[en] The luminescence spectra of CaS and SrS (as well as their solid solutions Ca_1-xSr_xS) doped with cerium have been measured under excitation by x-ray tube and x-ray synchrotron radiation. The comparison of these spectra with those of photoluminescence has revealed two types of radiation-induced Ce³⁺ emission centre. We suppose that the first type is an activator ion displaced from the regular site as a result of the Coulomb interaction with charged defects (trapped electrons and holes) created in its neighbourhood by the excitation in the fundamental adsorption range, whereas the second type is probably due to the creation, by x-ray exciting radiation, of sulphur vacancies in the first coordination shell of Ce³⁺. (author)

[en] Full text.The rare earth ions embedded in crystalline matrices are the subject of intense research in materials chemistry. During these last years, the Pr³⁺ doped compounds acquired a particular importance because of the possibilities of application in new scintillator materials or tunable UV lasers. The LiYF₄ crystal is one of the most promising candidates for this last application. The aim of this work is the study of the luminescence properties of this compound in the visible and ultraviolet part of the spectrum. The LiYF₄ compound is isostructural with the calcium tungstate (scheelite) CaWO₄ which exhibits a tetragonal structure. the space group of symmetry is C_4h⁶ with four LiYF₄ molecules by elementary cell. The Li atom is placed in the centre of a tetrahedron formed by 4 atoms of fluorine; 8 such tetrahedrons surround each Y³⁺ ion. When the Pr³⁺ ions are embedded in the LiYF₄ matrices they substitute certain Y³⁺ ions, no compensator of charge being then necessary. Only one site of symmetry characterizes the ions Pr³⁺, it is of S₄ type. trivalent praseodymium is an attractive optical activator, for its energy levels spectrum contains several metastable states ¹G₄, ¹D₂, ³P_0,1,2. Therefore, it is possible to stimulate the emission in the blue, the green, the orange, the red and the infra-red. Moreover broad bands corresponding to interconfigurational 4f5d-4f² transitions can also be observed. We have measured and analyzed the specter of ray diffraction X for the purpose to characterize the material studied. We have carried out emission and excitation spectra measurements using a Perkin-Elmer LS 55 luminescence spectrometer at different temperature (room, liquids azoth). the observed emission and ax citation bands were analyzed and interpreted

[en] Full text.In the last few years, the Pr³⁺ doped compounds acquired a particular importance because of the possibilities of application in new scintillator materials or tunable UV lasers. The LiYF₄ crystal is one of the most promising candidates for this application. The aim of this work is the study of LiYF₄: Pr³⁺ luminescence properties in the visible and ultraviolet part of the spectrum and the calculation of energy levels of Pr³⁺ 4f² configuration in this host matrix by using the crystal field method. X-ray diffraction analysis revealed that LiYF₄ is isostructural with the calcium tungstate (scheelite) CaWO₄ which exhibits a tetragonal structure. The space group of symmetry is C_4h⁶ with four LiYF₄ molecules by elementary cell. The Li atom is placed in the centre of a tetrahedron formed by 4 atoms of fluorine; 8 such tetrahedrons surround each Y³⁺ ion. When the Pr³⁺ ions are embedded in the LiYF₄ matrices they substitute certain Y³⁺ ions, no compensator of charge being then necessary. Only one site of symmetry characterizes the ions Pr³⁺, it is of S₄ type. We have carried out emission and excitation spectra measurements using a Perkin-Elmer LS 55 luminescence spectrometer at room and liquid nitrogen temperatures. The observed emission and excitation bands were analyzed and interpreted. In the visible region, the emissions observed were attributed to the 4f² - 4f² intra configurational transitions with low intensities, as they are parity forbidden. The majority of the peaks observed in the visible part corresponds to forced electric dipole transitions, because Pr³⁺ site in LiYF₄ does not exhibit inversion symmetry. it has been shown that the transitions take place per dominantly from ³P₀ level and nonradiative relaxation rate between ³P₀ and ¹D₂ is weak even at room temperature. the emission and excitation bands observed in the ultraviolet region were assigned to the 4f²-4fd inter configurational transitions which are characterized by broad bands because of the strong electron-phonon interaction and significant intensities, for they are parity allowed. It has been shown that excitation in the lowest level of 4f5d configuration yields in the ultraviolet and blue regions intense broad emission bands which can be attributed in order of decreasing energy, to the emission from this level to the following groups of 4f² ground configuration levels; ³H₄, ³H₅, (³H₆, ³F₂), (³F₃, ³F₄), (³P_j + ¹I₆) and ¹D₂. Thus, following 4f5d excitation, the higher levels of 4f² configuration are populated, in particular ³P_j triplet. The emission starting from these levels yields spectra identical to those measured by exciting directly towards ³P_j triplet, but with lower intensity. The 4f² energy level diagram calculations were carried out using the SPECTRA program developed at Argonne national Laboratory (USA). This program is based on the standard phenomenological model for a 4f^N ion embedded in a crystalline environment. As usual, the parameters to be adjusted involve the F², F⁴ and F⁶ Slater parameters describing the inter electronic repulsion between the two electrons of the 4f² configuration, the ζ_4f spin-orbit coupling constant (ζ_4f=<4f/r/4f>), the Trees parameters α, β, γ and the (B_q^K)_4∫crystal field parameters. The sets of free-ion and crystal field parameters corresponding to the best fits allow to reproduce satisfactorily the experimental spectra

[en] The luminescence of three compounds: LuBO₃, with calcite and vaterite structures, and YBO₃ (vaterite structure) doped with Pr³⁺ ions has been investigated. The emission was ascribed to four spin-allowed transitions and one spin-forbidden transition from the 4f5d to the 4f² configuration. Two additional broad emission bands with very weak intensity at 440 and 580 nm were studied. Evidence of the presence of Ce³⁺ traces is reported and their influence on Pr³⁺ luminescence is discussed. (authors)

[en] Luminescence of LiYF₄: Pr³⁺ in the visible and near ultraviolet region is investigated. In the visible region, the emissions observed were attributed to intra-4f²-configuration transitions. It has been shown that these transitions take place predominantly from ³P₀ level and nonradiative relaxation rate between ³P₀ and ¹D₂ is weak even at room temperature. The emission and excitation bands observed in the ultraviolet region were assigned to 4f² 4f5d inter configurational transitions. The energy levels of Pr³⁺ 4f² configuration are fitted by using the crystal field method. The set of free-ion and crystal field parameters corresponding to the best fits allow to reproduce satisfactorily the experimental spectra. (authors)

[en] Calcite, vaterite and aragonite type rare earth LnBO₃ (Ln=Lu, Y, La) orthoborate powders, doped with 1% cerium or praseodymium, were prepared by the classical solid state reaction method. The structure and the morphology of these powder materials were checked by X-ray Diffraction, Fourier Transform Infra Red Spectroscopy (FTIR) and Scanning Electron Microscopy. Room temperature excitation and emission spectra of four compounds: LuBO₃ with calcite and vaterite structure, YBO₃ (vaterite type structure) and LaBO₃ with aragonite, doped with 1% cerium or praseodymium ions, have been measured and investigated. The effect of Ce³⁺ and Pr³⁺ crystalline environment in these compounds on the position of their 5d (4f5d) levels has been discussed. This work is also devoted to the problem of the electron-vibrational interaction (EVI) in 4f–5d optical transitions in orthoborate materials. The emission and excitation 4f–5d transitions resulted in broad vibronic bands whose shape functions were described and found. The main EVI parameters, such as the Huang-Rhys factor, effective phonon energy, and zero-phonon line position, were estimated. These values are checked by modeling the Ce³⁺ 5d–4f and Pr³⁺ 4f5d–4f² emission lines shapes, in which good agreement with experimental spectra confirms validity of the performed analysis. - Highlights: ► Ce³⁺ and Pr³⁺-doped calcite, vaterite and aragonite LnBO₃ (Ln=Lu, Y, La) orthoborates were successfully prepared by the solid state reaction method. ► The effect of Ce³⁺ and Pr³⁺ crystalline environment in orthoborates on the position of their 5d levels has been discussed. ► The main electron-vibrational interaction parameters (EVI), such as the Huang-Rhys factor, effective phonon energy, and zero-phonon line position, were estimated and discussed.