Etudes structurales et propriétés de luminescence de nouveaux composés des systèmes Li2O-B2O3-Ln2O3 (Ln = terres rares)

The interest in borates has increased in recent years. The (Y,Gd)BO3: Eu3+ compound is currently used in the new plasma display panels as the red phosphor; Li6Ln(BO3)3: Ce3+ (Ln = raren earth) are a new class of scintillators for neutron imaging. In addition to the previously known borates Li6Ln(BO3)3 and Li3Ln2(BO3)3 the investigation of the Li2O-B2O3-Ln2O3 diagrams revealed the existence of four new compositions : the Li3Ln(BO3)2 borates and the LiLn6O5(BO3)3, LiLn2O2BO3 and Li2Ln5O4(BO3)3 oxyborates. All their structures weresolved on single crystals par X-ray diffraction. All oxyborates are characterized by the existence of anassembly of OLn4 tetrahedra, three-dimensional in LiLn6O5(BO3)3, bidimensional in Li2Ln5O4(BO3)3and LiLn2O2BO3, the (BO3)3- groups and Li+ ions being accommodated in holes or between the layersof these sub-lattices.The luminescent characteristics of trivalent europium, thulium and cerium were studied in order to evaluate their potential use in the previously described applications. Moreover charge transfer emissions of lanthanum and tetravalent cerium were observed.comme luminophore rouge dans les nouveaux dispositifs de visualisation à panneaux plasma ; le borate Li6(Y,Gd)(BO3)3: Ce3+ est particulièrement approprié pour la détection des neutrons. Outre les borates Li6Ln(BO3)3 (Ln = terre rare) et Li3Ln2(BO3)3 antérieurement connus, l'étude des diagrammes ternaires Li2O-B2O3-Ln2O3 a mis en évidence l'existence de quatre autres compositions : les borates Li3Ln(BO3)2 et les oxyborates LiLn6O5(BO3)3, LiLn2O2BO3 etLi2Ln5O4(BO3)3. Chacune des structures a été résolue sur monocristal par diffraction des rayons X.Une caractéristique des oxyborates est l'existence d'un assemblage de tétraèdres OLn4, tridimensionneldans les borates LiLn6O5(BO3)3 , bidimensionnel dans les phases Li2Ln5O4(BO3)3 et LiLn2O2BO3 , les groupes (BO3)3- et les ions Li+ se disposent dans des cavités ou entre les couches de ces sous-réseaux. Une étude de la luminescence des ions Eu3+, Tm3+ et Ce3+ a été effectuée afin notamment d'évaluer les performances de ces nouveaux matériaux en tant que luminophores ou scintillateurs. Pour la première fois dans des composés du bore, une luminescence de transfert de charge du lanthane et du cérium tétravalent a par ailleurs été observée

Crystal structures and luminescence properties of new compounds in the Li2O-B2O3-Ln2O3 systems (Ln = rare earth)

comme luminophore rouge dans les nouveaux dispositifs de visualisation à panneaux plasma ; le borate Li6(Y,Gd)(BO3)3: Ce3+ est particulièrement approprié pour la détection des neutrons. Outre les borates Li6Ln(BO3)3 (Ln = terre rare) et Li3Ln2(BO3)3 antérieurement connus, l'étude des diagrammes ternaires Li2O-B2O3-Ln2O3 a mis en évidence l'existence de quatre autres compositions : les borates Li3Ln(BO3)2 et les oxyborates LiLn6O5(BO3)3, LiLn2O2BO3 etLi2Ln5O4(BO3)3. Chacune des structures a été résolue sur monocristal par diffraction des rayons X.Une caractéristique des oxyborates est l'existence d'un assemblage de tétraèdres OLn4, tridimensionneldans les borates LiLn6O5(BO3)3 , bidimensionnel dans les phases Li2Ln5O4(BO3)3 et LiLn2O2BO3 , les groupes (BO3)3- et les ions Li+ se disposent dans des cavités ou entre les couches de ces sous-réseaux. Une étude de la luminescence des ions Eu3+, Tm3+ et Ce3+ a été effectuée afin notamment d'évaluer les performances de ces nouveaux matériaux en tant que luminophores ou scintillateurs. Pour la première fois dans des composés du bore, une luminescence de transfert de charge du lanthane et du cérium tétravalent a par ailleurs été observée.The interest in borates has increased in recent years. The (Y,Gd)BO3: Eu3+ compound is currently used in the new plasma display panels as the red phosphor; Li6Ln(BO3)3: Ce3+ (Ln = raren earth) are a new class of scintillators for neutron imaging. In addition to the previously known borates Li6Ln(BO3)3 and Li3Ln2(BO3)3 the investigation of the Li2O-B2O3-Ln2O3 diagrams revealed the existence of four new compositions : the Li3Ln(BO3)2 borates and the LiLn6O5(BO3)3, LiLn2O2BO3 and Li2Ln5O4(BO3)3 oxyborates. All their structures weresolved on single crystals par X-ray diffraction. All oxyborates are characterized by the existence of anassembly of OLn4 tetrahedra, three-dimensional in LiLn6O5(BO3)3, bidimensional in Li2Ln5O4(BO3)3and LiLn2O2BO3, the (BO3)3- groups and Li+ ions being accommodated in holes or between the layersof these sub-lattices.The luminescent characteristics of trivalent europium, thulium and cerium were studied in order to evaluate their potential use in the previously described applications. Moreover charge transfer emissions of lanthanum and tetravalent cerium were observed

ZnAl2O4 as potential sensor : variation of luminescence with thermal history

ZnAl2O4 spinel powders were prepared using the Pechini or co-precipitation synthetic route and were then treated at different temperatures (600-1350 °C). These powders were characterised by X-ray diffraction, scanning electron microscopy (SEM), diffuse reflectance and luminescence measurements. SEM investigations and the X-ray patterns showed that the spinel crystallite size was dependent on the synthetic route and the treatment temperature. In addition, the structural evolution was investigated by Rietveld refinements. The inversion rate decrease was correlated with the temperature, leading to a direct spinel phase for the sample treated at high temperature. Furthermore, luminescence measurements showed various emissions linked to the presence of defects in the matrix structure. The two main emissions observed were attributed to oxygen vacancy and Zn in the interstitial positions (as revealed by differential Fourier maps). The luminescence spectra exhibited strong differences between 1200 °C and 1350 °C. At the higher temperature, the characteristic emission spectra can be attributed to the direct spinel phase. The indirect-direct spinel transformation can be monitored through the change in the optical properties and correlated to the thermal history of the sample

ZnAl2O4 as potential sensor : variation of luminescence with thermal history

ZnAl2O4 spinel powders were prepared using the Pechini or co-precipitation synthetic route and were then treated at different temperatures (600-1350 °C). These powders were characterised by X-ray diffraction, scanning electron microscopy (SEM), diffuse reflectance and luminescence measurements. SEM investigations and the X-ray patterns showed that the spinel crystallite size was dependent on the synthetic route and the treatment temperature. In addition, the structural evolution was investigated by Rietveld refinements. The inversion rate decrease was correlated with the temperature, leading to a direct spinel phase for the sample treated at high temperature. Furthermore, luminescence measurements showed various emissions linked to the presence of defects in the matrix structure. The two main emissions observed were attributed to oxygen vacancy and Zn in the interstitial positions (as revealed by differential Fourier maps). The luminescence spectra exhibited strong differences between 1200 °C and 1350 °C. At the higher temperature, the characteristic emission spectra can be attributed to the direct spinel phase. The indirect-direct spinel transformation can be monitored through the change in the optical properties and correlated to the thermal history of the sample

Cristallogenèse, caractérisation spectroscopique et potentialité laser de borates et d'oxyborates de terre rare dopés ytterbium

Facilement excitable au moyen de diode InGaAs, l ion Ytterbium trivalent présenteune émission infrarouge intéressante pour des applications laser. Les matériaux étudiés sontnombreux. Parmi eux, les borates sont des composés stables, transparents dans l UV et présentant un seuil élevé au dommage. Ils sont par conséquent des bons candidats pour desapplications optiques. D un point de vue structural, ils présentent une diversité de sitesd accueil pour la terre rare ce qui permet de moduler les émissions obtenues en fonction de lacomposition.Nous avons mené une étude d élaboration et de caractérisation sur un borate (à fusioncongruente) et deux oxyborates (à fusion non congruente) de composition : Li6Ln(BO3)3 ;LiGd6O5(BO3)3 ; Na3La9O3(BO3)8.Les différentes croissances cristallines, par la méthode Czochralski ou par la méthodedu flux, nous ont permis d obtenir des cristaux de taille centimétrique comportant de largeszones transparentes, utiles pour les tests en cavité laser. L étude spectroscopique de l ionytterbium nous a permis d évaluer l éclatement des niveaux d énergie et de localiser les ionsterres rares dans ces matrices. Différentes caractérisations thermiques, mécaniques et optiquesont aussi été réalisées sur les cristaux afin de pouvoir estimer les paramètres laser.Finalement, de premiers tests lasers ont été réalisés et ont montré la potentialité de cesmatériaux en tant que matériaux amplificateurs.By the mean of InGaAs diode, trivalent ytterbium ions can give rise to an interestinginfrared emission for laser applications. Numerous compounds are in study. Among them,borates are stable, UV-transparent compounds and they have a high damage threshold. Thusthey are good candidates for optical applications. From a structural point of view, they exhibita diversity of host sites for the rare earth which allows to modulate emission withcomposition.We carry out an elaboration and characterization study on a borate and two oxyboratecompounds of formula Li6Ln(BO3)3 ; LiGd6O5(BO3)3 ; Na3La9O3(BO3)8.Crystal growth, by Czochralski or flux method depending on the meltingcharacteristic, enable us to obtain centimetric sized crystals with large transparent areas.Ytterbium ion spectroscopy study enables us to estimate energy levels splitting and to locaterare earth ions in these matrices. Several thermal, mechanical and optical characterizations were also performed on the crystals to estimate laser parameters. Finally, first laser tests werecarried out and have shown the potentiality of these materials as amplifier media.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Charge transfer emission of ytterbium-doped oxyborates

The ultraviolet spectroscopic properties of trivalent ytterbium have been studied at low temperature in one borate and two oxyborates in the ternary diagram Li2O–Y2O3–B2O3. The UV luminescence was detected in the two ytterbium-doped oxyborates. The evolution of these emissions was studied as a function of the temperature. A determination of the configurational coordinate diagrams based on the Struck and Fonger model is proposed to calculate the position in energy of the charge transfer band afforded by the simulation of spectral distribution of these excitation and emission bands. The strong correlation between the luminescent properties and the environments of the rare earth is pointed out

Luminescence properties of tungstates and molybdates phosphors: Illustration on ALn(MO₄)₂ compounds (A = alikaline cation, Ln = lanthanides, M = W, Mo)

The photoluminescence characteristics of compounds ALn(MO4)2, with A = Li, Na, K; Ln = Y, La, Gd, Lu, M = Mo, W, are analyzed as a function of their structure. The influences of cation size and electronegativity are discussed in relation with the Struck and Fonger theory, with a view to developing one or more of those compounds as phosphors in relation with the development of white LEDs. It appears that there are various excited states implied in the absorption process, with partially radiative transfer between them. All the radiative mechanisms are strongly related to temperature. Due to their electronegativity, tungstate compounds are the most promising, compared to molybdate ones, especially those in the monoclinic P2/n structure

Charge transfer emission of ytterbium-doped oxyborates

The ultraviolet spectroscopic properties of trivalent ytterbium have been studied at low temperature in one borate and two oxyborates in the ternary diagram Li2O–Y2O3–B2O3. The UV luminescence was detected in the two ytterbium-doped oxyborates. The evolution of these emissions was studied as a function of the temperature. A determination of the configurational coordinate diagrams based on the Struck and Fonger model is proposed to calculate the position in energy of the charge transfer band afforded by the simulation of spectral distribution of these excitation and emission bands. The strong correlation between the luminescent properties and the environments of the rare earth is pointed out

Luminescence switch of Mn-Doped ZnAl2O4 powder with temperature

Manganese-doped ZnAl2O4 phosphors were prepared by the Pechini synthesis route and treated at various temperatures from 600 to 1350 °C. The samples were characterized by TEM-EDX, XRD, EPR, and their diffuse reflectance and luminescence properties were investigated. The structural analysis showed the high solubility limit of manganese in this spinel matrix and allowed the determination of the global inversion rate, which characterizes the cation distribution in the A and B sites of the spinel structure. As the annealing temperature increased, this factor decreased leading to a more direct matrix. EPR analysis showed that, besides Mn3+ to Mn2+ reduction, the local environment of Mn2+ cations changed with the annealing temperature, which was also reflected in the evolution of the optical properties. As the annealing temperature increased, the red luminescence related to the presence of divalent manganese in octahedral sites faded and was replaced by a new green emission due to Mn(II) ions located in tetrahedral sites within the spinel structure. For 0.5% Mn-doped ZnAl2O4, this red to green luminescence switch occurred for samples treated between 1200 and 1350 °C. Moreover, the Al-overstoichiometric samples (Mn:ZnAl2.2O4+δ) showed that it is possible to modify the temperature range and the kinetics of this variation in emission wavelength. These tuneable properties suggest that Mn-doped spinels are potential candidates for developing stable and highly sensitive thermal sensors.Matériaux avancés pour capteurs optiques

Progress on lanthanide sesquioxide phase transition

The present work aims to bring an original approach to the understanding of the polymorphic transition of RE2O3 from the low-temperature polymorph to the higher-temperature polymorph: from the cubic C to the hexagonal A form or from the monoclinic C to the monoclinic B form, for which the transition temperatures depend intimately on the rare earth element. The view proposed here, focusing on the lanthanide sesquioxide series, from Nd2O3 to Dy2O3, includes crystallographic and energetic considerations, using both bulk and surface energy calculations. After a complete description of the polymorphic filiations based on A, B and C unit cells described on simplified RE cation stacking schemes, it will be shown that the growth of the crystallite size as a function of temperature explains the existence of such transitions for these sesquioxides