[en] The Eu²⁺ doped alkaline earth aluminates, MAl₂O₄:Eu²⁺ (M = Ca and Sr) co-doped with selected R³⁺ ions are efficient new persistent luminescence materials. The detailed mechanisms of persistent luminescence are not yet known. The importance of the defects to induce persistent luminescence was studied here by EPR, Moessbauer, thermoluminescence (TL) and persistent luminescence measurements. The EPR results show that even the non-doped material, CaAl₂O₄, contains trapped electrons, probably in anion vacancies. Similar EPR properties are shown by the Eu²⁺ doped and R³⁺ co-doped materials. Other paramagnetic defects are present, too. The Moessbauer results indicated complex distribution of the Eu²⁺ ions in the CaAl₂O₄ host. Different R³⁺ ions affect the thermoluminescence and persistent luminescence properties of the CaAl₂O₄:Eu²⁺, R³⁺ materials in a very different manner, from enhancing to total suppression

[en] The UV excited and persistent luminescence properties as well as thermoluminescence (TL) of Eu²⁺ doped strontium aluminates, SrAl₂O₄:Eu²⁺ were studied at different temperatures. Two luminescence bands peaking at 445 and 520 nm were observed at 20 K but only the latter at 295 K. Both Sr-sites in the lattice are thus occupied by Eu²⁺ but at room temperature efficient energy transfer occurs between the two sites. The UV excited and persistent luminescence spectra were similar at 295 K but the excitation spectra were different. Thus the luminescent centre is the same in both phenomena but excitation processes are different. Two TL peaks were observed between 50 and 250 deg. C in the glow curve. Multiple traps were, however, observed by preheating and initial rise methods. With longer delay times only the high temperature TL peak was observed. The persistent luminescence is mainly due to slow fading of the low temperature TL peak but the step-wise feeding process from high temperature traps is also probable

[en] For around ten years, intense attention has been devoted to materials which exhibit strong persistent luminescence. The Eu²⁺ doped SrAl₂O₄ is probably the best one. Co-doping with other trivalent rare earth ions, especially with Dy³⁺, enhances the persistent luminescence. Co-doping with Sm³⁺ leads to a drastic decrease in the efficiency of persistent luminescence, however. In this paper, the results of a careful analysis of the dynamic luminescence properties of SrAl₂O₄:Eu²⁺ and co-doped either with Dy³⁺ or Sm³⁺ under different excitation schemes are presented. The temporal transients and time resolved luminescence spectra were analysed with pulsed UV excitation at 355 nm provided by the third harmonic of a Nd:YAG laser. Then continuous wave excitation at 360 nm was also applied simultaneously with the pulsed UV excitation. The profile of the transient luminescence was found to be strongly dependent on the excitation scheme, especially on the pumping power at 355 nm. In addition to the Eu²⁺ decay, other population processes were found to occur. Two components were observed for all the materials studied. In order to carry out a complete analysis of the data, the positions of the 4f and 5d energy levels for the R³⁺ and R²⁺ ions in the energy gap of SrAl₂O₄ were determined

[en] The electronic structures of the distrontium magnesium disilicate (Sr₂MgSi₂O₇(:Eu²⁺)) materials were studied by a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu²⁺ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu²⁺ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available.

[en] The Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ materials were prepared with a solid state reaction and their microscopic structure (at 295 K only) and luminescence were studied at selected temperatures between 150 and 295 K. Undisturbed Sr crystal planes were common in the TEM images of the undoped Sr₂MgSi₂O₇ material, whereas with Eu²⁺ doping more disturbed planes were observed even in the nanometer scale. With Dy³⁺ co-doping, a large number of small lattice domains created by the discontinuities in the crystal structure was observed. The domains with different orientations seem to be centered around point defects. The decay curves of Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ showed fast (ms scale) persistent luminescence. The intensity of persistent luminescence increased considerably between 200 and 250 K while remaining constant in the ranges of 150-200 and 250-295 K. The changes were used to study the depth of the traps. In general, Dy³⁺ co-doping was found to deepen the traps

[en] The Y₂O₃:Eu³⁺,Mg²⁺,Ti^IV materials (x_Eu: 0.02, x_Mg: 0.08, x_Ti: 0.04) were prepared by solid state reaction. The purity and crystal structure of the material was studied with the X-ray powder diffraction. Luminescence properties were studied in the UV-VUV range with the aid of synchrotron radiation. The emission of Y₂O₃:Eu³⁺,Mg²⁺,Ti^IV had a maximum at 612 nm (λ_exc: 250 nm) due to the ⁵D₀→⁷F₂ transition of Eu³⁺. The excitation spectra (λ_em: 612 nm) showed a broad band at 233 nm, due to the charge transfer transition between O^2- and Eu³⁺, and at 297 nm due to the Ti→Eu³⁺ energy transfer. Only very weak persistent luminescence was discovered. In the room and 10 K temperature excitation spectra, the line at 208 nm is due to the formation of a free exciton (FE) and a broad band at 199 nm was related to the valence-to-conduction band absorption of the Y₂O₃ host lattice. The absorption edge was ca. 205 nm giving 6.1 eV as the energy gap of Y₂O₃.

[en] The electronic structure of the polycrystalline CaAl₂O₄:Eu²⁺,Ce³⁺ persistent luminescence materials were studied with X-ray absorption (XANES) and UV-VUV emission and excitation spectroscopy by using synchrotron radiation. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (E_g) value of 6.7 eV agrees very well with the DFT value of 6.4 eV. From the 4f⁷→4f⁶5d¹ excitation bands of Eu²⁺, the positions of the 4f⁷ ground as well as the 4f⁶5d¹ excited levels were established. The excitonic fine structure which could act as trap levels close to the bottom of the conduction band could not be observed, however. The different processes contributing to the mechanism of persistent luminescence from CaAl₂O₄:Eu²⁺ were constructed and discussed.

[en] Ba₂MgSi₂O₇:Eu²⁺,R³⁺ (R=Y, La-Nd, Sm-Lu) materials were prepared with a solid state reaction. The UV excited (λ_exc=355nm) and persistent luminescence bands of Ba₂MgSi₂O₇:Eu²⁺ were both centered at 500nm and thus they are due to the same Eu²⁺ ions. The Tm³⁺ co-doping induced by far the strongest enhancement of the persistent luminescence intensity and duration. The next strongest enhancement was achieved by the Er³⁺, Nd³⁺ and Pr³⁺ co-doping. The enhancement by the remaining R³⁺ co-doping ions was quite similar. The decay of the persistent luminescence consists of at least three processes. First process is very fast whereas the lifetime of the second process is in the range from 3.7 to 9min depending on the co-doping ion. For the Pr³⁺, Nd³⁺, Er³⁺ and Tm³⁺ co-doped materials a third process with a lifetime varying from 14 to 28min is observed. The persistent luminescence is suggested to involve lattice defects, i.e. oxygen (cation) vacancies, which create trapping levels for electrons (holes). The details of the mechanism(s) are still, however, under further investigations