Marcos V. dos S. Rezende

Influence of co-dopant in the europium reduction in SrAl2O4 host

Xerogels of strontium chlorate and aluminium chlorate doped with europium (un-co-doped) and co-doped with rare earth ions (Ln = Gd, Dy, Er and Y) were prepared using the proteic sol-gel route. Synchrotron radiation was used to investigate the effect of different co-dopants on the Eu(3+) → Eu(2+) reduction process during the synthesis of the samples. Samples were excited at the Eu LIII-edge and the XANES regions were analyzed. The results suggest that some of the Eu ions can be stabilized in the divalent state and that it is difficult to completely reduce Eu(3+) to Eu(2+) during thermal treatment. The mechanisms of the Eu reduction processes are explained by a proposed model based on the incorporation of charge-compensation defects.

Atomistic Simulation of Intrinsic Defects and Trivalent and Tetravalent Ion Doping in Hydroxyapatite

Atomistic simulation techniques have been employed in order to investigate key issues related to intrinsic defects and a variety of dopants from trivalent and tetravalent ions. The most favorable intrinsic defect is determined to be a scheme involving calcium and hydroxyl vacancies. It is found that trivalent ions have an energetic preference for the Ca site, while tetravalent ions can enter P sites. Charge compensation is predicted to occur basically via three schemes. In general, the charge compensation via the formation of calcium vacancies is more favorable. Trivalent dopant ions are more stable than tetravalent dopants.

The effect of the host composition on the lifetime decay properties of barium/strontium aluminates compounds

This paper reports the influence of the structural change on the luminescence of Eu-doped barium/strontium aluminates when excited with monochromatic X-rays (also known as X-ray excited optical luminescence—XEOL). Ba1−xSrxAl2O4 samples, with 0 < x < 1, were produced via proteic sol-gel route and it was observed that the XEOL emission spectra are composed by the Eu2+ and Eu3+ transitions, although no Eu2+ was observed in the X-ray absorption spectra. The XEOL intensities while the sample is under irradiation decreased as a function of the irradiation time, indicating the buildup of radiation damage. The saturation level of the XEOL is directly correlated to the amount of damages induced by the irradiation and the sample composition. The Ba-rich samples are the ones with higher XEOL yield. X-ray induced long lasting phosphorescence (LLP) was also observed for all samples and it was found that the duration of the phosphorescence emission also depends on the sample composition. In Sr-rich samples, the LLP has...

Effect of lithium excess on the LiAl 5 O 8 :Eu luminescent properties under VUV excitation

The influence of lithium excess and calcination temperature on the luminescence properties of the LiAl5O8:Eu under VUV excitation was investigated. The presence of both broad bands and sharp peaks in the VUV and X-ray emission spectra suggests the presence of Eu2+, even in absence of reducing atmosphere in the synthesis. The VUV excitation spectra indicated a band gap of 8.5 eV while the UV excited one showed the Eu3+ charge transfer transition starting at 3.9 eV. These values indicate that Eu2+ is stable in this host since its ground state is below the Fermi level of the host (ca. 4.1 eV). The relation between the intensity of Eu2+ and Eu3+ emissions showed that the reduction is favored at higher temperatures and lower Li content, leading to the proposition of a reduction mechanism based on the incorporation of charge compensation defects formed in the aliovalent doping of Eu3+ in Li+ sites.

Optical properties of Pr-doped BaY2F8

Crystalline samples of Pr-doped BaY2F8 (BaYF) were prepared by zone melting technique. The pure phase obtained was identified by X-ray diffraction measurement. Optical absorption result was evaluated and it showed that the formation of the absorption bands can be connected to color centers generated by radiation in the matrix. Radioluminescence emission measurements after excitation by X-ray showed that the material exhibited components responsible for long lasting phosphorescence. Short decay times were also evaluated, the measurements showed a fast component around 70 ns associated to the 4f15d1 → 4f2 transition of the Pr3+ ion. The Thermoluminescence (TL) results indicate the presence of two trapping centers.