions at the Ca2 site. For Eu3+ substitution, charge charge compensation by NaCa2′ defect is the most provable. The solution energies for the other configuration, for the two Eu3+ dopant ions, are very similar. This means that the Eu3+ dopant can be found in all Ca sites in the NaCaPO4. The theoretical spectroscopic study showed that the symmetry of the Eu3+ ions, based on the amount of non-null crystal field parameters, is a C4 symmetry for all multi-site of Eu3+. A set of phenomenological charge factors and the 7F1 manifold split were satisfactorily reproduced in agreement with the experimental data.
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