Bernard Piriou

Study of sites occupation and chemical environment of Eu3+ in phosphate-silicates oxyapatites by luminescence

Abstract Silicate apatites called britholites have been studied due to their potential application as materials in the form of nuclear waste for the containment of actinides. The luminescence study of Eu 3+ in the solid solution Ca 10− x La x (SiO 4 ) y (PO 4 ) 6− y O z □ 2− z (with z =1+1/2( x − y )) allows us to predict the location of an eventual actinide ion and provide structural information on the expected behaviour of the structure towards irradiation damage. The luminescence study confirms the preferential location of the Eu 3+ ion in the 6 h site ( C s point symmetry) of the space group P6 3 /m , where a strong crystal field due to Eu 3+ –O(4) bond occurs. This bond competes with oxygen SiO 4 and PO 4 tetrahedra and the competing is found to be stronger when silicate groups are present. This is because silicate tetrahedra are less compact than phosphate tetrahedra and are able to approach Eu 3+ much closer. It is also shown that the crystal field strength decreases with silicate content. The luminescence spectra tend towards those which are more common of Eu 3+ luminescence in which phosphates are doped with rare earth ions. The spectra of luminescence versus temperature confirms that the crystal field between Eu 3+ (6h)–O(4) is stronger when the temperature is low and the cell parameters are small.

Evidence of Eu3+O2− associates by luminescence study of some silicates and aluminosilicates

Abstract Unusual luminescence spectra with a strong 5D0 arr 7F0 line, a high position of the 5D0 level and the occurrence of two lines close to 16550 cm−1, i.e. in a frequency domain at the border between 5D0 → 7F2 and 5D0 → 7F1 transitions, have been evidenced in the case of cordierite, Mg2Al4Si5O18:Eu, mullite, 2Al2O3.SiO2:Eu and lanthanum disilicate, La2Si2 O7:Eu. By comparison with results on apatites and some fluorides, these unusual spectra have been attributed to the presence of a strong and anisotropic crystal field due to an EuO bonding.

Time-resolved fluorescence spectroscopy of Cr3+ in mullite

Abstract Mullites doped with Cr2O3 concentrations ranging from 2 to 10 wt% were investigated by both timeresolved emission spectroscopy and site-selective excitation. The emission decays are non-exponential. They depend on Cr concentration and emission frequency and, hence, it is possible to distinguish different kinds of Cr3+ sites: at least two different types of low field site, one type of high field site and one intermediate field site. The emission decay time for Cr3+ in intermediate field sites was found to be the longest. In Cr2O3-poor mullites Cr3+ occurs dominantly at high field sites, i.e. at structural M(1) positions replacing Al3+, whereas in Cr2O3-rich mullites the majority of Cr3+ is incorporated in interstitial sites.

Free silicon and crystallization in silicon nitride based ceramics and in oxynitride glasses

Changes in microstructure induced by heat treatments between 1000 and 1500 °C in Si3N4 based glasses and ceramics were characterized using x‐ray diffraction and Raman scattering. The presence of free silicon appears to be a common feature of these families of materials. Si precipitates were observed by transmission electron microscopy in a bulk glass and in a polycrystalline ceramic. It is shown that heat treatments affect the free silicon content and that its decrease upon annealing is closely related to the crystallization of secondary phases. The incidence of the amount of free silicon on the material color strongly suggests that silicon microcrystals are responsible for most of the color fluctuations reported in silicon nitride glasses and ceramics. The higher the free Si content, the darker they are.

Vibrational spectra of lead alkali apatites Pb8M2(PO4)6 with M=Ag and Na

Abstract Powders of lead alkali apatites with formula Pb 8 M 2 (PO 4 ) 6 (M=Ag, Na) were prepared by using the solid state method. Infrared (4000–350 cm −1 ) and Raman (1200–25 cm −1 ) spectra of these samples were recorded at room temperature and analyzed. Fundamental vibrational modes were identified and band assignments were made in comparison with the Calcium hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 .

Localization of Eu3+ in KGd1 9Eu0.1F7 and KGd1.9Eu0.1F6.97O0.015 by site-selective excitation and time-resolved spectroscopy

Abstract This paper reports site-selective excitation (or observation) and time-resolved spectroscopy of the host matrices KGd 2 F 7 and KGd 2 F 6.97 O 0.015 doped by Eu 3+ . The crystal structure of KGd 2 F 7 derives from that of the fluorite-type, by an ordering of the cations and anions. The luminescence spectra and site-selective excitation in the 5 D 0 → 7 F 0 region, have allowed to identify several distributions of discrete sites for the Eu 3+ ions, in which two sites are characterized by unusual spectra which are attributed to Eu–O bonds. Another kind of site, characteristic of fluorinated surroundings close to the centrosymmetry, exhibits a very long lifetime (6.8 ms) of the 5 D 0 level. Although the accurate structures of these compounds are not yet known, they are very close to that of KGd 2.77 F 9.32 previously solved and the spectroscopic results are in agreement with the number and the symmetries of the rare earth crystallographic sites.

Electronic structure of Pr3+ and Tm3+ doped LiYO2

Abstract An optical investigation of LiYO 2 doped with 2% Pr 3+ and 5% Tm 3+ is presented. The unit cell is monoclinic and the site symmetry at the rare earth site is C 1 . The absorption, excitation and fluorescence spectra at 20 K are utilized to determine the electronic structure of the ground 4f 2 and 4f 12 configurations respectively. For LiYO 2 :Pr 3+ , 64 energy levels from 3 H 4 up to 3 P 2 at 22 000 cm −1 are reported. For LiYO 2 Tm −1 , 58 energy levels from 3 H 6 up to 3 P 2 at 38 000 cm −1 and the relative intensities for absorption from the 3 H 6 ground state are given. A crystal field analysis of the data is performed. Configuration interaction is applied to LiYO 2 :Pr 3+ and improves the results. While Eu 3+ and Tb 3+ doped LiYO 2 are good red and green phosphors respectively, LiYO 2 :5%Tm 3+ luminesces only weakly. The reason for that is the existence of many possible paths for up-conversion processes.

Spectroscopic study of crystalline Nd2O2S: Optical absorption measurements, crystal field analysis and intensity calculations

Room temperature and low temperature optical absorption measurements are performed on Nd2O2S in crystal and powder form. The experimental oscillator strengths are deduced and compared with calculations at 8 and 300 K including the contribution of Judd-Ofelt and Jorgensen-Judd mechanisms. The global oscillator strengths are satisfactorily reproduced by considering the sum of both contributions but the relative intensities of the strong transitions are best reproduced by the Jorgensen-Judd mechanism alone.

Anti-Stokes photoluminescence of yellow band in GaN: evidence of two-photon excitation process

Time-dependent photoluminescence of yellow band in GaN has been investigated under various excitation conditions using a Q-switched Nd : YAG laser as an excitation source. An anti-Stokes photoluminescence of the yellow band that dominates the photoluminescence (PL) spectra has been observed under the below band-gap excitation. The intensity of the anti-Stokes PL shows a non-linear dependence on the excitation intensity. Our results suggest that the energy up-conversion is via a two-step two-photon absorption process.

Etude du Systeme LiPO3-Pr(PO3)3 Donnees sur LiPr(PO3)4

The LiPO3-Pr(PO3)3 system was studied by micro-differential thermal analysis. The only new compound observed in the system was LiPr(PO3)4, melting incongruently at 1246 K. An eutectic appears at 926 K. Crystallographic data and powder diagram of the new compound are given. LiPr(PO3)4 crystallizes in the C2/c monoclinic system with unit cell:a=16.428(6), b=7.054(3), c=9.747(4) Å, β=126°31′(3), V=910.2 Å3, Z=4.The IR and Raman spectra of this compound are given.