Environment of the Eu Ion within Nanocrystalline Eu-Doped BaAlO: Correlation of X-ray Diffraction, Mössbauer Spectroscopy, X-ray Absorption Spectroscopy, and Photoluminescence Investigations.

Powder samples of pure BaAlO and doped with 4.9 atom % Eu in relation to Ba were prepared by a hydrothermal route. The samples were characterized by X-ray diffraction, Eu Mössbauer spectroscopy, synchrotron-based X-ray absorption spectroscopy at the Ba L- and Eu L-edges, and photoluminescence measurements. Diffraction lines were broadened, indicating that the samples were nanocrystallline. The samples possessed a hexagonal crystal structure, space group P6. Eu Mössbauer spectroscopy revealed the presence of Eu in the 3+ oxidation state. The same information on the Eu oxidation state was also obtained by the Eu L-edge X-ray absorption near-edge structure of the doped sample. Extended X-ray absorption fine structure showed an Eu ion substituted for Ba on the Ba2 site in the BaAlO host structure, with charge compensation by an interstitial O in the vicinity of the Ba2 site. That was confirmed by a Rietveld structure refinement for the Eu-doped BaAlO sample. Analysis of the diffraction line broadening for the prepared samples was performed simultaneously with the structure refinement. Both the dopant Eu and the interstitial O acted as defects in the host BaAlO lattice, which increased the lattice strain from 0.02% for pure BaAlO to 0.17% for the Eu-doped sample. Crystallite sizes in the samples increased with Eu doping from 32 nm for pure BaAlO to 36 nm for Eu-doped BaAlO. This could likely be related to the increase in the diffusion rate of the cations in the sample when a part of the Ba cation content was exchanged with smaller Eu cations. The Eu-doped BaAlO sample exhibited red photoluminescence under excitation with λ = 308 nm. The observed emission spectrum indicated that Eu ions occupied the Ba site with lower symmetry in the doped sample.