Tuomas Aitasalo

Application of lanthanide (Eu, Nd) spectroscopy as a structural probe of diluted double phosphates

Abstract The present studies are focused on the application of europium(III) emission as a structural probe of diluted double phosphate and the determination of the number of metal sites, as well as their symmetries. The influence of the M(I) ions on the structure of phosphates is shown in the emission spectra at 77 K. The IR and Raman spectra were measured and used in the assignment of the vibronic components of electronic transitions. The electron–phonon coupling was analyzed in the emission and excitation spectra of europium double phosphate. Moreover, absorption spectra at 293 and 4 K as well as intensity analysis of Nd(III) f–f transition were used in the detection of structural modifications in both types of phosphates—rubidium (1) and sodium (2) salts. The energy level diagram was proposed and compared to the earlier reported data for different types of phosphates.

Synchrotron radiation investigations of the Sr2MgSi2O7:Eu2+,R3+ persistent luminescence materials

Abstract The electronic and defect energy level structure of polycrystalline Sr2MgSi2O7:Eu2+,R3+ persistent luminescence materials were studied with thermoluminescence and different synchrotron radiation spectroscopies (UV-VUV emission and excitation, X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)). Special attention was paid on the effect of the R3+ co-dopants on the persistent luminescence properties of the materials. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (Eg) value of ca. 7.1 eV agreed very well with the DFT value of 6.7 eV. The variation of the Eg value was attempted to relate with the trap structure as well as with the different properties of the R3+ co-dopants. The trap level energy distribution depended strongly on the R3+ co-dopant except for the shallowest trap energy above the room temperature remaining practically the same, however. The different processes in the mechanism of persistent luminescence from Sr2MgSi2O7:Eu2+,R3+ were assembled and their contributions discussed.

Spectroscopic and Structural Properties of Ca1-xSrxAl2O4:Eu2☎, RE3☎ Persistent Luminescence Materials

The preparation, structure and luminescence of the Ca1-xSrxAl2O4:Eu2☎, RE3☎ system were studied. Monoclinic CaAl2O4 was the major phase when the strontium content x was from 0 to 0.6, but hexagonal SrAl2O4 was obtained when x was 0.8 and monoclinic SrAl2O4 when x was 1. Only slight Ca/Sr cation solid solubility was observed. The strontium ions dissolved better into the CaAl2O4 phase than vice versa. Two luminescence bands were observed for mixed compositions, peaking at 440 and 530 nm, corresponding to those of the monoclinic CaAl2O4:Eu2☎ and hexagonal SrAl2O4:Eu2☎ ones. The persistent luminescence was enhanced by the Ca/Sr replacement. This observation supports the mechanisms where the lattice defects act as traps.

Persistent luminescence of Eu2+ and Na+ doped alkaline earth aluminates

The luminescence and afterglow properties of the Eu2☎ and Na☎ doped alkaline earth aluminales, stoichiometric and non-stoichiometric (Mx Al2O4:Eu2☎, Na☎; M = Ca or Sr, x = 0.97, 1.00 or 1.03, XNa, = 0.02), were studied. Broad band luminescence and afterglow of the Eu2☎ ion were observed in the blue (λmax = 440nm) and green (λmax = 520nm) region for the calcium and strontium aluminates, respectively. Both Na☎ co-doping and strontium excess quenched the afterglow efficiently. The results supported the mechanism of the persistent luminescence where the cation vacancies act as traps. The results for the calcium aluminates were ambiguous, probably due to the slightly larger ionic radius of the Na☎ with respect to that of Ca2☎. The sodium ions may not fit into the calcium sites and thus form (an) independent compound(s).

Persistent Luminescence Materials

The luminescent efficiency of rare earth ions is usually drastically lowered when defects are present in the host lattice. Persistent luminescence is the most recent rare earth application based on lattice defects. Typical materials are the Eu2+ doped alkaline earth aluminates, MAl2O4:Eu2+ (M = Ca and Sr). The trivalent R3+ ions as codopants enhance greatly the duration and intensity of persistent luminescence. As a result of very slow thermal bleaching of the excitation energy from the lattice defects acting as traps, the new persistent luminescent materials yield luminescence still visible to naked eye for more than ten hours. Despite the seemingly simple stoichiometry and structure of these materials, the determination of persistent luminescence mechanism(s) presents a very complicated problem. This report presents in detail some of the factors affecting the luminescence properties of the Eu2+,R3+ doped MAl2O4. The possible mechanisms involved with different defect centers and interactions between them and the emitting Eu2+ ion are discussed based on the results of systematic investigations carried out on the preparation, composition, structure and different luminescence properties.